Invasive species are significant drivers of global environmental change, altering the stability and functioning of numerous ecosystems. The exotic shrub Lonicera maackii is an aggressive invader throughout much of the eastern United States. While much is known about its population and community impacts, little is known about effects on ecosystem processes. This dissertation documents changes in ecosystem processes associated with L. maackii growing beneath three native tree species (Fraxinus quadrangulata, Quercus muehlenbergii, Carya ovata) in a savanna in Kentucky. Like many invasive plants, L. maackii litter decomposed and lost nitrogen (N) rapidly, especially in comparison with native tree litter. In comparison to the soils beneath the trees where the exotic shrub was absent, soils beneath L. maackii had a lower bulk density, elevated soil organic matter, C:N, and total soil N and a modified soil microbial community. Inorganic N deposition from spring throughfall was also altered by L. maackii, with higher NO3-N deposition beneath shrubs located beneath the tree canopy relative to canopy locations without L. maackii. While many exotic plant species have been shown to alter ecosystem processes, their impact is often not uniform. This variability is attributed to among-site differences (soil, climate, plant community): within site variability is often ignored. While many of L. maackii’s alterations to ecosystem processes were uniform across the site, several were dependent upon interactions between the exotic and the native tree species. Litter from L. maackii decomposed and lost N more rapidly under C. ovata than under the other native tree species. Soils beneath L. maackii shrubs located under C. ovata also had a greater fungal:bacterial ratio and a greater abundance of the saprophytic fungal lipid biomarker 18:1ω9c. These results demonstrate that L. maackii’s impact extends to ecosystem processes and suggests that invasive plants may have variable effects within a given environment depending on their interactions with the dominant native species. Identifying native species or communities that are more vulnerable to alterations of ecosystem function upon invasion may prove useful to land managers and foster a better understanding of the role that community dynamics play in moderating or enhancing invasive species impacts.

Doctor of Philosophy / Department of Biology / John M. Blair / Juniperus virginiana, an important woody plant invader in the U.S. Central Plains, has increased considerably in density and cover in large areas previously dominated by tallgrass prairie. Change in the phenology and nitrogen use efficiency of the dominant plant communities as J. virginiana replaces native prairies may lead to increased plant productivity and biomass accumulation, but may also alter the microclimate and litter quality that affect soil microbial communities responsible for key soil processes. I have focused my investigations on changes in key soil processes that could lead to differences in soil N availability, as well as changes in ecosystem C and N pools and fluxes as J. virginiana expands into native grasslands. Juniperus virginiana forest soils exhibit greater cumulative annual net N mineralization (11.52 ± 0.38 µg N g¯1 soil y¯1) compared to prairie soils (7.90 ± 0.26 µg N g¯1 soil y¯1) (F = 60.67, P = 0.016), yet slightly reduced potential soil C flux. Examination of internal soil N cycling revealed that both J. virginiana and prairie soils minimize potential soil N losses, by rapid microbial immobilization of inorganic N, and constraining nitrification via substrate limitation or environmental constraints. Leaf-level photosynthetic nitrogen use efficiency (NUE) was over a magnitude higher in the dominant grass, Andropogon gerardii, but high annual ecosystem-level NUE and greater soil N availability may contribute to the higher productivity and rapid accrual of C in newly established J. virginiana forests. Increased plant productivity and elimination of fire in J. virginiana forests have allowed at least 80,000 kg ha-1 increase in ecosystem C storage in about half a century. Soil organic C, an important long-term sink, has also increased significantly in J. virginiana forests, with approximately 34% replacement of C4 grass-derived soil C with new C from trees in the A-horizon. The observed high productivity of J. virginiana and increased N availability necessary to support continued plant biomass accumulation are possible because of substantial (~ 44%) increase in ecosystem N in measured pools, which is a likely a result of reduced volatilization of N from biomass burning, possible increased exogenous N inputs, and/or N translocation from deeper soil horizons. Reduced fire return intervals in prairie provide an opportunity for J. virginiana to establish and facilitate N accrual, which may allow this species to accelerate is own establishment through creating conditions of increased N availability and efficient utilization of N.

Wild boars (Sus scrofa) have increased rapidly in population size and home range since their reintroduction to the Swedish fauna. Through their rooting behaviour, the boars can cause great disturbance to the ecosystem. The consequences of this disturbance have been recorded in many parts of the world but the effects they have on boreal forest ecosystems have been less studied.In this study I examined to what extent wild boars affect (i) the soil microbial community, (ii) the soil respiration, (iii) the soil C and nutrient content, as well as (iv) the growth of trees in a boreal forest in central Sweden. Enclosures that have harboured wild boars for seven years were compared with nearby areas without wild boars in both a pine forest and a spruce forest. Using phospholipid fatty acid (PLFA) analyses, the soil microbial biomass, community, and fungi:bacteria ratio were investigated in the treatments. The soil respiration was measured monthly from June to November. The total soil concentrations of C, N and P were examined as well as the availability of N in the form of NO3-and NH4+. Tree growth was measured by measuring tree ring width and comparisons were made by looking at the relative ring width of tree cores before and after the introduction of wild boars. I found that the soil microbial community did generally not differ between the treatments, except for a higher fungi:bacteria ratio within the enclosures. Soil respiration was consistently lower in the enclosures throughout all months. Soil nutrient levels were similar between the treatments except for a slightly lowertotal P content and NO3-content in the pine forest enclosure. Lastly, the tree growth was severely decreased by the presence of wild boars. In summary my study suggests that anincrease of the wild boar population in boreal forests may have a minor impact on many soil processes, while tree growth can be hampered. However, studies in natural ecosystems are needed to investigate this effect further.

Termites are important ecosystem engineers in tropical and sub-tropical terrestrial regions where they influence ecosystem processes by altering the physical and chemical structure of the habitat. Termites affect nutrient availability by decomposition and comminution (shredding) of organic matter and act as agents of bioturbation as they re-work substrates during the construction of nests, tunnels and runways. At present we have a relatively good understanding of termite diversity patterns in the tropics through the extensive use of the standardised transect sampling protocol by Eggleton et al. (1995). These diversity data suggest that there is a functional difference in termite assemblage structure, and potentially in termite abundance and biomass, among comparable habitats across continents. However due to the lack of comparable abundance and biomass data from South America this has not previously been confirmed. In this thesis I, therefore, collected extensive data on termite taxonomic and functional assemblage structure in a South American site in Peru. The data were used to compare termite abundance and biomass from two comparable sites in Africa (Cameroon) and south east Asia (Malaysia) in order to gain better understanding of the role termites play in ecosystem processes. I found that there was an intercontinental difference in the abundance and biomass of termite feeding-groups mainly due to the dominance of soil-feeding termites in Cameroon and the absence of fungus-growing termites from Peru. The impact of certain lineages on the intercontinental differences suggests that the differences may be due to biogeographical evolution. Moreover, Eggleton et al. (1998) show that larger-bodied soil-feeding termites in Cameroon process more energy per unit area than predicted by their body size. Due to the need for an examination of the allometric relationships in termite assemblages outside Africa and the development of a more sophisticated feeding-group classification I explore the findings in Eggleton et al. (1998) further using population density - body mass relationships in three termite feeding-groups among the three continental sites in Cameroon, Peru and Malaysia. I found that large-bodied soil-feeding termites in Cameroon and large-bodied wood-feeding termites in Peru had higher population densities than expected by their body masses. As the population density - body mass relationship is inverse to that of the energy - body mass relationship the results suggest that the two feeding-groups also use more energy than expected by their body masses. Further, we have a relatively good understanding of the role termites play as ecosystem engineers e.g. in nutrient cycling and distribution, however, compared with our understanding of wood and litter decomposition in tropical forests quantitative data on the impact of termites in soil processes is poorly understood. In this thesis I conducted, to our knowledge, the first in situ soil macrofauna exclusion experiment using translocated soil in Peru to examine the impact of termites on soil C and N loss. I found that termites promote soil C and N loss which may be linked to the increase in microbial activity due to the passage of soil through the termite gut as well as the affect termites have on bioturbation and nutrient distribution. To conclude, in this thesis I present the first intercontinental comparison of abundance and biomass as well as the first in situ soil macrofauna exclusion experiment to date. The link between termite ecology, biogeography and evolution is discussed as well as the contribution of this thesis to the field of termite ecology.

To date, there has been little research into the role of microbial community structure in the functioning of the soil ecosystem and on the links between microbial biomass size, microbial activity and key soil processes that drive nutrient availability. The maintenance of structural and functional diversity of the soil microbial community is essential to ensure the sustainability of agricultural production systems. Soils of the same type with similar fertility that had been under long-term organic and conventional crop management in Canterbury, New Zealand, were selected to investigate relationships between microbial community composition, function and potential environmental impacts. The effects of different fertilisation strategies on soil biology and nitrogen (N) dynamics were investigated under field (farm site comparison), semi-controlled (lysimeter study) and controlled (incubation experiments) conditions by determining soil microbial biomass carbon (C) and N, enzyme activities (dehydrogenase, arginine deaminase, fluorescein diacetate hydrolysis), microbial community structure (denaturing gradient gel electrophoresis following PCR amplification of 16S and 18S rDNA fragments using selected primer sets) and N dynamics (mineralisation and leaching). The farm site comparison revealed distinct differences between the soils in microbial community structure, microbial biomass C (conventional > organic) and arginine deaminase activity (organic > conventional). In the lysimeter study, the soils were subjected to the same crop rotation (barley (Hordeum vulgare L.), maize (Zea mays L.), rape (Brassica napus L. ssp. oleifera (Moench)) plus a lupin green manure (Lupinus angustifolius L.) and two fertiliser regimes (following common organic and conventional practice). Soil biological properties, microbial community structure and mineral N leaching losses were determined over 2½ years. Differences in mineral leaching losses were not significant between treatments (total organic management: 24.2 kg N per ha; conventional management: 28.6 kg N per ha). Crop rotation and plant type had a larger influence on the microbial biomass, activity and community structure than fertilisation. Initial differences between soils decreased over time for most biological soil properties, while they persisted for the enzyme activities (e.g. dehydrogenase activity: 4.0 and 2.9 µg per g and h for organic and conventional management history, respectively). A lack of consistent positive links between enzyme activities and microbial biomass size indicated that similarly sized and structured microbial communities can express varying rates of activity. In two successive incubation experiments, the soils were amended with different rates of a lupin green manure (4 or 8t dry matter per ha), and different forms of N at 100 kg per ha (urea and lupin) and incubated for 3 months. Samples were taken periodically, and in addition to soil biological properties and community structure, gross N mineralisation was determined. The form of N had a strong effect on microbial soil properties. Organic amendment resulted in a 2 to 5-fold increase in microbial biomass and enzyme activities, while microbial community structure was influenced by the addition or lack of C or N substrate. Correlation analyses suggested treatment-related differences in nutrient availability, microbial structural diversity (species richness or evenness) and physiological properties of the microbial community. The findings of this thesis showed that using green manures and crop rotations improved soil biology in both production systems, that no relationships existed between microbial structure, enzyme activities and N mineralisation, and that enzyme activities and microbial community structure are more closely associated with inherent soil and environmental factors, which makes them less useful as early indicators of changes in soil quality.

Thesis (MScConEcol (Conservation Ecology and Entomology))--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: Heuweltjies are below ground termitaria found along the west coast and the south-western and -eastern Cape. They traversing different climates, soil and vegetation types within the Succulent Karoo and Fynbos biomes. From a birds eye view, heuweltjies appear as near-circular landscape features, demonstrating clear difference in vegetation structure to their surroundings. A study of aspects of the structure and function of heuweltjies, mima-like mounds of south-western South Africa was undertaken. The main aim of the study was to investigate topsoil properties and processes and vegetation structure and the ecophysiology of plants growing on these mounds, and compare it to areas off these patches. Four study sites across a rainfall gradient were chosen for soil and vegetation studies (two within the Succulent Karoo and two at the Fynbos biome). Ten sampling sites on heuweltjies and 10 off heuweltjie (interpatch) were chosen at each study site for these analyses. Soils samples from the top 10 cm were analysed for total nitrogen (N) and carbon (C) content, phosphorus (P) concentrations, particle-size distributions, pH and electrical conductivity (related to salinity). Soil biogeochemistry studies (i.e. potential N mineralization, potential soil respiration and phosphatase activity) were also done in the laboratory. In the field, the cover of soil cryptogams, selected plant growth forms and selected physical variables at each site, on and off heuweltjies, were estimated on 1m2 grids. Two of the study sites, one in the Succulent Karoo and one in Fynbos, were chosen for field measurements of stomatal conductance, predawn and midday xylem water potentials of two growth forms (deciduous and evergreen non-succulent shrubs) at selected times in the duration of the study. Leaf samples were taken to make once-off measurements of specific leaf area, C and N isotope compositions (δ13C, δ15N), and foliar N content. Furthermore, a fertilization experiment that has been running at the Tierberg Karoo Research Centre (Succulent Karoo) since 1997, where 10g m-2 of N have been added to interpatch vegetation twice annually, was included in the stomatal conductance and xylem water potential studies. The aim of including the fertilization experiment was to ascertain whether interpatch vegetation will eventually resemble heuweltjie vegetation in ecophysiology following N fertilization. Soil water probes were inserted, 10 cm deep, on one on heuweltjie and one off heuweltjie site for the long-term estimation of gravimetric water content and temperature at the Tierberg Karoo Research Centre. Heuweltjie soils were found to be hotspots for nutrients and biogeochemical activity. They had elevated levels of total % C, % N and available P (mg kg-1) compared to surrounding interpatch soils. pH and electrical conductivity (μS cm-1) of these soils were also high compared to interpatch areas. Heuweltjie soils have higher silt content and retain more water, and for a longer period following a rainstorm. Potential soil respiration, potential N mineralization and alkaline and acid phosphatase activity were generally higher on heuweltjies.Cover of the selected plant growth forms differed little between heuweltjies and the surrounding vegetation but site differences were observed. Heuweltjies differed from interpatches in the cover of soil cryptogams, with higher cover of mosses and lower cover of lichens on mounds. There was higher cover of plant litter and termite frass on the heuweltjies.There was a decrease in soil available P and increase in non-succulent shrub cover from low to high rainfall areas. On the other hand no significant differences were found in the xylem water potential of interpatch and heuweltjie plants at either of the study sites. At the Tierberg Karoo Research, stomatal conductance of interpatch plants was higher and water use efficiency (δ13C) lower compared to that of heuweltjie plants suggesting some level of water stress for heuweltjie plants at this site. Here, heuweltjie plants also had higher foliar N, lower C/N ratio values, signs of high photosynthetic potential and highly decomposable litter and higher δ15N. At Jonaskop, stomatal conductance and water use efficiency did not differ between heuweltjies and interpatch plants. There were also no differences in foliar N concentrations, C/N ratios and δ15N between heuweltjie and interpatch plants at this site. Plants growing in high N plots (the fertilizer plots) had higher xylem water potential and more variable stomatal conductance than those growing in control plots. The results of this study have shown heuweltjies to be hotspots for nutrients and biogeochemical cycling, and given enough water, will be ideal patches for plant growth. In the Succulent Karoo, high salinity (and finer textured soils) on the heuweltjies may contribute to water stress during dry periods, and together with disturbance, provide bottom-up controls on plant physiology, and ultimately, vegetation structure. In the Fynbos, plants may be limited by nutrients on heuweltjies, possibly because of increased interspecific competition, while increased rainfall may also lead to more nutrients leaching from the topsoils. The results presented are especially relevant to restoration programs, and may shed some light on restoration trajectories after overgrazing, cultivation and mining activities. / AFRIKAANSE OPSOMMING: Heuweltjies is ondergrondse termitaria wat langs die weskus, die suid-wes en oos Kaap van Suid Afrika voorkom. Hulle kom voor onder verskillende klimaatstreke, grondtipes en planttipes wat binne die Sukkulente Karoo en Fynbos biome voorkom. Vanuit die lug uitgesien, verskyn heuweltjies as sirkelvormige landskapseienskappe, wat duidelike verskille in plant struktuur en die omringende omgewing demonstreer. ʼn Studie van aspekte van struktuur enfunksie van ʼn heuweltjies, mima-agtige hope vansuidwestelike Suid-Afrikais onderneem. Die hoofdoel van die studie was om die eienskappe van oppervlakgrondeen -prosesse, plant struktuur, en die ekofisiologie van plante wat op heuweltjies groei te ondersoek, en dit te vergelyk met areas af van hierdie kolle. Vier studieareas wat oor „n reënvalgradient strek is gekies vir grond en plantegroei studies (twee binne die Sukkulente Karoo en twee binne die Fynbos biome). Tien monsterareas op heuweltjies en 10 af van heuweltjies (omringende areas) is gekies binne elke studiearea vir die bogenoemde analises. Grondmonsters van die boonste 10 cm van die grond is geneem en geanalieseer vir totale stikstof (N) en koolstof (C), fosfor (P) konsentrasies, gronddeeltjiegrootte, pH, en elektriese geleidingsvermoë, (hou verband met soutgehalte). Grond biogeochemie studies (d.i. potensiële stikstof mineralisasie, potensiële grond respirasie en fosfatase aktiwiteit) is ook in die laboratorium uitgevoer. In die veld was die dekking van grond kriptogame, geselekteerde plant groeivorms en fisiese veranderlikes by elke lokaliteit, beide op en van heuweltjies op 1m2vierkante bereken. Twee studie lokaliteite, een in die Sukkulent Karoo en een in die Fynbos, is gekies vir veld metings van huidmondjiegeleiding, voor dagbreek en middag xileem potensiale van twee groei vorms (bladwisselende en immergroen nie-sukkulente struike) op geselekteerde tye gedurende die studie. Blaarmonsters is geneem om eenmalige metings te maak van die spesifieke blaararea, C en N isotoop komposisies (δ13C, δ15N), en blaar N inhoud. Verder is „nbemestingseksperiment wat by die Tierberg Karoo Navorsing Sentrum (Sukkulente Karoo) loop sedert 1997, waar 10gm-2 van N jaarliks twee keer bygevoeg word by die plante wat tussen heuweltjies groei ingesluit in die stomata geleiding en xileem water potensiaal studies. Die doel van die bevrugting eksperiment was om vas te stel of die omliggende plante (af van heuweltjies) uiteindelik soos die van die heuweltjiessal lyk in terme van ekofisologie nadat dit met stikstof bemes is. Grondwater monitoreringsapparaat was ingesit (10cm diep) op een op heuweltjie en een lokaliteitaf van die heuweltjie vir die langtermyn bepaling van gravimetriese water inhoud (m3m-3) en temperatuur by die Tierberg Karoo Navorsing Sentrum. Daar is gevind dat grond op heuweltjies “skerpkolle” is vir voedingstowwe en biogeochemiese aktiwiteit. Hulle het ook verhoogte vlakke van totale %C, % N, en beskikbare P (mg kg-1) teenoor die omliggende areas om die heuweltjies. pH en elektriese geleidingsvermoë (μS cm-1) van hierdie gronde was ook hoër in vergelyking met die omliggende areas. Heuweltjie grond het „n hoër sout inhoud gehad en behou meer water vir ʼn langer periode na ʼn reënstorm. Potensiële grond respirasie, potensiële stikstof mineralisasie, en alkaliese en suur fosfatase aktiwiteit is oor die algemeen hoër op heuweltjies. Die voorkoms van die geselekteerde plant groeivorms verskil min tussen heuweltjies en die omliggende omgewing, maar verskille tussen studie areas is opgemerk. Heuweltjies het verskil van areas af van heuweltjies in die voorkoms van grond kriptogame, met ʼn hoër voorkoms van mosse en ʼn laer voorkoms van ligene op die heuweltjies. Daar was hoër voorkoms van plantafvalmateriaal en termiet uitwerpsel op die heuweltjies. Daar was ʼn afname in grond P (mg kg-1) en ʼn toename in nie-sukkulente struik bedekking van lae tot hoë reënval areas. Daar was geen verskil in die xileem water potensiaal van plante op die heuweltjies of af van die heuweltjies by enige van die studie lokaliteite nie. By die Tierberg Karoo Navorsing Sentrum was huidmondjie geleiding van plant in die areas tussen heuweltjies hoër en die water gebruik doeltreffendheid (δ13C) laer in vergelyking met die van die heuweltjie plante, wat op„n sekere vlak waterstres dui in heuweltjie plante by hierdie studiearea. Hier het heuweltjie plante ook hoër blaar N, laer C/N waardes, tekens van hoër fotosintetiese potensiaal en hoogs afbreekbare afvalmaterialgehad asook hoër δ15N. By Jonaskop het huidmondjie geleiding en water gebruik doeltreffendheid nie verskil tussen heuweltjie en omliggende plante nie. Daar was ook geen verskille in blaar N konsentrasies, C/N verhoudings en δ15N tussen heuweltjie en omliggende plante by hierdiestudiearea nie. Plante wat groei in hoë stikstof plotte, het hoër xileem water potensiaal (minder stres) en meer veranderlike stomatal geleiding as die plante wat in die kontrole plotte groei. Die resultate van die studie het getoon dat heuweltjies “skerpkolle” is van voedingstowwe en biogeochemiese siklusse, en gegewe genoeg grondwater, sal ideale areas wees vir die groei van plante. In die Sukkulente Karoodra hoë soutgehalte (en fyner grond teksture) op die heuweltjies by tot water stres gedurende droë tye, en saam met versteurings dra dit by tot grond tot plant kontrole op plantfisiologie, en uiteindelik plant gemeenskapsstruktuur. In die Fynbos word plante op die heuweltjies beperk deur te min voedingstowwe, moontlik omdat daar ʼn toename is in intraspesifieke kompetisie, terwyl hoër reenvalkan lei tot hoër loging van voedingstowwe vanuit die bogrond. Die resultate wat hier gerapporteer word is veral relevant vir restourasieprogramme, en mag lig werp op restourasie trajekte na oorbeweiding, aanplanting van gewasse en mynbou-aktiwiteite.

To date, there has been little research into the role of microbial community structure in the functioning of the soil ecosystem and on the links between microbial biomass size, microbial activity and key soil processes that drive nutrient availability. The maintenance of structural and functional diversity of the soil microbial community is essential to ensure the sustainability of agricultural production systems. Soils of the same type with similar fertility that had been under long-term organic and conventional crop management in Canterbury, New Zealand, were selected to investigate relationships between microbial community composition, function and potential environmental impacts. The effects of different fertilisation strategies on soil biology and nitrogen (N) dynamics were investigated under field (farm site comparison), semi-controlled (lysimeter study) and controlled (incubation experiments) conditions by determining soil microbial biomass carbon (C) and N, enzyme activities (dehydrogenase, arginine deaminase, fluorescein diacetate hydrolysis), microbial community structure (denaturing gradient gel electrophoresis following PCR amplification of 16S and 18S rDNA fragments using selected primer sets) and N dynamics (mineralisation and leaching). The farm site comparison revealed distinct differences between the soils in microbial community structure, microbial biomass C (conventional>organic) and arginine deaminase activity (organic>conventional). In the lysimeter study, the soils were subjected to the same crop rotation (barley (Hordeum vulgare L.), maize (Zea mais L.), rape (Brassica napus L. ssp. oleifera (Moench)) plus a lupin green manure (Lupinus angustifolius L.) and two fertiliser regimes (following common organic and conventional practice). Soil biological properties, microbial community structure and mineral N leaching losses were determined over 2½ years. Differences in mineral leaching losses were not significant between treatments (total organic management: 24.2 kg N ha⁻¹; conventional management: 28.6 kg N ha⁻¹). Crop rotation and plant type had a larger influence on the microbial biomass, activity and community structure than fertilisation. Initial differences between soils decreased over time for most biological soil properties, while they persisted for the enzyme activities (e.g. dehydrogenase activity: 4.0 and 2.9 µg g⁻¹ h⁻¹ for organic and conventional management history, respectively). A lack of consistent positive links between enzyme activities and microbial biomass size indicated that similarly sized and structured microbial communities can express varying rates of activity. In two successive incubation experiments, the soils were amended with different rates of a lupin green manure (4 or 8t dry matter ha⁻¹), and different forms of N at 100 kg ha⁻¹ (urea and lupin) and incubated for 3 months. Samples were taken periodically, and in addition to soil biological properties and community structure, gross N mineralisation was determined. The form of N had a strong effect on microbial soil properties. Organic amendment resulted in a 2 to 5-fold increase in microbial biomass and enzyme activities, while microbial community structure was influenced by the addition or lack of C or N substrate. Correlation analyses suggested treatment-related differences in nutrient availability, microbial structural diversity (species richness or evenness) and physiological properties of the microbial community. The findings of this thesis showed that using green manures and crop rotations improved soil biology in both production systems, that no relationships existed between microbial structure, enzyme activities and N mineralisation, and that enzyme activities and microbial community structure are more closely associated with inherent soil and environmental factors, which makes them less useful as early indicators of changes in soil quality.