Long-term watershed and field nitrogen (N) balances were used in this study to quantify the surface (baseflow) and ground water lag times and effects of BMPs on N discharge from a Virginia Coastal Plain watershed. Ten-year water quantity/quality data (1986-1996) collected at the Nomini Creek (NC) watershed were used. Field (Field-N) and watershed (Watershed-N) scale N models were developed for computing the N balances. BMPs evaluated in this study included no-till corn and split N application. The role of atmospheric N (atm-N) deposition (dry+wet) in masking the effects of BMPs on watershed N loading was also investigated. Nitrogen retention and discharge from the forest areas in the NC watershed were simulated using the 5-year water and N input and output data from forested subwatersheds. Field and watershed N balances (WNBAL) were used to evaluate the effects of BMPs on measured surface and ground water N in the NC watershed. A 6-month laboratory study was conducted to develop N mineralization (Nmin) models for agricultural, forest, and fallow soils in the NC watershed. Mineralization potential (N0) and rate constants (k) for surface and subsurface soils from agricultural, forest, and fallow soils were estimated by fitting the laboratory measured data to a first-order model, using the nonlinear regression procedure. A large variability (300%, 163 - 471 kg/ha) in N0 of agricultural surface soils was observed. On average, forest soils had much higher potentially mineralizable N than agricultural soils. The first-order model was incorporated into the Field-N model to predict daily Nmin using the measured N0 and k and daily values of soil water and temperature. Atmospheric deposition was a major source of N in the NC watershed, accounting for 23% of the total N input. Variation in atm-N deposition during the 10-year period was from 10 to 42 kg/ha (average = 25 kg/ha); much larger than the variation in fertilizer N (37 to 51 kg/ha). Atm-N deposition was found to be a controlling factor affecting surface water DIN (dissolved inorganic N) and TDN (total dissolved N) loading in the NC watershed; an indication that atm-N deposition is a masking factor in the BMP impact evaluation. Large uncertainty in atm-N deposition existed due to uncertainty involved in quantifying dry N deposition. Forested areas of the NC watershed retained 77% of the atm-N deposition. Forest area N discharge was simulated using the 77% retention and annual atmospheric deposition. Comparison of Field-N predicted N balance and leaching (steady-state and transient conditions) with observed ground water NO3 concentration revealed that the ground water lag time ranged from 2 to 8 months. Unusually rapid transport of solute in the watershed was facilitated by the network of discontinuous clay lenses. Based on the lag time, the pre-BMP (1986-1990) and post-BMP (1991-1995) periods were defined. Results from Field-N indicated that implementation of split fertilizer N on corn reduced the post-BMP ground water NO3 concentration by 10-12% at two of the four ground water monitoring sites. The split N application reduced the frequency of detection of high NO3 (> 9 mg/l) concentration by 44% during the post-BMP period. Considerably large uncertainty existed in evaluating the effects of BMPs on ground water NO3 due to N contributions from neighboring agricultural and forest areas. Effects of no-till corn could not be evaluated since this BMP was already implemented at the sites prior to the beginning of the study. Results of statistical trend analysis of the ground water N supported the modeling results. Watershed-N model was able to accurately predict the effects of land use activities on watershed N balances (WNBAL) and baseflow and ground water N. A one-to-one relationship between the WNBAL and observed N loading and concentration time series was observed. Comparison of WNBAL and measured baseflow N revealed that the baseflow lag time or residence time was between 4-11 months. Multivariate regression models were developed to predict baseflow N using Watershed-N results. The multivariate model predicted the N loading and concentration exceptionally well (R2 > 90%). Corn N input and output and acreage was an important predictor of ground water N and baseflow N loading and concentration. Post-BMP WNBAL was considerably less than the WNBAL for the pre-BMP period. However, these reductions were mainly due to the 43% reductions in atm-N deposition and 31% increase in the plant uptake during the post-BMP period. Reductions in WNBAL caused by BMPs were only 5%. Reductions in N loading caused by BMPs were 10%. Statistical trend analysis of monitoring and modeling results indicated significant post-BMP reductions in WNBAL and DIN and TDN loading. However, poor to moderate evidence was available to suggest that BMPs caused a significant reductions in WNBAL and N loading. Marginal effects of BMPs could mainly be attributed to insufficient BMP implementation. Watershed-N was used to evaluate N reduction scenarios and to design BMPs. Irrigating corn was one of the best BMPs, as it could reduce N loading from NC watershed by 50%. Quantification of lag time and long-term watershed N balances from this study provide crucial information for understanding N cycling and factors controlling N discharges which is essential for designing programs for controlling N discharges from Mid-Atlantic Coastal Plain watersheds. / Ph. D.

Anthropogenic activities have increased nitrogen (N) emissions. However, higher N deposition is associated with negative fallouts in ecosystems, highlighting the necessity to define its critical load (CLO). Lichens, with their lack of roots, stomata, and cuticles, are helpful air pollution indicators and can be used to establish N CLO. Several studies report epiphytic lichens’ reaction to N deposition, but few have also focused on their recovery. In this study, I reproduced a deposition gradient to investigate epiphytic lichen biomass response to long-term N deposition. As such, whole trees were fertilized with 0.6, 6, 12.5, 25, or 50 kg N/ha/year from 2006 to 2012. In 2012, after six years of irrigation, Hypogymnia species and A. sarmentosa had altered biomass, and based on those results, a CLO below 6 kg N/ha/year is recommended for epiphytic lichens in the boreal forest. I also investigated the recovery of epiphytic lichens nine years after the simulated N deposition ceased. In 2021, a high increase in lichen biomass was recorded, suggesting that lichen growth rate is faster than previously assumed by other studies. However, biomass recovery was lower in high N treatments in Hypogymnia sp., Bryoria sp., and A. sarmentosa, suggesting those species recall the historical treatments possibly due to retained impairments. Since differences among treatments increased in the nine years after the simulated N deposition ceased, my results indicate that short-term negative effects of N deposition can have long-lasting effects on the epiphytic lichen community. Thus, I stress the importance of considering the recovery capability of different species during CLO determination to avoid underestimating the impact of N deposition.

Nitrogen (N) is an essential element to plants for growth, maintenance and reproduction, however most N does not exist in a form that is biologically available to plants. In order to maximise the acquisition and retention of N, plants have evolved a variety of morphological and physiological adaptations and life history strategies, as well as the ability to respond plastically to changes in resource availability in ecological time. Determining the ecophysiological responses of plants to changes in root N availability is crucial to further understanding of the mechanisms underlying competitive interactions between plants, and between plants and other organisms, that ultimately contribute to community structure and ecosystem functioning. Carnivorous plants are ideal systems for investigating ecophysiological responses to N availability as:- (i) they share a unique adaptation for obtaining supplemental N from captured prey, therefore ecological stoichiometry and energetic cost/benefit models may be explored; (ii) the trait of botanical carnivory is widely considered to have independently co-evolved as a response to N-deficient, sunny and wet environments, therefore resource allocation trade-offs between plant investment in N and carbon (C) acquisition may be observed, and (iii) they are extremely sensitive to changes in root N availability in ecological time. In this research, the carnivorous plant Drosera rotundifolia (round-leaved sundew) was used to address several unanswered ecophysiological and evolutionary questions relating to patterns and processes of prey capture and the N nutrition of carnivorous plants. Furthermore, the potential for reducing uncertainty in the calculation of plant reliance on carnivory using a δ15N natural abundance multi-level linear mixing model was explored. A combined approach of in-situ and ex-situ studies was employed, using co-occurring non-carnivorous plants or carnivorous plant species with differing evolutionary lineages or prey capture mechanisms respectively to provide context. Results show that the adaptations of carnivory, high reproductive investment and a relatively short life span enable Drosera rotundifolia to survive and thrive in an extreme, N deficient environment. Phenotypically plastic responses by the plant to light and root N availability provide evidence of resource allocation trade-offs between investment in carnivory for N acquisition and in photosynthesis for C acquisition. Plants invested less heavily in prey capture (measured as the stickiness of leaf mucilage) as N availability increased or light availability decreased. These results show that the energetic costs associated with carnivory are avoided by the plant when less costly sources of N are available for uptake and that the production of carbon-rich mucilage is only made under nutrient-limited and well-lit conditions. Results obtained from the comparison of captured insect prey with background invertebrates of potential prey indicate that Drosera rotundifolia is a dietary generalist, where the quantity of prey captured per plant is positively correlated with leaf stickiness and total leaf area. Plant reliance on prey-derived N decreased with increasing root N availability, providing evidence that carnivory is only of net benefit to the plant in N-deficient and well-lit environments, as the photosynthetic costs of investment in the trait are not exceeded by the energetic gain from prey N uptake in shady or dry habitats. A more accurate and precise method for calculating plant reliance on botanical carnivory is presented which incorporates the insect diet of the plant. This method has wider significance for reducing uncertainty in the calculation of relative source contributions to a mixture for most natural abundance applications using a multi-level linear mixing model. To conclude, results from this research further understanding of the ecophysiological mechanisms underlying plant responses to changes in resource availability and the selective pressures driving the evolution of plant adaptations. These results therefore assist with predicting how plants and plant communities may respond to sustained N deposition inputs and future environmental scenarios.

In the work described in this thesis I studied how increases in nitrogen (N) inputs may affect plant community structure in boreal forest understorey vegetation. These phenomena were investigated in N fertilization experiments and along a national N deposition gradient. After five years of N additions, large changes in understorey vegetation composition were observed in the fertilization study. In plots that received 50 kg N ha'1 year"1 (N2), the abundance of the dominant species, Vaccinium myrtillus, decreased on average by 32 %. No decrease was observed in control plots during the same period. In contrast, the grass Deschampsia flexuosa responded positively to increased N input, being on average more than five times as abundant in the N2 treatments as in controls. Also an increase was seen in the incidence of disease caused by the parasitic fungus Valdensia heterodoxa on leaves of V. myrtillus following N additions. The parasite was on average nearly twice as abundant in N2 plots than in control plots. This could be explained by increased N concentrations in host plant tissue. Disease incidence also increased following experimental additions of glutamine to leaf surfaces of V. myrtillus, suggesting a causal connection between plant N concentration and performance of the fungus. The parasite also played a key role in the observed changes in understorey species composition. D. flexuosa was more abundant in patches in which V. myrtillus was severely affected by V heterodoxa. This suggests that V heterodoxa mediates the increased abundance of D. flexuosa following increased N additions. The fungus mediates changes in the composition of understorey vegetation mainly by increasing light availability via premature leaf loss of V. myrtillus. The incidence of disease due to the parasite was on average higher in large than in smaller N-treated plots, indicating that the response to N fertilization is spatially scale dependent. This shows that using small plot sizes in experiments that simulate changed environmental conditions may be problematic, as important interactions may be underestimated. Comparison of the occurrence of understorey species between regions with different rates of N deposition revealed that the occurrence of the two dwarf shrubs V. myrtillus and V. vitis- idaea was lower in regions with high N deposition compared to regions with low deposition. The opposite pattern was found for V heterodoxa. This is consistent with expectations from N fertilization experiments. For D. flexuosa no differences in occurrence were found between the different regions investigated. The effects on vegetation and mycorrhizal fungi observed following N additions were also found to be long lasting. Nine years after termination of the fertilization, no signs of recovery were detected, and nearly 50 years after termination characteristic signs of N fertilization were found among bryophytes and mycorrhizal fungi. This suggests that the time needed for re-establishment of the original biota following N-induced changes may be substantial. / <p>Diss. (sammanfattning) Umeå : Umeå universitet, 2002, härtill 5 uppsatser</p> / digitalisering@umu

Biodiversity rich regions worldwide face threats from various global change agents. This research quantifies environmental influences on vegetation, and the impacts of exotic woody plant invasion and anthropogenic nitrogen (N) deposition in a global biodiversity hotspot. The study was conducted in the montane grasslands of the Nilgiris, Western Ghats, and outlines potential management options for this region. Specifically, I examined (1) the role of environmental factors in influencing native plant distribution and ecosystem properties, (2) the status and impact of exotic shrub (Scotch broom, henceforth broom) invasion, (3) the role of disturbances in the success of broom, (4) the role of fire in restoring invaded grasslands, and (5) the impacts of terrestrial N loading on the grassland ecosystem. I used experiments and surveys to assess these. Distributions of several key species were explained by a few complex environmental gradients. In invaded-grasslands, broom populations consisted mainly of intermediate size and age classes, with no clear indication of population decline. Invasion negatively impacted plant community structure and drastically changed composition, favoring shade-tolerant and weedy species. However, invasion did not greatly alter ecosystem function. Fire successfully eliminated mature broom stands, but resulted in a short-term increase in broom seedling recruitment. At the end of 18 months, the fire effects on uninvaded-grasslands were not apparent, but there was no conclusive evidence of the formerly invaded patches attaining the composition of uninvaded-grasslands following burning. N fertilization strongly influenced soil N dynamics, and shoot N concentrations, but effects on aboveground production were weak. Surprisingly, N enrichment had positive effects on diversity in the short-term. It is clear that these grasslands need immediate management intervention to forestall degradation from invasion. Fire could be used to eliminate mature broom stands and deplete persistent seedbanks, which will facilitate colonization by shade-intolerant grassland plants. Active restoration should be mindful of environmental preferences of framework species. Long-term studies of the impacts of N deposition in the context of disturbances will help determine realistic critical thresholds and utilize disturbances to buffer the potential adverse effects of increasing N loading.

Anthropogenic activities are increasing inorganic nitrogen (N) loadings to lakes in the northern hemisphere. In many boreal lakes phytoplankton are N limited, wherefore enhanced N input may affect the productivity of pelagic food webs. Simultaneously, global change causes increased inflows of terrestrial dissolved organic carbon (DOC) to boreal lakes. Between clear and humic lakes, whole lake primary and consumer production naturally differs. However, research is inconclusive as to what controls pelagic production in these lakes. Further, it is unclear how DOC affects the response of the pelagic food web to enhanced inorganic N availability. The overarching goal of this thesis was to study the effects of inorganic N and organic C for pelagic food webs in boreal lakes. In the thesis, I first identified the main drivers of pelagic production during summer in eight non-manipulated Swedish boreal lakes with naturally low or high DOC. Then I investigated how increased N availability affects the pelagic food chain, and how the response differs with DOC. Therefore, whole lake inorganic N fertilization experiments were conducted in six Swedish boreal lakes across a DOC gradient (low, medium, high) divided into three lake pairs (control, N enriched) with one reference and two impact years. In each lake, I also investigated the response of zooplankton growth using in situ mesocosm experiments excluding planktivores. I found that humic boreal lakes had lower phytoplankton production and biomass than clear water lakes. Further, phytoplankton community composition and food quality differed with DOC. However, high DOC did not reduce pelagic energy mobilization or zooplankton biomass, but promoted a higher dominance of cladoceran relative to copepod species. N addition clearly enhanced phytoplankton biomass and production in the experimental lakes. However, this stimulating N effect decreased with DOC as caused by light limitation. Further, the newly available phytoplankton energy derived from N addition was not efficiently transferred to zooplankton, which indicates a mismatch between producer energy supply and consumer energy use. Indeed, the mesocosm experiment revealed that decreased food quality of phytoplankton in response to N addition resulted in reduced food web performance, especially in clearer lakes. In humic lakes, zooplankton production and food web efficiency were clearly more resilient to N addition. In summary, my thesis suggests that any change in the landscape that enhances inorganic N availability will especially affect pelagic food webs in clear water lakes. In contrast, brownification will result in more lakes being resilient to eutrophication caused by enhanced N deposition.

Mikrobielle Gemeinschaften werden direkt und indirekt von einem komplexen System verschiedenster Interaktionen zwischen biotischen und abiotischen Faktoren beeinflusst. So zum Beispiel von verschiedenen Pflanzenarten und ihren jeweiligen Eigenschaften, dem Nährstoffgehalt des Bodens, sowie dem pH-Wert. Im Gegenzug gestalten Mikroorganismen als wichtige Treiber der C- und N-Kreisläufe ihre Umwelt. Im Rahmen der vorliegenden Arbeit wurden mehrere Studien unter kontrollierten Feld- und Laborbedingungen, sowie unter natürlichen Bedingungen im Freiland durchgeführt, um verschiedene Einflussfaktoren zu bestimmen und den Grad ihres Einflusses zu ermitteln. Die Zusammensetzung der prokaryotischen Gemeinschaften in den verschiedenen Bodenproben wurden mit Hilfe phylogenetischer Marker, der 16S-rRNA Gene und der 16S-rRNA, analysiert. Die erhaltenen Amplikon-basierten Daten wurden dann prozessiert und die Indices für Artenvielfalt und Artenreichtum berechnet. Zusätzlich wurden Betadiversitätsanalysen durchgeführt, um Unterschiede in der Zusammensetzung der bakteriellen Gemeinschaft zwischen den verschiedenen Behandlungen sichtbar zu machen. Des Weiteren wurden die erhaltenen DGGE Profile für Clusteranalysen verwendet, um Ähnlichkeiten oder Unterschiede in der Struktur der Bakteriengemeinschaft zwischen den verschiedenen Behandlungen aufzuzeigen. Die vorliegende Arbeit gibt einen Einblick über den Einfluss der Baumarten, Baumartendiversität, des Laubes und des Probennahmezeitpunktes auf die Zusammensetzung und Diversität von Bakteriengemeinschaften in Böden. Die erhaltenen Daten zeigten, dass die Laubschicht der Haupteinflussfaktor auf die Zusammensetzung der bakteriellen Gemeinschaft in der Rhizosphäre von jungen Buchen und Eschen ist. Des Weiteren zeigte sich, das verschiedene Baumarten, deren Diversität, sowie saisonale Unterschiede nur einen geringen Einfluss auf die Struktur der bakteriellen Gemeinschaft haben. Zusätzlich konnte gezeigt werden, dass die mikrobielle Gemeinschaftsstruktur nicht signifikant von Buchen- und Eschensetzlingen beeinflusst wird, vermutlich aufgrund des frühen Entwicklungsstadiums der verwendeten Baumsetzlinge. Weiterhin konnte gezeigt werden, dass die Buchensetzlinge das Wachstum von Bakterien inhibierten, während das Pilzwachstum gefördert wurde. Dies wurde vermutlich hervorgerufen durch eine Verschiebung des pH-Wertes im Boden verursacht durch buchenspezifische Wurzelausscheidungen. Morphologisch unterschiedliche Baumarten beeinflussen die Struktur und Diversität mikrobieller Gemeinschaften auf verschiedenen Wegen. Die Analyse der Bakterien- und Pilzgemeinschaften in natürlichen Waldböden unter erwachsenen Buchen und Fichten zeigte einen signifikanten Einfluss der untersuchten Baumarten auf deren Zusammensetzung. Es konnte ein Einfluss des pH-Werts auf die Bakterien- und Pilzvielfalt unter den analysierten Fichtenbeständen gezeigt werden. Des Weiteren wurden die Auswirkungen hoher NO3- Depositionen auf die CH4 und N2O Gasflüsse und die aktiven Bakterien- und Archeengemeinschaften in gemäßigten Laubwaldböden mit Hilfe von Mesokosmen untersucht. Es konnte ein starker Effekt der NO3- Düngung auf die CH4 Aufnahmeraten und N2O Emissionen des gedüngten Laubwaldbodens gezeigt werden. Die N-Düngung hemmte die CH4 Aufnahme des Bodens, während die N2O Emission stieg. Die Bakteriengemeinschaft in den gedüngten Mikrokosmen verschob sich im Verlauf des Versuches in Richtung einer denitrifizierenden Gemeinschaft, dominiert durch die Gattung Rhodanobacter. Darüber hinaus konnte eine Reduzierung der bakteriellen Vielfalt und der CO2 Emission innerhalb der N-gedüngten Mikrokosmen gezeigt werden. Des Weiteren sanken die CO2 Emissionsraten in beiden Behandlungen im Verlauf des Experiments. Dies deutet auf eine reduzierte Aktivität der vorhandenen Bodenmikroorganismen hin, möglicherweise hervorgerufen durch eine C Limitierung des verwendeten Waldbodens. Obwohl eine Verschiebung in der relativen Häufigkeit der auftretenden nitrifizierenden Archeen der Gattung Nitrosotalea nachgewiesen wurde, konnte eine signifikante Veränderung in der Zusammensetzung der gesamten Archeengemeinschaft nicht beobachtet werden. Die Ergebnisse zeigten jedoch einen erheblichen Beitrag methylotropher, methanotropher und nitrifizierender Bakterien, welche in geringer Zahl auftraten, in Bezug auf die gemessene CH4 Aufnahme. Des Weiteren wurden die Auswirkungen der Anwesenheit von Ameisen und ihrer Aktivitäten auf die Aktivität und Vielfalt der mikrobiellen Gemeinschaften im Boden und Ameisennest untersucht. Ameisen transportierten den von Läusen gewonnenen Honigtau in den Boden und verursachten damit eine Abnahme der mikrobiellen Biomasse in der Streuschicht, während die δ15N-Signatur, die basale Atmung und die mikrobielle Biomasse im Boden erhöht wurden. Im Gegensatz dazu konnten mittels Cluster-Analyse der erstellten DGGE Profile keine deutlichen Unterschiede der mikrobiellen Gemeinschaftsstruktur in den untersuchten Mikrokosmen gezeigt werden. Im Gegensatz dazu beeinflusste die Nestbauaktivität und der Eintrag von organischen Substanzen in den Boden durch die Ameisen jedoch die Struktur der Bakteriengemeinschaften im Freiland. Die Cluster-Analyse der erhaltenen DGGE Profile zeigte Unterschiede in der Zusammensetzung der bakteriellen Gemeinschaft in Abhängigkeit vom Probenentnahmeort und der Ameisenaktivität. Außerdem konnte gezeigt werden, dass sich die Struktur der Bakteriengemeinschaft in den Ameisennestern von der im Umgebungsboden unterschied. Ein sekundäres Projekt dieser Arbeit war die Erfassung und der Vergleich der mikrobiellen Gemeinschaften in biologischen Bodenkrusten zweier unterschiedlicher Standorte in extrazonalen, trockenen Bergsteppen der nördlichen Mongolei. Die Studie zeigte deutliche Unterschiede in der mikrobiellen Gemeinschaftsstruktur der beiden Standorte, welche sich im Grad der Störung unterschieden.

L’impact de l’homme sur l’environnement mondial et sur la diversité et le fonctionnement des écosystèmes terrestres fait l’objet d’une attention croissante. De nombreuses études ont évalué les effets de facteurs du changement global tels sur les processus de cycle de l'azote du sol dans les prairies. Cependant, ces études n'ont pas pris en compte le fait que la modification du régime de précipitations avait également une influence sur le régime de dépôt d'azote. En outre, la réponse du cycle de l'azote dans les sols des prairies à de multiples facteurs du changement global agissant ensemble et parfois en même temps que des perturbations telles que les incendies, doit encore être étudiée. Cela limite fortement notre capacité à comprendre et à prévoir les effets du changement global sur les prairies. Dans ce travail de doctorat, deux expériences ont été menées: (i) une expérience en mésocosme pour évaluer les effets combinés d'une augmentation des dépôts d'azote et de changements dans la quantité et la fréquence des précipitations sur le cycle de l'azote édaphique dans une prairie semi-aride; (ii) une expérience in situ pour évaluer les effets combinés de l'augmentation de la concentration en CO2, du réchauffement, d’une modification des précipitations, du dépôt d'azote et d’un feu sur le cycle de l'azote du sol dans une prairie méditerranéenne. Cela permet d'étudier les effets de la combinaison de plusieurs facteurs de changement global (et d’une perturbation feu) sur l'abondance des communautés microbiennes du cycle de l'azote.Les groupes microbiens étudiés étaient les bactéries et les archées oxydant l'ammoniac (AOB et AOA, respectivement), les réducteurs de nitrite porteuses des gènes nirK ou nirS, et les réducteurs de N2O porteurs des gènes nosZI- et nosZII, plus les bactéries oxydant le nitrite du genre Nitrobacter et Nitrospira pour la prairie méditerranéenne. Les principaux résultats et conclusions sont les suivants: 1) Les réactions des différents groupes de (dé)nitrifiants aux scénarios de changement global différaient fortement quel que soit le type de prairie. Les AOB étaient principalement dépendant de la disponibilité en azote En revanche, dans les deux prairies, les AOA étaient plus sensibles à la dynamique de l'eau du sol que la dynamique de l'azote. L'abondance des Nitrobacter étaient principalement affectée par les facteurs de changement global affectant l'abondance de l'AOB, tandis que l'abondance des Nitrospira était davantage liée aux changements d'abondance des AOA dans la prairie méditerranéenne. 2）Dans la prairie californienne où deux dépôts d'azote élevés avaient lieu chaque année, l'effet de l'azote dominait les effets du changement global. En revanche, dans la prairie chinoise, les dépôts d’azote simulés par des apports chroniques couplés aux événements de précipitation n’augmentaient pas l’abondance des dénitrifiants et ne faisaient que légèrement augmenter les émissions de N2O. 3） Pour les deux prairies, l'interaction entre les facteurs du changement global sur le cycle de l'azote du sol ne pouvait pas être prédite simplement en étudiant les effets d'un ou de deux facteurs. Ces effets interactifs ont pu être expliqués par des effets sur des variables environnementales clés telles que l'humidité du sol, la disponibilité de l'azote minéral, le pH et la croissance des racines. Ces résultats démontrent qu'il est impossible de prédire comment les (dé)nitrifiants et la (dé)nitrification répondent aux scénarios de changement global impliquant de multiples facteurs uniquement à partir de la connaissance d'effets de facteurs étudiés isolément. Cela nécessite donc des études plus approfondies dans le domaine de la biologie des changements globaux. La modélisation et l'évaluation de la généralité de ces effets d'interaction complexes constituent donc une priorité majeure pour les chercheurs qui veulent prédir les réponses du cycle de l'azote dans le sol au changement global et les rétroactions sur le climat. / The impact of global environmental changes on the diversity and functioning of terrestrial ecosystems has received increasing attention. Many studies evaluated the effects of single -and less often multiple- global change factors on soil N cycling processes in grasslands. However, these studies have not recognized that altered precipitation regime also has an influence on wet N deposition regime. Further, the response of grassland soil N cycling to co-occurring multiple global change factors and disturbance like fire, and how N cycling response to fire could differ under different global change scenarios, remains unclear. This strongly restricts our ability to understand and predict global change effect on grasslands. In this work, two experiments were conducted: (i) a mesocosm experiment to assess the combined effects of increased N deposition and changes in both the amount and frequency of rainfall on soil N cycling in a semi-arid Monsoon grassland; and (ii) an in situ experiment to assess the combined effects of elevated CO2, warming, increased precipitation, N deposition and fire on soil N cycling in a Mediterranean grassland. This allows studying the -possibly interactive- effects of several global change factors on the abundances of soil N-cycling microbial communities. The microbial groups studied were ammonia oxidizing bacteria and archaea (AOB and AOA, respectively), nirK- and nirS-nitrite reducers, nosZI- and nosZII-N2O reducers, plus Nitrobacter and Nitrospira for the Mediterranean grassland. The main results and conclusions are: 1）The responses of different groups of soil (de)nitrifiers to global change scenarios differed strongly regardless the grassland type. AOB were mostly driven by N. In contrast, AOA were more sensitive to soil water dynamics than N dynamics in both grasslands. Nitrobacter abundance was mostly affected by global change factors through their effects on AOB abundance, whereas Nitrospira abundance was more related to changes of AOA in the Mediterranean grassland. Similarly, nirK- and nirS-harboring nitrite reducers and nosZI-harboring N2O reducers were more sensitive to N deposition than nosZII-harboring N2O reducers, and nirK- and nirS-bacteria positively responded to reduced precipitation. This highlights niche differentiation between them and indicates that the balance between them may be altered in the future; 2）In the Mediterranean grassland, where high N deposition was simulated by two N addition events each year, the N effect dominated global change effects. In contrast, in the Monsson grassland, chronic wet N deposition did not increase denitrifier abundance and only weakly increased soil N2O emissions. This was explained by the efficient capture of added N by the dominant grass species and by the increased plant growth leading to increased transpiration and decreased soil moisture. 3）For both grasslands, the interaction between global change factors on soil N cycling could not be predicted simply by studying the effects of one or two factors. These interactive effects were explained by effects on key environmental variables like soil moisture, mineral N availability, pH and belowground plant growth.These results demonstrates the limitation of predicting how (de)nitrifiers respond to global change scenarios involving multiple factors only from studying single factor effects. Particularly, interactive effects were observed between N deposition, decreased precipitation amount and altered precipitation frequency in the Monsoon grassland; and between fire, N deposition, warming, elevated precipitation and elevated CO2 in the Mediterranean grassland. This calls for more comprehensive studies in the global change biology domain. Modelling and evaluating the generality of these complex interaction effects is thus a high priority for research to predict the responses of soil N cycling processes to global change and feedbacks on climate in the future

No description available.

No description available.