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The effect of pulse crops on arbuscula mycorrhizal fungi in a durum-based cropping systemFraser, Tandra 07 April 2008
Pulses are an important component in crop rotations in the semiarid Brown soil zone of southern Saskatchewan, Canada. Besides their capability to fix nitrogen, pulse crops establish a strong symbiotic relationship with arbuscular mycorrhizal fungi (AMF), which have been shown to increase nutrient and water uptake through hyphal extensions in the soil. Incorporating strongly mycorrhizal crops in a rotation may increase inoculum levels in the soil and benefit the growth of a subsequent crop. The objective of this study was to determine if AMF potential and colonization of a durum crop is significantly affected by cropping history and to assess the impact of pulses in crop rotations on the abundance and diversity of AMF communities in the soil. In 2004 and 2005, soil, plant, and root samples were taken on Triticum turgidum L. (durum) with preceding crops of Pisum sativum L. (pea), Lens culinaris Medik (lentil), Cicer arietinum L. (chickpea), Brassica napus L. (canola) or Triticum turgidum L. (durum). Although there were few differences in soil N and P levels, previous crop had a significant effect (p<0.05) on durum yields in both years. A previous crop of pea was associated with the highest yields, while the durum monocultures were lowest. Arbuscular mycorrhizal potential and colonization were significantly affected (p<0.05) by cropping history, but not consistently as a result of inclusion of a pulse crop. Phospholipid and neutralipid fatty acids (PLFA/NLFA) were completed to analyse the relative abundance of AMF (C16:1ù5), saprophytic fungi (C18:2ù6), and bacteria in the soil. The effect of treatment on the abundance of AMF, saprotrophic fungi and bacteria were not significant (p<0.05), but the changes over time were. These results demonstrate that although previous crop may play a role in microbial community structure, it is not the only influencing factor.
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The effect of pulse crops on arbuscula mycorrhizal fungi in a durum-based cropping systemFraser, Tandra 07 April 2008 (has links)
Pulses are an important component in crop rotations in the semiarid Brown soil zone of southern Saskatchewan, Canada. Besides their capability to fix nitrogen, pulse crops establish a strong symbiotic relationship with arbuscular mycorrhizal fungi (AMF), which have been shown to increase nutrient and water uptake through hyphal extensions in the soil. Incorporating strongly mycorrhizal crops in a rotation may increase inoculum levels in the soil and benefit the growth of a subsequent crop. The objective of this study was to determine if AMF potential and colonization of a durum crop is significantly affected by cropping history and to assess the impact of pulses in crop rotations on the abundance and diversity of AMF communities in the soil. In 2004 and 2005, soil, plant, and root samples were taken on Triticum turgidum L. (durum) with preceding crops of Pisum sativum L. (pea), Lens culinaris Medik (lentil), Cicer arietinum L. (chickpea), Brassica napus L. (canola) or Triticum turgidum L. (durum). Although there were few differences in soil N and P levels, previous crop had a significant effect (p<0.05) on durum yields in both years. A previous crop of pea was associated with the highest yields, while the durum monocultures were lowest. Arbuscular mycorrhizal potential and colonization were significantly affected (p<0.05) by cropping history, but not consistently as a result of inclusion of a pulse crop. Phospholipid and neutralipid fatty acids (PLFA/NLFA) were completed to analyse the relative abundance of AMF (C16:1ù5), saprophytic fungi (C18:2ù6), and bacteria in the soil. The effect of treatment on the abundance of AMF, saprotrophic fungi and bacteria were not significant (p<0.05), but the changes over time were. These results demonstrate that although previous crop may play a role in microbial community structure, it is not the only influencing factor.
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Soilborne disease suppressiveness / conduciveness : analysis of microbial community dynamics / by Johannes Hendrikus HabigHabig, Johannes Hendrikus January 2003 (has links)
Take-all is the name given to the disease caused by a soilborne fungus
Gaeumannomyces graminis (Sacc.) von Arx and Olivier var. tritici Walker (Ggt), an
ascomycete of the family Magnaportheaceae (Cook, 2003). This fungus is an
aggressive soil-borne pathogen causing root rot of wheat (primary host), barley and rye
crops (secondary host). The flowering, seedling, and vegetative growth stages can be
affected by the infection of the whole plant, leaves, roots, and stems. Infections of roots
result in losses in crop yield and quality primarily due to a lowering in nutrient uptake.
Take-all is most common in regions where wheat is cultivated without adequate crop
rotation. Crop rotation allows time between the planting dates of susceptible crops,
which causes a decrease in the inoculum potential of soilborne plant pathogens to
levels below an economic threshold by resident antagonistic soil microbial communities.
Soilborne disease suppressiveness is an inherent characteristic of the physical,
chemical, and/or biological structure of a particular soil which might be induced by
agricultural practices and activities such as the cultivation of crops, or the addition of
organisms or nutritional amendments, causing a change in the microfloral environment.
Disturbances of soil ecosystems that impact on the normal functioning of microbial
communities are potentially detrimental to soil formation, energy transfers, nutrient
cycling, and long-term stability. In this regard, an overview of soil properties and
processes indicated that the use of microbiological and biochemical soil properties,
such as microbial biomass, the analysis of microbial functional diversity and microbial
structural diversity by the quantification of community level physiological profiles and
signature lipid biomarkers are useful as indicators of soil ecological stress or restoration
properties because they are more responsive to small changes than physical and
chemical characteristics. In this study, the relationship between physico-chemical
characteristics, and different biological indicators of soil quality of agricultural soils
conducive, suppressive, and neutral with respect to take-all disease of wheat as caused
by the soilborne fungus Gaeumannomyces graminis var. tritici (Ggt), were investigated
using various techniques. The effect of crop rotation on the functional and structural
diversity of soils conducive to take-all disease was also investigated. Through the
integration of quantitative and qualitative biological data as well as the physico-chemical
characteristics of the various soils, the functional and structural diversity of microbial
IV
communities in the soils during different stadia of take-all disease of wheat were
characterised. All results were evaluated statistically and the predominant physical and
chemical characteristics that influenced the microbiological and biochemical properties
of the agricultural soils during different stadia of take-all disease of wheat were identified
using multivariate analyses. Although no significant difference @ > 0.05) could be
observed between the various soils using conventional microbiological enumeration
techniques, the incidence of Gliocladium spp. in suppressive soils was increased.
Significant differences @ < 0.05) were observed between agricultural soils during
different stadia of take-all disease of wheat. Although no clear distinction could be made
between soils suppressive and neutral to take-all disease of wheat, soils suppressive
and conducive to take-all disease of wheat differed substantially in their community level
physiological profiles (CLPPs). Soils suppressive / neutral to take-all disease were
characterised by enhanced utilisation of carboxylic acids, amino acids, and
carbohydrates, while conducive soils were characterised by enhanced utilisation of
carbohydrates. Shifts in the functional diversity of the associated microbial communities
were possibly caused by the presence of Ggt and associated antagonistic fungal and
bacterial populations in the various soils. It was evident that the relationships amongst
the functionality of the microbial communities within the various soils had undergone
changes through the different stages of development of take-all disease of wheat, thus
implying different substrate utilisation capabilities of present soil microbial communities.
Diversity indices were calculated as Shannon's diversity index (H') and substrate
equitability (J) and were overall within the higher diversity range of 3.6 and 0.8,
respectively, indicating the achievement of very high substrate diversity values in the
various soils. A substantial percentage of the carbon sources were utilised, which
contributed to the very high Shannon-Weaver substrate utilisation indices. Obtained
substrate evenness (equitability) (J) indices indicated an existing high functional
diversity. The functional diversity as observed during crop rotation, differed significantly
(p < 0.05) from each other, implying different substrate utilisation capabilities of present
soil microbial communities, which could possibly be ascribed to the excretion of root
exudates by sunflowers and soybeans. Using the Sorenson's index, a clear distinction
could be made between the degrees of substrate utilisation between microbial
populations in soils conducive, suppressive, and neutral to take-all disease of wheat, as
well as during crop rotation. Furthermore, the various soils could also be differentiated
on the basis of the microbial community structure as determined by phospholipid fatty
acid (PLFA) analysis. Soil suppressive to take-all disease of wheat differed significantly
(p < 0.05) from soils conducive, and neutral to take-all disease of wheat, implying a shift
in relationships amongst the structural diversity of microbial communities within the
various soils. A positive association was observed between the microbial phospholipid
fatty acid profiles, and dominant environmental variables of soils conducive,
suppressive, and neutral to take-all disease of wheat. Soils conducive and neutral to
take-all disease of wheat were characterised by high concentrations of manganese, as
well as elevated concentrations of monounsaturated fatty acids, terminally branched
saturated fatty acids, and polyunsaturated fatty acids which were indicative of Gram-negative
bacteria, Gram-positive bacteria and micro eukaryotes (primarily fungi),
respectively. These soils were also characterised by low concentrations of
phosphorous, potassium, percentage organic carbon, and percentage organic nitrogen,
as well as low soil pH. Soil suppressive to take-all disease of wheat was characterised
by the elevated levels of estimated of biomass and elevated concentrations of normal
saturated fatty acids, which is ubiquitous to micro-organisms. The concentration of
normal saturated fatty acids in suppressive soils is indicative of a low structural
diversity. This soil was also characterised by high concentrations of phosphorous,
potassium, percentage organic carbon, and percentage organic nitrogen, as well as
elevated soil pH. The relationship between PLFAs and agricultural soils was
investigated using principal component analysis (PCA), redundancy analysis (RDA) and
discriminant analysis (DA). Soil suppressive to take-all disease of wheat differed
significantly (p < 0.05) from soils conducive, and neutral to take-all disease of wheat,
implying a shift in relationships amongst the structural diversity of microbial communities
within the various soils. A positive association was observed between the microbial
phospholipid fatty acid profiles, and dominant environmental variables of soils
conducive, suppressive, and neutral to take-all disease of wheat. Hierarchical cluster
analysis of the major phospholipid fatty acid groups indicated that the structural diversity
differed significantly between soils conducive, suppressive, and neutral to take-all
disease of wheat caused by Gaeumannomyces graminis var. tritici. The results indicate
that the microbial community functionality as well as the microbial community structure
was significantly influenced by the presence of take-all disease of wheat caused by
Gaeumannomyces graminis var. tritici, and that the characterisation of microbial
functional and structural diversity by analysis of community level physiological profiles
and phospholipid fatty acid analysis, respectively, could be successfully used as an
assessment criteria for the evaluation of agricultural soils conducive, suppressive, and
neutral to take-all disease of wheat, as well as in crop rotation systems. This
methodology might be of significant value in assisting in the management and
evaluation of agricultural soils subject to the prevalence of other soilborne diseases. / Thesis (M.Sc. (Microbiology))--North-West University, Potchefstroom Campus, 2004.
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Soilborne disease suppressiveness / conduciveness : analysis of microbial community dynamics / by Johannes Hendrikus HabigHabig, Johannes Hendrikus January 2003 (has links)
Take-all is the name given to the disease caused by a soilborne fungus
Gaeumannomyces graminis (Sacc.) von Arx and Olivier var. tritici Walker (Ggt), an
ascomycete of the family Magnaportheaceae (Cook, 2003). This fungus is an
aggressive soil-borne pathogen causing root rot of wheat (primary host), barley and rye
crops (secondary host). The flowering, seedling, and vegetative growth stages can be
affected by the infection of the whole plant, leaves, roots, and stems. Infections of roots
result in losses in crop yield and quality primarily due to a lowering in nutrient uptake.
Take-all is most common in regions where wheat is cultivated without adequate crop
rotation. Crop rotation allows time between the planting dates of susceptible crops,
which causes a decrease in the inoculum potential of soilborne plant pathogens to
levels below an economic threshold by resident antagonistic soil microbial communities.
Soilborne disease suppressiveness is an inherent characteristic of the physical,
chemical, and/or biological structure of a particular soil which might be induced by
agricultural practices and activities such as the cultivation of crops, or the addition of
organisms or nutritional amendments, causing a change in the microfloral environment.
Disturbances of soil ecosystems that impact on the normal functioning of microbial
communities are potentially detrimental to soil formation, energy transfers, nutrient
cycling, and long-term stability. In this regard, an overview of soil properties and
processes indicated that the use of microbiological and biochemical soil properties,
such as microbial biomass, the analysis of microbial functional diversity and microbial
structural diversity by the quantification of community level physiological profiles and
signature lipid biomarkers are useful as indicators of soil ecological stress or restoration
properties because they are more responsive to small changes than physical and
chemical characteristics. In this study, the relationship between physico-chemical
characteristics, and different biological indicators of soil quality of agricultural soils
conducive, suppressive, and neutral with respect to take-all disease of wheat as caused
by the soilborne fungus Gaeumannomyces graminis var. tritici (Ggt), were investigated
using various techniques. The effect of crop rotation on the functional and structural
diversity of soils conducive to take-all disease was also investigated. Through the
integration of quantitative and qualitative biological data as well as the physico-chemical
characteristics of the various soils, the functional and structural diversity of microbial
IV
communities in the soils during different stadia of take-all disease of wheat were
characterised. All results were evaluated statistically and the predominant physical and
chemical characteristics that influenced the microbiological and biochemical properties
of the agricultural soils during different stadia of take-all disease of wheat were identified
using multivariate analyses. Although no significant difference @ > 0.05) could be
observed between the various soils using conventional microbiological enumeration
techniques, the incidence of Gliocladium spp. in suppressive soils was increased.
Significant differences @ < 0.05) were observed between agricultural soils during
different stadia of take-all disease of wheat. Although no clear distinction could be made
between soils suppressive and neutral to take-all disease of wheat, soils suppressive
and conducive to take-all disease of wheat differed substantially in their community level
physiological profiles (CLPPs). Soils suppressive / neutral to take-all disease were
characterised by enhanced utilisation of carboxylic acids, amino acids, and
carbohydrates, while conducive soils were characterised by enhanced utilisation of
carbohydrates. Shifts in the functional diversity of the associated microbial communities
were possibly caused by the presence of Ggt and associated antagonistic fungal and
bacterial populations in the various soils. It was evident that the relationships amongst
the functionality of the microbial communities within the various soils had undergone
changes through the different stages of development of take-all disease of wheat, thus
implying different substrate utilisation capabilities of present soil microbial communities.
Diversity indices were calculated as Shannon's diversity index (H') and substrate
equitability (J) and were overall within the higher diversity range of 3.6 and 0.8,
respectively, indicating the achievement of very high substrate diversity values in the
various soils. A substantial percentage of the carbon sources were utilised, which
contributed to the very high Shannon-Weaver substrate utilisation indices. Obtained
substrate evenness (equitability) (J) indices indicated an existing high functional
diversity. The functional diversity as observed during crop rotation, differed significantly
(p < 0.05) from each other, implying different substrate utilisation capabilities of present
soil microbial communities, which could possibly be ascribed to the excretion of root
exudates by sunflowers and soybeans. Using the Sorenson's index, a clear distinction
could be made between the degrees of substrate utilisation between microbial
populations in soils conducive, suppressive, and neutral to take-all disease of wheat, as
well as during crop rotation. Furthermore, the various soils could also be differentiated
on the basis of the microbial community structure as determined by phospholipid fatty
acid (PLFA) analysis. Soil suppressive to take-all disease of wheat differed significantly
(p < 0.05) from soils conducive, and neutral to take-all disease of wheat, implying a shift
in relationships amongst the structural diversity of microbial communities within the
various soils. A positive association was observed between the microbial phospholipid
fatty acid profiles, and dominant environmental variables of soils conducive,
suppressive, and neutral to take-all disease of wheat. Soils conducive and neutral to
take-all disease of wheat were characterised by high concentrations of manganese, as
well as elevated concentrations of monounsaturated fatty acids, terminally branched
saturated fatty acids, and polyunsaturated fatty acids which were indicative of Gram-negative
bacteria, Gram-positive bacteria and micro eukaryotes (primarily fungi),
respectively. These soils were also characterised by low concentrations of
phosphorous, potassium, percentage organic carbon, and percentage organic nitrogen,
as well as low soil pH. Soil suppressive to take-all disease of wheat was characterised
by the elevated levels of estimated of biomass and elevated concentrations of normal
saturated fatty acids, which is ubiquitous to micro-organisms. The concentration of
normal saturated fatty acids in suppressive soils is indicative of a low structural
diversity. This soil was also characterised by high concentrations of phosphorous,
potassium, percentage organic carbon, and percentage organic nitrogen, as well as
elevated soil pH. The relationship between PLFAs and agricultural soils was
investigated using principal component analysis (PCA), redundancy analysis (RDA) and
discriminant analysis (DA). Soil suppressive to take-all disease of wheat differed
significantly (p < 0.05) from soils conducive, and neutral to take-all disease of wheat,
implying a shift in relationships amongst the structural diversity of microbial communities
within the various soils. A positive association was observed between the microbial
phospholipid fatty acid profiles, and dominant environmental variables of soils
conducive, suppressive, and neutral to take-all disease of wheat. Hierarchical cluster
analysis of the major phospholipid fatty acid groups indicated that the structural diversity
differed significantly between soils conducive, suppressive, and neutral to take-all
disease of wheat caused by Gaeumannomyces graminis var. tritici. The results indicate
that the microbial community functionality as well as the microbial community structure
was significantly influenced by the presence of take-all disease of wheat caused by
Gaeumannomyces graminis var. tritici, and that the characterisation of microbial
functional and structural diversity by analysis of community level physiological profiles
and phospholipid fatty acid analysis, respectively, could be successfully used as an
assessment criteria for the evaluation of agricultural soils conducive, suppressive, and
neutral to take-all disease of wheat, as well as in crop rotation systems. This
methodology might be of significant value in assisting in the management and
evaluation of agricultural soils subject to the prevalence of other soilborne diseases. / Thesis (M.Sc. (Microbiology))--North-West University, Potchefstroom Campus, 2004.
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Implications of land-use change and pasture management on soil microbial function and structure in the mountain rainforest region of southern EcuadorPotthast, Karin 07 June 2013 (has links) (PDF)
In the present thesis, implications of pasture establishment, fertilization and abandonment on soil C and nutrient dynamics were investigated for the mountain rainforest region of southern Ecuador. Over the past decades the natural forest of the study area has been threatened by conversion to cattle pastures. However, the soil fertility of these extensively grazed pastures (active pastures) declines continuously during pasture use. The invasion of bracken fern (Pteridium arachnoideum) leads to pasture abandonment when bracken becomes dominant. In order to reveal the mechanisms behind the deterioration of soil fertility, biotic and abiotic soil properties and their interaction were analyzed along a land‐use gradient (natural forest – active pasture – abandoned pasture).
The ecosystem disturbance of the mountain rainforest through pasture use changed the microbial function and structure, and affected soil CO2‐C fluxes. Annually, 2 Mg soil CO2‐C ha‐1 were additionally emitted from the pasture land. This acceleration in soil respiration rates was related to accelerated rates of microbial C mineralization and fine‐root respiration. The high‐quality, N‐rich above‐ and belowground residues of the pasture grass (S. sphacelata, C4‐plant), especially the huge fine‐root biomass, provided a high C and N availability for soil microbes. Compared to the forest, increased soil pH and accelerated base saturation were further factors beneficial for soil microbial growth and metabolism of the upper mineral soil at active pastures. Three times higher amounts of microbial biomass C and a significant shift in the microbial community structure towards a higher relative abundance of Gram(‐)‐ bacteria and fungi were observed.
Long‐term pasture use and the invasion of bracken (C3‐plant) diminished beneficial effects for microbes, causing a significant decrease in the C, net, and gross N mineralization rates as well as a two‐third reduction in the microbial biomass. A preferential substrate utilization of grass‐derived C4 by the soil microbes resulted in a rapid decline of the C4‐pool. As a consequence, the less available C3‐pool from bracken and former forest increased its dominance in the SOC‐pool, further decreasing pasture productivity and finally causing pasture abandonment. The lower quality and quantity of above‐ and belowground residues of the bracken (high lignin content, C/N) resulted in resource‐limited conditions that influenced the microbial function to greater extent than their structure. The microbial structure seemed to be sensitive mainly to soil pH along the land‐use gradient. Thus, a disconnection between microbial structure and function was identified.
Fertilization experiments were conducted both in the lab and in the field to evaluate the impact of urea and/or rock phosphate amendment on SOM dynamics and on pasture productivity of active pastures. After combined fertilization the pasture yield was most efficiently increased by 2 Mg ha−1 a−1, indicating a NP‐limitation of grass growth. Furthermore, the fodder quality was improved by a higher content of P and Ca in the grass biomass. The microorganisms of the active pasture soil responded with an adaptation of their structure to the increased substrate availability in the short term, but did not change their initial functions in the long term. After urea/ rock phosphate addition a significant increase in the relative fungal abundance was detected, but neither a microbial limitation of energy nor of N or P was observed. However, urea addition accelerated gaseous losses of soil CO2‐C in the short term.
In the study area, pronounced alterations in ecosystem functioning due to land‐use changes were detected, especially in soil C and N cycling rates. For a sustainable land‐use in this region it is crucial to prevent pasture degradation and to rehabilitate degraded pastures in order to protect the prevailing mountain rainforest ecosystem. It is of crucial importance for active pasture soils to maintain or even increase resource availability, being one indicator of soil fertility. In this context, the soil organic matter has to be retained in the long‐term to maintain high microbial activity and biomass, and thus pasture productivity. A moderate fertilization with urea and rock phosphate can be a first step to provide continuous nutrient supply for grass growth and to strengthen livestock health through increased fodder quality. However, the risk of further additional emissions of soil CO2‐C due to increased loads of urea fertilizer application has to be kept in mind. Overall, for the establishment of a sustainable land‐use management the control of bracken invasion and an adjusted nutrient management are needed. Further investigations on the reduction of soil nutrient losses and increased nutrient use efficiencies of plants, such as combined planting with legumes or the usage of cultivars with special nutrient acquisition strategies, should be in the focus of future work. / In der vorliegenden Dissertation werden die Auswirkungen der Weideetablierung, ‐düngung sowie des Verlassens von Weiden auf Bodenkohlenstoff‐ und Nährstoffdynamik in einer tropischen Bergregenwaldregion Ecuadors zusammenfassend dargestellt und diskutiert. Der Naturwald des Untersuchungsgebietes ist seit Jahrzehnten durch Brandrodung und die Umwandlung in extensiv genutztes Weideland (aktive Weide) in seinem flächenhaften Bestand bedroht. Als Problem hat sich der Verlust an Fruchtbarkeit der Weideböden während ihrer Bewirtschaftung herausgestellt. Des Weiteren führt die Einwanderung des Tropischen Adlerfarns (Pteridium arachnoideum, C3‐Pflanze) zu einer Reduktion der oberirdischen Grasbiomasse. Nimmt diese Entwicklung überhand, werden die betroffenen Flächen von den Bauern nicht mehr aktiv genutzt, verlassen und neuer Regenwald gerodet. Um mehr über die Mechanismen der Verringerung der Bodenfruchtbarkeit zu erfahren, wurden biotische und abiotische Bodeneigenschaften und deren Interaktion entlang eines Landnutzungsgradienten (Naturwald – aktive Weide – verlassene Weide) untersucht.
Die Zerstörung des Bergregenwaldökosystems und die Überführung der gerodeten Flächen zur Weidebewirtschaftung verändert die Funktion und Struktur der Bodenmikroorganismen und beeinflusst den CO2‐C Fluss aus dem Boden. Jährlich werden 2 t CO2‐C ha‐1 zusätzlich vom Weideland emittiert. Diese Erhöhung der Bodenatmungsraten kann mit erhöhten Raten der mikrobiellen C‐Mineralisierung und Feinwurzelatmung in Verbindung gebracht werden. Das Weidegras (S. sphacelata, C4‐Pflanze) liefert C‐ und N‐reiche ober und unterirdische organische Substanz (z.B. durch die Feinwurzelbiomasse) und trägt damit zu einer Erhöhung der C‐ und N‐Verfügbarkeit für die mikroorganismen bei. Darüber hinaus stellen ein höherer pH‐Wert und eine erhöhte Basensättigung im oberen Mineralboden der aktiven Weide günstige Bedingungen für mikrobielles Wachstum und Metabolismus dar. Als Konsequenz sind die Gehalte an mikrobiellem Biomassekohlenstoff um das Dreifache erhöht und die mikrobiellen Gemeinschaftsstrukturen signifikant in Richtung einer höheren relativen Abundanz der Gram(‐)‐Bakterien und Pilze verschoben.
Eine längerfristige Weidebewirtschaftung ohne Kompensation von Nährstoffverlusten sowie die Einwanderung des Tropischen Adlerfarnes verschlechterte die Bedingungen für die Mikroorganismen, was zu einem signifikanten Rückgang des SOC, der Netto‐ und Brutto‐N‐Mineralisierungsraten sowie zu einer Halbierung der mikrobiellen Biomasse führt. Eine bevorzugte Substratnutzung von Graskohlenstoff (C4) durch die Mikroorganismen hat einen schnellen Abbau des C4‐Pools zur Folge. Somit dominiert nun der mikrobiell schlechter verfügbare C3‐Pool den Bodenkohlenstoffpool. Dies führt zu einem weiteren Rückgang der Weideproduktivität und schließlich zum Offenlassen der Weide. Die geringere Qualität und Quantität der vom Farn stammenden ober‐ und unterirdischen organischen Substanz (hoher Ligninanteil, weites C/N), führten zu einer Limitierung der Ressourcen für die Mikroorganismen, welche deren Funktionen in größerem Maße beeinflussen als deren Gemeinschaftsstruktur. Im Gegensatz dazu wird entlang des Landnutzungsgradienten die Struktur hauptsächlich durch den pH‐Wert beeinflusst. Daraus folgt, dass Struktur und Funktion der Bodenmikroorganismen voneinander entkoppelt auf Veränderungen reagieren können.
Um den Einfluss von Harnstoff‐ und/ oder Rohphosphatdüngung aktiver Weiden auf die Dynamik der organischen Bodensubstanz und auf die Weideproduktivität zu untersuchen, wurden sowohl Labor‐ als auch Feldversuche durchgeführt. Im Feldexperiment wurde gezeigt, dass eine NP‐Limitierung der Grasbiomasseproduktion vorliegt und durch eine geringe NP‐Kombinationsdüngung die oberirdische Phytomasseproduktion um 2 t ha−1 a−1 gesteigert und die Futterqualität durch eine Erhöhung der P‐ und Ca‐ Gehalte verbessert werden kann. Die Mikroorganismen reagierten mit einer Anpassung ihrer Struktur an die kurzzeitig erhöhte Substratverfügbarkeit. Nach Gabe von Harnstoff und/ oder Rohphosphat wurde weder eine N‐ noch eine P‐Limitierung der Bodenmikroorganismen festgestellt, und die mikrobiellen Funktionen wurden langfristig nicht verändert. Dagegen bewirkte die Düngergabe einen erhöhten relativen Anteil der Pilzabundanz. Im Labor sowie im Feld kam es nach Harnstoffdüngung kurzzeitig zu verstärkten gasförmigen Verlusten des Bodenkohlenstoffs.
Aufgrund der Landnutzungsänderungen im Untersuchungsgebiet veränderten sich die Ökosystemfunktionen stark, speziell die Boden‐C‐ und Boden‐N‐Umsatzraten. Für eine nachhaltige Landnutzung in der Region, d. h., für den Schutz der noch verbliebenen natürlichen Bergregenwaldflächen, ist es von entscheidender Bedeutung, dass die Weidedegradierung verhindert wird und degradierte Flächen wieder in Nutzung genommen werden. Als entscheidend für die Weideproduktivität hat sich in dieser Studie die Ressourcenverfügbarkeit für Bodenmikroorganismen herausgestellt. Daher ist es sehr wichtig, diese Ressourcenverfügbarkeit in Böden aktiv‐genutzter Weiden zu erhalten oder noch zu erhöhen, denn sie wirkt sich vor allem auf die organische Bodensubstanz und im Wechselspiel damit auf die mikrobielle Biomasse und Aktivität aus. Eine moderate Kombinationsdüngung aus Harnstoff und Rohphosphat ist ein erster Schritt in diese Richtung. Dabei sollte jedoch das Risiko zusätzlicher bodenbürtiger CO2‐C Emissionen in Folge höherer Düngergaben berücksichtigt werden. Für ein nachhaltiges Landnutzungsmanagement sind Maßnahmen gegen die Einwanderung des Adlerfarnes und ein angepasstes Nährstoffmanagement notwendig. Weitere Untersuchungen sollten auf eine Minimierung der Nährstoffverluste und eine erhöhte Nährstoffnutzungseffizienz der Pflanzen fokussiert werden. Weidemischkulturen aus Gräsern mit Leguminosen sowie der Einsatz von Kulturen mit speziellen Nährstoffaneignungsstrategien könnten dabei eine große Rolle spielen und sollten in der Region erprobt werden. / La tesis presentada investiga el impacto del establecimiento de pasto, de su fertilización y de su manejo tradicional (abandono del pastizal) a la dinámica del carbono y de los nutrientes de suelo en la región de los bosques tropicales montañosos en el Sur de Ecuador. Durante las últimas décadas el bosque natural en el área de estudio ha estado amenazada por su conversión a pastizales. Sin embargo, la fertilidad del suelo en pastos de tipo extensivo (pastos activos) decrece frecuentemente durante el uso de los pastos. La invasión de Llashipa (Pteridium arachnoideum) conduce al abandono de los pastos cuando la ésta se vuelve dominante. Con la finalidad de revelar los mecanismos detrás de esta disminución de la fertilidad de suelo, se analizaron las propiedades bióticas y abióticas del suelo y sus interacciones, a lo largo de una gradiente del uso de la tierra (bosque natural —pasto activo — pastos abandonados).
La perturbación del ecosistema de bosque tropical montañoso por su cambio de uso, mediante el establecimiento de pastizales, ha alterado la función y la estructura de los microorganismos y ha afectado el flujo de CO2‐C del suelo. Cada año 2 Mg CO2‐C ha‐1 fueron emitidas adicionalmente por el establecimiento de pastos. Esta aceleración en la tasa de respiración del suelo está relacionada con el aumento de las tasas de mineralización microbiana de carbono y de la respiración de las raíces. La alta calidad y abundancia de N de los residuos orgánicos del suelo con pasto Mequeron (S. sphacelata, C4‐planta), especialmente debido a la gran biomasa de las raíces finas, ofrecen una disponibilidad alta de C y N para los microorganismos. En comparación con el bosque natural, el aumento del pH y la saturación bases acelerada fueron condiciones más favorables para el crecimiento microbiano y para el metabolismo microbiano en el parte superior del suelo mineral en pastos activos. La cantidad de C de la biomasa de los microorganismos fue tres veces mayor que la del bosque y se ha observado un cambio significativo de la estructura de la comunidad microbiana, en donde la abundancia relativa de los hongos y de las bacterias Gram(‐) ha aumentado.
El uso de pasto a largo plazo y la invasión de Llashipa (C3‐planta) han reducido los efectos benéficos para los microorganismos, que resultaron en una reducción significativa de las tasas de la mineralización de C y N, y en una reducción en dos tercios de la biomasa microbiana. El uso preferencial de los microorganismos por sustrato de pasto C4 han resultado en una rápida disminución de la reserva de C4. Como consecuencia, la menor disponibilidad de la reserva de C3 de las plantas de Llashipa y de la cobertura anterior de
bosque ha incrementado su dominancia en la reserva de materia orgánica del suelo. Eso resulta, en una mayor disminución de la productividad de los pastos, conduciendo finalmente al abandono de los campos de pastos. La menor calidad y cantidad de los residuos acumulados sobre y bajo el suelo provenientes de la Llashipa han dado como resultado un sustrato de limitadas condiciones que están afectando más a las funciones microbiales antes que a su estructura. La estructura microbiana parece ser más sensible al pH del suelo a largo de la gradiente del uso de la tierra; de manera que se ha identificado una desconexión entre la estructura y función microbial.
Experimentos de fertilización en laboratorio y en campo han sido realizados para evaluar el impacto de la aplicación de enmiendas (urea y/o roca fosfórica) a la dinámica de la materia orgánica y a la productividad de los pastos activos. El resultado del experimento de campo ha demostrado que la fertilización combinada es más efectiva, mostrando un aumento en la producción de biomasa de 2 Mg ha−1 a−1, lo que indica una limitación de N y P para el crecimiento del pasto. Además, la calidad de forraje se mostró incrementada ya que el contenido de P y de Ca han aumentado significativamente. Los microorganismos del suelo en el pasto activo han respondido a corto plazo con una adaptación de su estructura ante la disponibilidad de sustrato, pero no han mostrado un cambio de sus funciones iniciales a largo plazo. Después de la aplicación de urea y de la roca fosfórica, se detectó un incremento significativo en la abundancia de los hongos, pero tampoco se observó una limitación de energía microbial ni de N o P. Sin embargo, la aplicación de urea ha aumentado la pérdida gaseosa de CO2‐C del suelo a corto plazo.
Debido al cambio de uso de la tierra en la área de investigación, se ha detectado una alteración notable de la función del ecosistema, especialmente en el ciclo de C y N de suelo. Para un uso sostenible de la tierra en esta región, es crucial el prevenir la degradación de pastos y rehabilitar aquellos degradados. En el suelo de pastos activos es de gran importancia el mantener o aún mejor el aumentar la disponibilidad del sustrato, que es uno de los indicadores de la fertilidad del suelo. En este contexto, la materia orgánica se debe ser retenida a largo plazo para mantener la actividad y biomasa microbiana alta y por ende la productividad de pasto. Una moderada fertilización con urea y roca fosfórica puede ser un primer paso para proveer un continuo suministro de nutrientes por el crecimiento del pasto y para reforzar la sanidad pecuaria por medio de un forraje de mayor calidad. Sin embargo, el riesgo de emisiones adicionales de CO2‐C del suelo debido a una aplicación más alta de urea debe tenerse en cuenta. Se puede concluir que para un manejo sostenible del uso de la tierra, tanto el control de la invasión de Llashipa y como un suministro adecuado de nutrientes son necesarios. Adicionalmente se podría decir que es necesario profundizar el estudio de la reducción de las pérdidas de los nutrientes de suelo y de la eficiencia del uso de los nutrientes en las plantas, así como las asociaciones de pastos con leguminosas o el uso de cultivos de absorción selectiva de nutrientes, que serían estrategias importantes para el futuro.
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Implications of land-use change and pasture management on soil microbial function and structure in the mountain rainforest region of southern EcuadorPotthast, Karin 10 April 2013 (has links)
In the present thesis, implications of pasture establishment, fertilization and abandonment on soil C and nutrient dynamics were investigated for the mountain rainforest region of southern Ecuador. Over the past decades the natural forest of the study area has been threatened by conversion to cattle pastures. However, the soil fertility of these extensively grazed pastures (active pastures) declines continuously during pasture use. The invasion of bracken fern (Pteridium arachnoideum) leads to pasture abandonment when bracken becomes dominant. In order to reveal the mechanisms behind the deterioration of soil fertility, biotic and abiotic soil properties and their interaction were analyzed along a land‐use gradient (natural forest – active pasture – abandoned pasture).
The ecosystem disturbance of the mountain rainforest through pasture use changed the microbial function and structure, and affected soil CO2‐C fluxes. Annually, 2 Mg soil CO2‐C ha‐1 were additionally emitted from the pasture land. This acceleration in soil respiration rates was related to accelerated rates of microbial C mineralization and fine‐root respiration. The high‐quality, N‐rich above‐ and belowground residues of the pasture grass (S. sphacelata, C4‐plant), especially the huge fine‐root biomass, provided a high C and N availability for soil microbes. Compared to the forest, increased soil pH and accelerated base saturation were further factors beneficial for soil microbial growth and metabolism of the upper mineral soil at active pastures. Three times higher amounts of microbial biomass C and a significant shift in the microbial community structure towards a higher relative abundance of Gram(‐)‐ bacteria and fungi were observed.
Long‐term pasture use and the invasion of bracken (C3‐plant) diminished beneficial effects for microbes, causing a significant decrease in the C, net, and gross N mineralization rates as well as a two‐third reduction in the microbial biomass. A preferential substrate utilization of grass‐derived C4 by the soil microbes resulted in a rapid decline of the C4‐pool. As a consequence, the less available C3‐pool from bracken and former forest increased its dominance in the SOC‐pool, further decreasing pasture productivity and finally causing pasture abandonment. The lower quality and quantity of above‐ and belowground residues of the bracken (high lignin content, C/N) resulted in resource‐limited conditions that influenced the microbial function to greater extent than their structure. The microbial structure seemed to be sensitive mainly to soil pH along the land‐use gradient. Thus, a disconnection between microbial structure and function was identified.
Fertilization experiments were conducted both in the lab and in the field to evaluate the impact of urea and/or rock phosphate amendment on SOM dynamics and on pasture productivity of active pastures. After combined fertilization the pasture yield was most efficiently increased by 2 Mg ha−1 a−1, indicating a NP‐limitation of grass growth. Furthermore, the fodder quality was improved by a higher content of P and Ca in the grass biomass. The microorganisms of the active pasture soil responded with an adaptation of their structure to the increased substrate availability in the short term, but did not change their initial functions in the long term. After urea/ rock phosphate addition a significant increase in the relative fungal abundance was detected, but neither a microbial limitation of energy nor of N or P was observed. However, urea addition accelerated gaseous losses of soil CO2‐C in the short term.
In the study area, pronounced alterations in ecosystem functioning due to land‐use changes were detected, especially in soil C and N cycling rates. For a sustainable land‐use in this region it is crucial to prevent pasture degradation and to rehabilitate degraded pastures in order to protect the prevailing mountain rainforest ecosystem. It is of crucial importance for active pasture soils to maintain or even increase resource availability, being one indicator of soil fertility. In this context, the soil organic matter has to be retained in the long‐term to maintain high microbial activity and biomass, and thus pasture productivity. A moderate fertilization with urea and rock phosphate can be a first step to provide continuous nutrient supply for grass growth and to strengthen livestock health through increased fodder quality. However, the risk of further additional emissions of soil CO2‐C due to increased loads of urea fertilizer application has to be kept in mind. Overall, for the establishment of a sustainable land‐use management the control of bracken invasion and an adjusted nutrient management are needed. Further investigations on the reduction of soil nutrient losses and increased nutrient use efficiencies of plants, such as combined planting with legumes or the usage of cultivars with special nutrient acquisition strategies, should be in the focus of future work.:Contents
Acknowledgement I
Table of content III
List of Tables V
List of Figures VI
Abbreviations VII
Summary (English/German/Spanish) .................................................... 1
1 Introduction ................................................................................... 6
1.1 Impact of land‐use changes on C and nutrient dynamics ............... 6
1.1.1 Soil organic carbon and soil CO2 flux 7
1.1.2 The role of soil microbes 8
1.1.3 Plant‐microbe interactions 10
1.1.4 Impact of soil environment on soil microbes 11
1.2 Pasture establishment in the tropics .......................................... 13
1.3 Research area ....................................................................... .... 15
2 Objectives and research questions ......................... ................... 19
2.1 Land‐use change ........................................................................ 19
2.2 Pasture management ............................................................. ... 21
3 Methodology ................................................................................. 22
3.1 Study sites ............................................................................... 22
3.1.1 Land‐use gradient 22
3.1.2 Pasture Fertilization Experiment (FERPAST) 23
3.2 General analyses ....................................................................... 24
3.2.1 Laboratory experiments 25
3.2.2 In situ measurements 26
3.2.3 Statistics 27
4 Results ............................................................................................ 28
4.1 Soil C and nutrient dynamics along a land‐use gradient ............. 28
Potthast, K., Hamer, U., Makeschin, F., 2011. Land‐use change in a tropical mountain rainforest region of southern Ecuador affects soil microorganisms and nutrient cycling. Biogeochemistry, 1‐17.
4.2 Impact of pH and ongoing succession on microbial function and structure .......... 29
4.3 Response of soil microbes to bracken‐invasion ........................... 32
Potthast K., Hamer U., Makeschin F. 2010. Impact of litter quality on mineralization processes in managed and abandoned pasture soils in Southern Ecuador. Soil Biology and Biochemistry 42, 56‐64.
4.4 Response of soil microbes and pasture grass to fertilization ........33
Hamer, U., Potthast, K., Makeschin, F., 2009. Urea fertilisation affected soil organic matter dynamics and microbial community structure in pasture soils of Southern Ecuador. Applied Soil Ecology 43, 226‐233.
Potthast, K., Hamer, U., Makeschin, F., 2012. In an Ecuadorian pasture soil the growth of Setaria sphacelata, but not of soil microorganisms, is co‐limited by N and P. Applied Soil Ecology 62, 103‐114.
5 Discussion .................................................................................... 34
5.1 Impact of land‐use changes ...................................................... 34
5.1.1 Soil CO2 fluxes 34
5.1.2 Microbial structure and function 34
5.2 Soil fertility loss of pastures ‐reasons and first prevention steps‐ . 37
5.2.1 Litter decay and SOM dynamics 37
5.2.2 Fertilization and SOM dynamics 39
5.3 Conclusions and Perspectives ...................................................... 42
References ..................................................................................... 46
Curriculum vitae......................................................................... 58 / In der vorliegenden Dissertation werden die Auswirkungen der Weideetablierung, ‐düngung sowie des Verlassens von Weiden auf Bodenkohlenstoff‐ und Nährstoffdynamik in einer tropischen Bergregenwaldregion Ecuadors zusammenfassend dargestellt und diskutiert. Der Naturwald des Untersuchungsgebietes ist seit Jahrzehnten durch Brandrodung und die Umwandlung in extensiv genutztes Weideland (aktive Weide) in seinem flächenhaften Bestand bedroht. Als Problem hat sich der Verlust an Fruchtbarkeit der Weideböden während ihrer Bewirtschaftung herausgestellt. Des Weiteren führt die Einwanderung des Tropischen Adlerfarns (Pteridium arachnoideum, C3‐Pflanze) zu einer Reduktion der oberirdischen Grasbiomasse. Nimmt diese Entwicklung überhand, werden die betroffenen Flächen von den Bauern nicht mehr aktiv genutzt, verlassen und neuer Regenwald gerodet. Um mehr über die Mechanismen der Verringerung der Bodenfruchtbarkeit zu erfahren, wurden biotische und abiotische Bodeneigenschaften und deren Interaktion entlang eines Landnutzungsgradienten (Naturwald – aktive Weide – verlassene Weide) untersucht.
Die Zerstörung des Bergregenwaldökosystems und die Überführung der gerodeten Flächen zur Weidebewirtschaftung verändert die Funktion und Struktur der Bodenmikroorganismen und beeinflusst den CO2‐C Fluss aus dem Boden. Jährlich werden 2 t CO2‐C ha‐1 zusätzlich vom Weideland emittiert. Diese Erhöhung der Bodenatmungsraten kann mit erhöhten Raten der mikrobiellen C‐Mineralisierung und Feinwurzelatmung in Verbindung gebracht werden. Das Weidegras (S. sphacelata, C4‐Pflanze) liefert C‐ und N‐reiche ober und unterirdische organische Substanz (z.B. durch die Feinwurzelbiomasse) und trägt damit zu einer Erhöhung der C‐ und N‐Verfügbarkeit für die mikroorganismen bei. Darüber hinaus stellen ein höherer pH‐Wert und eine erhöhte Basensättigung im oberen Mineralboden der aktiven Weide günstige Bedingungen für mikrobielles Wachstum und Metabolismus dar. Als Konsequenz sind die Gehalte an mikrobiellem Biomassekohlenstoff um das Dreifache erhöht und die mikrobiellen Gemeinschaftsstrukturen signifikant in Richtung einer höheren relativen Abundanz der Gram(‐)‐Bakterien und Pilze verschoben.
Eine längerfristige Weidebewirtschaftung ohne Kompensation von Nährstoffverlusten sowie die Einwanderung des Tropischen Adlerfarnes verschlechterte die Bedingungen für die Mikroorganismen, was zu einem signifikanten Rückgang des SOC, der Netto‐ und Brutto‐N‐Mineralisierungsraten sowie zu einer Halbierung der mikrobiellen Biomasse führt. Eine bevorzugte Substratnutzung von Graskohlenstoff (C4) durch die Mikroorganismen hat einen schnellen Abbau des C4‐Pools zur Folge. Somit dominiert nun der mikrobiell schlechter verfügbare C3‐Pool den Bodenkohlenstoffpool. Dies führt zu einem weiteren Rückgang der Weideproduktivität und schließlich zum Offenlassen der Weide. Die geringere Qualität und Quantität der vom Farn stammenden ober‐ und unterirdischen organischen Substanz (hoher Ligninanteil, weites C/N), führten zu einer Limitierung der Ressourcen für die Mikroorganismen, welche deren Funktionen in größerem Maße beeinflussen als deren Gemeinschaftsstruktur. Im Gegensatz dazu wird entlang des Landnutzungsgradienten die Struktur hauptsächlich durch den pH‐Wert beeinflusst. Daraus folgt, dass Struktur und Funktion der Bodenmikroorganismen voneinander entkoppelt auf Veränderungen reagieren können.
Um den Einfluss von Harnstoff‐ und/ oder Rohphosphatdüngung aktiver Weiden auf die Dynamik der organischen Bodensubstanz und auf die Weideproduktivität zu untersuchen, wurden sowohl Labor‐ als auch Feldversuche durchgeführt. Im Feldexperiment wurde gezeigt, dass eine NP‐Limitierung der Grasbiomasseproduktion vorliegt und durch eine geringe NP‐Kombinationsdüngung die oberirdische Phytomasseproduktion um 2 t ha−1 a−1 gesteigert und die Futterqualität durch eine Erhöhung der P‐ und Ca‐ Gehalte verbessert werden kann. Die Mikroorganismen reagierten mit einer Anpassung ihrer Struktur an die kurzzeitig erhöhte Substratverfügbarkeit. Nach Gabe von Harnstoff und/ oder Rohphosphat wurde weder eine N‐ noch eine P‐Limitierung der Bodenmikroorganismen festgestellt, und die mikrobiellen Funktionen wurden langfristig nicht verändert. Dagegen bewirkte die Düngergabe einen erhöhten relativen Anteil der Pilzabundanz. Im Labor sowie im Feld kam es nach Harnstoffdüngung kurzzeitig zu verstärkten gasförmigen Verlusten des Bodenkohlenstoffs.
Aufgrund der Landnutzungsänderungen im Untersuchungsgebiet veränderten sich die Ökosystemfunktionen stark, speziell die Boden‐C‐ und Boden‐N‐Umsatzraten. Für eine nachhaltige Landnutzung in der Region, d. h., für den Schutz der noch verbliebenen natürlichen Bergregenwaldflächen, ist es von entscheidender Bedeutung, dass die Weidedegradierung verhindert wird und degradierte Flächen wieder in Nutzung genommen werden. Als entscheidend für die Weideproduktivität hat sich in dieser Studie die Ressourcenverfügbarkeit für Bodenmikroorganismen herausgestellt. Daher ist es sehr wichtig, diese Ressourcenverfügbarkeit in Böden aktiv‐genutzter Weiden zu erhalten oder noch zu erhöhen, denn sie wirkt sich vor allem auf die organische Bodensubstanz und im Wechselspiel damit auf die mikrobielle Biomasse und Aktivität aus. Eine moderate Kombinationsdüngung aus Harnstoff und Rohphosphat ist ein erster Schritt in diese Richtung. Dabei sollte jedoch das Risiko zusätzlicher bodenbürtiger CO2‐C Emissionen in Folge höherer Düngergaben berücksichtigt werden. Für ein nachhaltiges Landnutzungsmanagement sind Maßnahmen gegen die Einwanderung des Adlerfarnes und ein angepasstes Nährstoffmanagement notwendig. Weitere Untersuchungen sollten auf eine Minimierung der Nährstoffverluste und eine erhöhte Nährstoffnutzungseffizienz der Pflanzen fokussiert werden. Weidemischkulturen aus Gräsern mit Leguminosen sowie der Einsatz von Kulturen mit speziellen Nährstoffaneignungsstrategien könnten dabei eine große Rolle spielen und sollten in der Region erprobt werden.:Contents
Acknowledgement I
Table of content III
List of Tables V
List of Figures VI
Abbreviations VII
Summary (English/German/Spanish) .................................................... 1
1 Introduction ................................................................................... 6
1.1 Impact of land‐use changes on C and nutrient dynamics ............... 6
1.1.1 Soil organic carbon and soil CO2 flux 7
1.1.2 The role of soil microbes 8
1.1.3 Plant‐microbe interactions 10
1.1.4 Impact of soil environment on soil microbes 11
1.2 Pasture establishment in the tropics .......................................... 13
1.3 Research area ....................................................................... .... 15
2 Objectives and research questions ......................... ................... 19
2.1 Land‐use change ........................................................................ 19
2.2 Pasture management ............................................................. ... 21
3 Methodology ................................................................................. 22
3.1 Study sites ............................................................................... 22
3.1.1 Land‐use gradient 22
3.1.2 Pasture Fertilization Experiment (FERPAST) 23
3.2 General analyses ....................................................................... 24
3.2.1 Laboratory experiments 25
3.2.2 In situ measurements 26
3.2.3 Statistics 27
4 Results ............................................................................................ 28
4.1 Soil C and nutrient dynamics along a land‐use gradient ............. 28
Potthast, K., Hamer, U., Makeschin, F., 2011. Land‐use change in a tropical mountain rainforest region of southern Ecuador affects soil microorganisms and nutrient cycling. Biogeochemistry, 1‐17.
4.2 Impact of pH and ongoing succession on microbial function and structure .......... 29
4.3 Response of soil microbes to bracken‐invasion ........................... 32
Potthast K., Hamer U., Makeschin F. 2010. Impact of litter quality on mineralization processes in managed and abandoned pasture soils in Southern Ecuador. Soil Biology and Biochemistry 42, 56‐64.
4.4 Response of soil microbes and pasture grass to fertilization ........33
Hamer, U., Potthast, K., Makeschin, F., 2009. Urea fertilisation affected soil organic matter dynamics and microbial community structure in pasture soils of Southern Ecuador. Applied Soil Ecology 43, 226‐233.
Potthast, K., Hamer, U., Makeschin, F., 2012. In an Ecuadorian pasture soil the growth of Setaria sphacelata, but not of soil microorganisms, is co‐limited by N and P. Applied Soil Ecology 62, 103‐114.
5 Discussion .................................................................................... 34
5.1 Impact of land‐use changes ...................................................... 34
5.1.1 Soil CO2 fluxes 34
5.1.2 Microbial structure and function 34
5.2 Soil fertility loss of pastures ‐reasons and first prevention steps‐ . 37
5.2.1 Litter decay and SOM dynamics 37
5.2.2 Fertilization and SOM dynamics 39
5.3 Conclusions and Perspectives ...................................................... 42
References ..................................................................................... 46
Curriculum vitae......................................................................... 58 / La tesis presentada investiga el impacto del establecimiento de pasto, de su fertilización y de su manejo tradicional (abandono del pastizal) a la dinámica del carbono y de los nutrientes de suelo en la región de los bosques tropicales montañosos en el Sur de Ecuador. Durante las últimas décadas el bosque natural en el área de estudio ha estado amenazada por su conversión a pastizales. Sin embargo, la fertilidad del suelo en pastos de tipo extensivo (pastos activos) decrece frecuentemente durante el uso de los pastos. La invasión de Llashipa (Pteridium arachnoideum) conduce al abandono de los pastos cuando la ésta se vuelve dominante. Con la finalidad de revelar los mecanismos detrás de esta disminución de la fertilidad de suelo, se analizaron las propiedades bióticas y abióticas del suelo y sus interacciones, a lo largo de una gradiente del uso de la tierra (bosque natural —pasto activo — pastos abandonados).
La perturbación del ecosistema de bosque tropical montañoso por su cambio de uso, mediante el establecimiento de pastizales, ha alterado la función y la estructura de los microorganismos y ha afectado el flujo de CO2‐C del suelo. Cada año 2 Mg CO2‐C ha‐1 fueron emitidas adicionalmente por el establecimiento de pastos. Esta aceleración en la tasa de respiración del suelo está relacionada con el aumento de las tasas de mineralización microbiana de carbono y de la respiración de las raíces. La alta calidad y abundancia de N de los residuos orgánicos del suelo con pasto Mequeron (S. sphacelata, C4‐planta), especialmente debido a la gran biomasa de las raíces finas, ofrecen una disponibilidad alta de C y N para los microorganismos. En comparación con el bosque natural, el aumento del pH y la saturación bases acelerada fueron condiciones más favorables para el crecimiento microbiano y para el metabolismo microbiano en el parte superior del suelo mineral en pastos activos. La cantidad de C de la biomasa de los microorganismos fue tres veces mayor que la del bosque y se ha observado un cambio significativo de la estructura de la comunidad microbiana, en donde la abundancia relativa de los hongos y de las bacterias Gram(‐) ha aumentado.
El uso de pasto a largo plazo y la invasión de Llashipa (C3‐planta) han reducido los efectos benéficos para los microorganismos, que resultaron en una reducción significativa de las tasas de la mineralización de C y N, y en una reducción en dos tercios de la biomasa microbiana. El uso preferencial de los microorganismos por sustrato de pasto C4 han resultado en una rápida disminución de la reserva de C4. Como consecuencia, la menor disponibilidad de la reserva de C3 de las plantas de Llashipa y de la cobertura anterior de
bosque ha incrementado su dominancia en la reserva de materia orgánica del suelo. Eso resulta, en una mayor disminución de la productividad de los pastos, conduciendo finalmente al abandono de los campos de pastos. La menor calidad y cantidad de los residuos acumulados sobre y bajo el suelo provenientes de la Llashipa han dado como resultado un sustrato de limitadas condiciones que están afectando más a las funciones microbiales antes que a su estructura. La estructura microbiana parece ser más sensible al pH del suelo a largo de la gradiente del uso de la tierra; de manera que se ha identificado una desconexión entre la estructura y función microbial.
Experimentos de fertilización en laboratorio y en campo han sido realizados para evaluar el impacto de la aplicación de enmiendas (urea y/o roca fosfórica) a la dinámica de la materia orgánica y a la productividad de los pastos activos. El resultado del experimento de campo ha demostrado que la fertilización combinada es más efectiva, mostrando un aumento en la producción de biomasa de 2 Mg ha−1 a−1, lo que indica una limitación de N y P para el crecimiento del pasto. Además, la calidad de forraje se mostró incrementada ya que el contenido de P y de Ca han aumentado significativamente. Los microorganismos del suelo en el pasto activo han respondido a corto plazo con una adaptación de su estructura ante la disponibilidad de sustrato, pero no han mostrado un cambio de sus funciones iniciales a largo plazo. Después de la aplicación de urea y de la roca fosfórica, se detectó un incremento significativo en la abundancia de los hongos, pero tampoco se observó una limitación de energía microbial ni de N o P. Sin embargo, la aplicación de urea ha aumentado la pérdida gaseosa de CO2‐C del suelo a corto plazo.
Debido al cambio de uso de la tierra en la área de investigación, se ha detectado una alteración notable de la función del ecosistema, especialmente en el ciclo de C y N de suelo. Para un uso sostenible de la tierra en esta región, es crucial el prevenir la degradación de pastos y rehabilitar aquellos degradados. En el suelo de pastos activos es de gran importancia el mantener o aún mejor el aumentar la disponibilidad del sustrato, que es uno de los indicadores de la fertilidad del suelo. En este contexto, la materia orgánica se debe ser retenida a largo plazo para mantener la actividad y biomasa microbiana alta y por ende la productividad de pasto. Una moderada fertilización con urea y roca fosfórica puede ser un primer paso para proveer un continuo suministro de nutrientes por el crecimiento del pasto y para reforzar la sanidad pecuaria por medio de un forraje de mayor calidad. Sin embargo, el riesgo de emisiones adicionales de CO2‐C del suelo debido a una aplicación más alta de urea debe tenerse en cuenta. Se puede concluir que para un manejo sostenible del uso de la tierra, tanto el control de la invasión de Llashipa y como un suministro adecuado de nutrientes son necesarios. Adicionalmente se podría decir que es necesario profundizar el estudio de la reducción de las pérdidas de los nutrientes de suelo y de la eficiencia del uso de los nutrientes en las plantas, así como las asociaciones de pastos con leguminosas o el uso de cultivos de absorción selectiva de nutrientes, que serían estrategias importantes para el futuro.:Contents
Acknowledgement I
Table of content III
List of Tables V
List of Figures VI
Abbreviations VII
Summary (English/German/Spanish) .................................................... 1
1 Introduction ................................................................................... 6
1.1 Impact of land‐use changes on C and nutrient dynamics ............... 6
1.1.1 Soil organic carbon and soil CO2 flux 7
1.1.2 The role of soil microbes 8
1.1.3 Plant‐microbe interactions 10
1.1.4 Impact of soil environment on soil microbes 11
1.2 Pasture establishment in the tropics .......................................... 13
1.3 Research area ....................................................................... .... 15
2 Objectives and research questions ......................... ................... 19
2.1 Land‐use change ........................................................................ 19
2.2 Pasture management ............................................................. ... 21
3 Methodology ................................................................................. 22
3.1 Study sites ............................................................................... 22
3.1.1 Land‐use gradient 22
3.1.2 Pasture Fertilization Experiment (FERPAST) 23
3.2 General analyses ....................................................................... 24
3.2.1 Laboratory experiments 25
3.2.2 In situ measurements 26
3.2.3 Statistics 27
4 Results ............................................................................................ 28
4.1 Soil C and nutrient dynamics along a land‐use gradient ............. 28
Potthast, K., Hamer, U., Makeschin, F., 2011. Land‐use change in a tropical mountain rainforest region of southern Ecuador affects soil microorganisms and nutrient cycling. Biogeochemistry, 1‐17.
4.2 Impact of pH and ongoing succession on microbial function and structure .......... 29
4.3 Response of soil microbes to bracken‐invasion ........................... 32
Potthast K., Hamer U., Makeschin F. 2010. Impact of litter quality on mineralization processes in managed and abandoned pasture soils in Southern Ecuador. Soil Biology and Biochemistry 42, 56‐64.
4.4 Response of soil microbes and pasture grass to fertilization ........33
Hamer, U., Potthast, K., Makeschin, F., 2009. Urea fertilisation affected soil organic matter dynamics and microbial community structure in pasture soils of Southern Ecuador. Applied Soil Ecology 43, 226‐233.
Potthast, K., Hamer, U., Makeschin, F., 2012. In an Ecuadorian pasture soil the growth of Setaria sphacelata, but not of soil microorganisms, is co‐limited by N and P. Applied Soil Ecology 62, 103‐114.
5 Discussion .................................................................................... 34
5.1 Impact of land‐use changes ...................................................... 34
5.1.1 Soil CO2 fluxes 34
5.1.2 Microbial structure and function 34
5.2 Soil fertility loss of pastures ‐reasons and first prevention steps‐ . 37
5.2.1 Litter decay and SOM dynamics 37
5.2.2 Fertilization and SOM dynamics 39
5.3 Conclusions and Perspectives ...................................................... 42
References ..................................................................................... 46
Curriculum vitae......................................................................... 58
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