Manure injection is an alternative manure application method that places manure in subsurface bands rather than spreading it evenly across the soil surface as done with the typical broadcasting method. The reduced exposure of manure to air under injection can lead to greater N retention when compared to broadcasting, but also alters the spatial distribution of manure. This altered spatial distribution of manure could alter soil nutrient dynamics and crop growth; however, literature exploring this subject is limited. Therefore, this dissertation aimed to compare soil nitrate and active carbon levels between manure injection and broadcasting, assess the spatial distribution of soil N under injection, and determine if the subsurface bands under injection cause differential crop growth. An 8-site on-farm study was conducted comparing spring manure applications under corn silage. This study found that soil NO3-N was the same under injection and broadcasting but did alter the spatial distribution of soil NO3-N as it was consistently elevated in the injection band compared to between bands. No differences in active carbon were observed, even when measuring the injection band directly. This finding calls into question the usefulness of measuring active carbon in manured systems. Corn silage yields were only significantly increased at 1 of 8 sites, and this occurred at the one site that did not receive a sidedress N application, which suggests that N was not limiting at the other seven sites. A small-scale research plot study examining fall manure applications under small grains found similar results to the previous study. No consistent differences in soil NO3-N were observed between injected, broadcast, and control plots; however, soil NO3-N was greater in the injection band compared to between, a difference that persisted for two months after manure application. Evidence of soil NO3-N leaching was observed in one study year, suggesting soil NO3-N leaching under fall manure applications should be examined. No consistent differences in soil active carbon were observed, either between manure application methods or injection bands. Furthermore, the alteration in soil NO3-N under injection did not lead to differential small grain growth. A 24 site on-farm study was conducted to assess potential differential growth of small grains following manure injection. This study found that soil NO3-N in the manure injection band compared to between bands was significantly increased in 13 of 24 sites and was on average 137% greater in-band at the 0-15 cm depth. This difference did not persist through small grain silage harvest as only 1 of 24 sites showed a significant difference in-band. Small grain maturity did not show any difference in 2021 due to late planting dates, but some differences were observed in the injection band compared to between bands one month after planting. As with soil NO3-N, these differences did not persist through silage harvest. Small grain forage quality parameters were not different in-band compared to between-band at harvest, while DM yield only differed in 3 of 24 sites, with 2 of those 3 sites being under wheat. The data presented in this dissertation indicates that manure injection causes differential soil NO3-N levels from banding. Accurately measuring soil NO3-N levels under injection was difficult due to the injection band being difficult to fully sample and suggests injected soil NO3-N levels were underestimated. No meaningful changes in crop growth were observed due to banding or different manure application methods. / Doctor of Philosophy / Manure injection is a more environmentally friendly method of manure application when compared to traditional surface broadcasting. While research is clear on the environmental benefits of manure injection, the agronomic benefits of injection are unclear. Therefore, this research aimed to compare soil nitrogen and crop response to manure injection. Manure injection did not result in consistently increased corn or small grain yields when compared to manure broadcasting. Soil nitrate was not typically altered between manure application methods, but this could have been due to our soil sampling method not sampling enough of the manure injection band. Manure injection did result in soil nitrate being concentrated in the area manure was injected. The elevated soil nitrate in the area manure was injected typically persisted 1-2 months after manure application but didn't persist to the end of the growing season. This early season increase in soil nitrate concentrations in the manure injection area did not result in differential small grain maturity in both a small-scale research plot study and a 24 site on-farm study. Three of 24 sites studied showed increased small grain yield when comparing the area manure was injected compared between injection bands, with two of these three sites being under wheat. This suggests small grain yield response to manure injection bands could be species dependent.

Soil is extremely important for providing food, biomass and raw materials, water and nutrient storage; supporting biodiversity and providing foundations for man-made structures. However, its health is threatened by human activities, which can greatly affect the potential of soils to fulfil their functions and, consequently, result in environmental, economic and social damage. These issues require the characterisation of the impact and spatial extent of the problems. This can be achieved through the creation of detailed and comprehensive soil maps that describe both the spatial and vertical variability of key soil properties. Detailed three-dimensional (3D) digital soil maps can be readily used and embedded into environmental models. Three-dimensional soil mapping is not a new concept. However, only with the recent development of more powerful computers has it become feasible to undertake such data processing. Common techniques to estimate soil properties in the three-dimensional space include geostatistical interpolation, or a combination of depth functions and geostatistics. However, these two methods are both partially flawed. Geostatistical interpolation and kriging in particular, estimate soil properties in unsampled locations using a weighted average of the nearby observations. In order to produce the best possible estimate, this form of interpolation minimises the variance of each weighted average, thus decreasing the standard deviation of the estimates, compared to the soil observations. This appears as a smoothing effect on the data and, as a consequence, kriging interpolation is not reliable when the dataset is not sampled with a sampling designs optimised for geostatistics. Depth function approaches, as they are generally applied in literature, implement a spline regression of the soil profile data that aims to better describe the changes of the soil properties with depth. Subsequently, the spline is resampled at determined depths and, for each of these depths, a bi-dimensional (2D) geostatistical interpolation is performed. Consequently, the 3D soil model is a combination of a series of bi-dimensional slices. This approach can effectively decrease or eliminate any smoothing issues, but the way in which the model is created, by combining several 2D horizontal slices, can potentially lead to erroneous estimations. The fact that the geostatistical interpolation is performed in 2D implies that an unsampled location is estimated only by considering values at the same depth, thus excluding the vertical variability from the mapping, and potentially undermining the accuracy of the method. For these reasons, the literature review identified a clear need for developing, a new method for accurately estimating soil properties in 3D – the target of this research, The method studied in this thesis explores the concept of soil specific depth functions, which are simple mathematical equations, chosen for their ability to describe the general profile pattern of a soil dataset. This way, fitting the depth function to a particular sample becomes a diagnostic tool. If the pattern shown in a particular soil profile is dissimilar to the average pattern described by the depth function, it means that in that region there are localised changes in the soil profiles, and these can be identified from the goodness of fit of the function. This way, areas where soil properties have a homogeneous profile pattern can be easily identified and the depth function can be changed accordingly. The application of this new mapping technique is based on the geostatistical interpolation of the depth function coefficients across the study area. Subsequently, the equation is solved for each interpolated location to create a 3D lattice of soil properties estimations. For this way of mapping, this new methodology was denoted as top-down mapping method. The methodology was assessed through three case studies, where the top-down mapping method was developed, tested, and validated. Three datasets of diverse soil properties and at different spatial extents were selected. The results were validated primarily using cross-validation and, when possible, by comparing the estimates with independently sampled datasets (independent validation). In addition, the results were compared with estimates obtained using established literature methods, such as 3D kriging interpolation and the spline approach, in order to define some basic rule of application. The results indicate that the top-down mapping method can be used in circumstances where the soil profiles present a pattern that can be described by a function with maximum three coefficients. If this condition is met, as it was with key soil properties during the research, the top-down mapping method can be used for obtaining reliable estimates at different spatial extents.

Master of Science / Department of Biology / John M. Blair / Climate change predictions for the Great Plains region of North America include increased temperatures, changes to annual precipitation, and reduced growing season precipitation, which will likely alter grassland soil systems. To date, few studies have examined belowground community responses to predicted climate change scenarios, with fewer assessing long-term changes. My research focused on the impacts of long-term changes in precipitation and associated soil water content on belowground grassland systems (belowground plant biomass, soil carbon (C) and nitrogen (N) pools, microbial biomass C and N, and invertebrate communities) using recently collected samples from a long-term (16-yr) reciprocal core transplant between Konza Prairie Biological Station (MAP = 850 mm) and Kansas State Agricultural Research Center at Hays (MAP = 580 mm), with the Hays site having a long-term average annual precipitation amount that is ~30% less than the Konza site. Results from the experiment indicate that either increases or decreases in annual precipitation can have profound effects on belowground grassland systems. Belowground plant biomass, microbial biomass, and potential C mineralization rates were greater at the wetter Konza site regardless of soil origin. Total C stored in soils incubated at Konza was significantly greater as well, likely due to greater root inputs. The effects of precipitation were most apparent in the surface soil layers (0-20 cm), while soil origin impacted soil properties to a greater extent with increasing depth. This contrasted with results for the soil mesofauna, where total microarthropods responded negatively and nematodes responded positively to increased annual precipitation. Results of this study indicate important changes in soil C and N pools, belowground plant biomass, and soil mesofauna within grassland systems subject to changing precipitation regimes, and suggest more mesic prairie systems are more sensitive to changes in soil water availability than those in more arid grassland systems.

Planting trees on agricultural land either as farm woodlands or agroforestry (trees integrated with farming) is one option for reducing the level of atmospheric carbon dioxide. Trees store carbon as biomass, and may increase carbon storage in the ground. A review of the literature outlined uncertainty relating to changes in carbon storage after planting trees on agricultural land. The aim of this thesis is to deter¬mine the impact of tree planting on arable and pasture land in terms of above and belowground carbon storage and thereby address these uncertainties, and assess the implications for the Woodland Carbon Code: a voluntary standard for carbon storage in UK woodlands. Measurements of soil organic carbon to a depth of 1.5 m were taken at two field sites in Bedfordshire in the UK: a 19 year old silvoarable trial, and a 14 year old silvopasture and farm woodland. On average 60% and 40% of the soil carbon (rel¬ative to 1.5 m) was found beneath 0.2 and 0.4 m in depth respectively. Whilst tree planting in the arable system showed gains in soil organic carbon (12.4 t C ha−1 at 0–40 cm), tree planting in the pasture was associated with losses of soil organic carbon (6.1–13.4 t C ha−1 at 0–10 cm). Evidence from a nearby mature grazed woodland indicate that these losses may be recovered. No differences associated with tree planting were found to the full 1.5 m, though this may be due to a lack of statistical power. Measurements of above and belowground biomass, and the root distribution of 19 year old poplar (Populus spp.) trees (at the silvoarable trial) and ash (Fraxinus excelsior) trees ranging from 7 to 21 years (at several field sites across Bedfordshire) were made, involving the destructive harvest of 48 trees. These measurements suggest that Forestry Commission yield tables overestimate yield for poplar trees grown in a silvoarable system. An allometric relationship for determining ash tree biomass from diameter measurements was established. The biophysical model Yield-SAFE was updated to take into account root growth, and was parameterised using field measurements. It was successfully used to describe existing tree growth at two sites, and was then used to predict future biomass carbon storage at the silvoarable trial. Measurements indicate that losses in soil carbon at relatively shallow depths can offset a large proportion of the carbon stored in tree biomass, but assessing changes on a site by site basis may be prohibitively expensive for schemes such as the Woodland Carbon Code.

Doctor of Philosophy / Department of Agronomy / Charles W. Rice / Biofuel feedstocks such as grains and cellulose are gaining increased attention as part of the U.S. portfolio of solutions to address climate change and improve energy security. As the future of biofuels unfolds, major concerns are emerging, including the sustainability of the soil resource in bioenergy cropping system. With a clear understanding of the sustainability risks that exist within the agricultural soil resources, it is now essential to develop metrics that document the soil health as well as the total biomass production of different cropping system. We tested the effectiveness of eight bioenergy plant species grouped between annual and perennial crops. Our main objective was to determine the sustainability of bioenergy cropping systems. There was significantly greater soil structural stability plus greater root biomass under the perennial crops but greater aboveground biomass in the annual crop. Differences in soil carbon measured to 1.2 m were not significant between energy crops after five years. A transparent, unbiased method to identify possible change in soil characteristics under bioenergy cropping practice was offered. Our next metrics were soil aggregate stability and microbial community structure as indicators of soil ecosystem health and environmental stability. The effects 24 years of differing levels of residue and fertilizer inputs on soil aggregate stability, aggregate C and microbial community structure were evaluated. A native, undisturbed prairie site, located nearby was used as a reference in this study. The results showed that greater inputs of inorganic N and increased returns of crop residues did not cause a proportionately greater increase in SOC. The abundance of microbial parameters generally followed their potential carbon pool in cultivated soils but a strong mismatch was observed in the native prairie site. Our results showed for the first time a clear disconnect between decomposers and macroaggregates; highlighting the role of soil structure in protecting organic matter. Soil carbon sequestration is one of the mechanisms that have been proposed as temporary measure to mitigate global climate change. However, there was a particularly large risk of negative effects of mitigation measures related to the increased removal of crop residues from cropping systems for use in bioenergy, if this means that soil carbon is reduced. Effective measurement of soil C at the field scale requires an understanding of the spatial variability of soil C on a landscape scale. Recent technological advances in soil C measurement offer new opportunities in this area. Our surface measurements of soil C by near infrared spectroscopy (NIRS) provided a quick assessment of soil C and, soil C predicted by NIRS and measured by dry combustion laboratory measurements was correlated with and R-squared of 0.84.

Os objetivos do presente trabalho foram avaliar o potencial de acúmulo de carbono (C) em solos de cerrado sob diferentes usos florestais (Eucalyptus, Pinus versus vegetação natural) e as possíveis alterações ocorridas na ciclagem de nutrientes sob esse tipo de coberturas. Para isso, foram coletadas 30 amostras de solo (0-5, 10-25 e 35-50 cm) e de serrapilheira acumulada em plantios de Eucalyptus, Pinus e remanescentes de cerradão em quatro municípios do estado de SP, nas quais se determinou: pH, matéria orgânica (MO), C, macronutrientes e densidade, além da granulometria no solo. Os estoques de C do solo foram calculados através do ajuste e integração de equações exponenciais, obtendo-se valores entre 3,4 e 8,6 kgC.m-2.(na camada de 0 a 30 cm) e entre 5,7 e 11,3 kgC.m-2.(até 1m). Os resultados mostraram que a silvicultura de Eucalyptus e Pinus afeta o acúmulo de C e a ciclagem de nutrientes em áreas de cerrado. As alterações nos estoques de C ocorreram principalmente nos horizontes orgânicos e na camada superficial do solo em decorrência da substituição da MO original por outra de pior qualidade química. A influência do tipo de vegetação sobre o C da camada superficial do solo variou em função de características do sítio, verificando ganhos em alguns dos locais estudados, perdas em outros e ainda diferenças não significativas. Em profundidades maiores, o conteúdo de C mostrou-se fortemente relacionado com o teor de argila e diminuiu sob cultura de Eucalyptus e Pinus, sendo mais fortes as depleções sob Pinus. Nos plantios houve formação de horizontes orgânicos espessos, com concentrações de C elevadas. A concentração de nitrogênio (N), cálcio, magnésio e potássio do material aí acumulado foi menor do que nas áreas naturais, enquanto que a acidez e a relação C/N foram maiores. Isso pode inibir a decomposição, o que explicaria o maior armazenamento de C na serapilheira. A incorporação desse material ao solo implica em alterações da MO, que é um dos principais fatores de estruturação e fertilidade dos solos tropicais e foi afetada em quantidade e qualidade. As relações C/N quantificadas na camada superficial do solo foram significativamente maiores do que em áreas de vegetação natural, indicando substituição da MO nessa camada no tempo de vida dos plantios (~40 anos). Os resultados demonstram a ocorrência de alterações na qualidade química da MO na serapilheira e no solo superficial sob uso silvicultural, as quais podem originar maiores estoques e tempos de residência do C, mas também diminuições de recursos tróficos para a comunidade decompositora, com implicações no resto do ecossistema. Os resultados sugerem que a dinâmica do carbono do solo varia ao longo do perfil, sendo necessário esclarecer melhor os fatores que definem o carbono da camada superficial, maior em quantidade e mais sensível aos efeitos do manejo. / Effects on soil organic carbon storage potential and possible biogeochemical changes of established forest plantations were assessed in southeast Brazil, in Eucalyptus and Pinus plantations compared with natural areas of native dry forest (cerradão). 30 plots were randomly distributed for soil (0-5, 10-25 and 35-50 cm) and forest floor litter collection in mature plantations (~40 years old) and adjacent native forest. The design was replicated in 4 localities in Sao Paulo, southeast Brazil. Organic matter, organic carbon, macro nutrients, pH, density, and soil texture were determined. Soil organic carbon stocks were calculated through exponential equations adjustment and integration, values ranged from 3,4 to 8,6 kgC.m-2.(on the 0 and 30 cm layer) and from 5,7 to 11,3 kgC.m-2.(up to 1m). Soil organic carbon and biogeochemical features were affected under Eucalyptus and Pinus plantations. Changes in carbon stocks were stronger in organic layers and topsoil, due to the replacement of the original organic matter, causing chemical quality decrease. Effects of vegetation on topsoil organic carbon were site dependent, as plantations results in gains, losses and no remarkable differences between natural and forested areas. Eucalyptus and Pinus establishment led to organic carbon losses, which possibly conducted by soil disturbances at implementation of plantations. Carbon content was strongly related to clay at deeper layers, but not at shallower ones. Thicker organic layers with higher carbon content were found under plantations. Lower calcium, magnesium and potassium concentrations and higher acidity and C/N (carbon to nitrogen) ratios were measured at implanted forest floor litter layers. These changes could inhibit decomposition, explaining larger litter carbon storage. Soil organic matter is an important factor in maintaining tropical soil structure and fertility. It was affected by Eucalyptus and Pinus forestation. While litter organic mater is incorporated into the soil, it leads to soil organic matter chemical quality decreases. The topsoil C/N ratio measured in plantations was significantly higher than in natural vegetation areas, indicating organic matter replacement on this layer in plantations lifetime (~40 years). We concluded that litter and topsoil organic matter chemical properties were affected by forestation with Eucalyptus and Pinus, which could result in larger C stocks and residence times, but could also decrease trophic resources for decomposers, with implications on the whole ecosystem. The results suggest that soil carbon dynamics changes along the soil profile. Factors controlling surface soil carbon dynamics must be clarified further, as they contained high carbon amounts, the most sensible to management practices.

Carbon (C) pools and fluxes in northern hemisphere forest ecosystems are attracting increasing attention concerning predicted climate change. This thesis studied C fluxes, particularly soil C dynamics, in spruce forest ecosystems in relation to interactions between physical/biological processes using a process-based ecosystem model (CoupModel) with data for Swedish conditions. The model successfully described general patterns of C and N dynamics in managed spruce forest ecosystems with both tree and field layers. Using regional soil and plant data, the change in current soil C pools was -3 g C m-2 yr-1 in northern Sweden and +24 g C m-2 yr-1 in southern Sweden. Simulated climate change scenarios resulted in increased inflows of 16-38 g C m-2 yr-1 to forest ecosystems throughout Sweden, with the highest increase in the south and the lowest in the north. Along a north-south transect, this increased C sequestration mainly related to increased tree growth, as there were only minor decreases in soil C pools. Measurements at one northern site during 2001-2002 indicated large soil C losses (-96 g C m-2 yr-1), which the model successfully described. However, the discrepancy between these large losses and substantially smaller losses obtained in regional simulations was not explained. A simulation based on Bayesian calibration successfully reproduced measured C, water and energy fluxes, with estimated uncertainties for major components of the simulated C budget. Site-specific measurements indicated a large contribution from field layer fine roots to total litter production, particularly in northern Sweden. Mean annual tree litter production was 66% higher at the most southerly site (240 g C m-2 yr-1 compared with 145 g C m-2 yr-1 in the north), but when field and bottom layers were included the difference decreased to 16% (total litter production 276 g C m-2 yr-1 and 239 g C m-2 yr-1 respectively). Regional simulations showed that decomposition rate for the stable soil C fraction was three times higher in northern regions compared with southern, providing a possible explanation why soil C pools in southern Sweden are roughly twice as large as those in the north. / QC 20100922

Two peat cores from two mires with different characteristics, but both containingpermafrost features and located in the eastern part of the Tavvavuoma mire complex innorthernmost Sweden, were analysed for macrofossils and geochemical properties. Local vegetationsequences and changes in geochemical properties of peat were used to reconstruct development ofthe studied mires during the Holocene. The study includes measurements of water/ice content, bulkdensity, loss-on-ignition and C/N ratio. Radiocarbon dates for peatland inception and permafrostaggradation are available. The main purpose of the study is to verify permafrost history in thepeatlands. The results of the macrofossil analysis and values of C/N ratio indicate nutrient poor tointermediate fen environments in both studied mires until recently. Signs of permafrost upheavalwhich caused formation of xerophilic peat can be proved only since late 1950’s. The study resultscorroborate with other studies from Northern Fennoscandia and infer peatland initiation soon afterthe deglaciation of the area and permafrost-free conditions throughout entire Holocene untilrecently.

Forested riparian zones, which function as a buffer between agricultural fields and streams, filter out contaminants and sediment from the fields thereby improving water quality, cool the water with shade from trees, stabilize the stream bank and provide habitat for wildlife. However, in many agricultural areas, riparian vegetation has been removed for crop production or pasture purposes. Riparian restoration or rehabilitation is a way of restoring riparian ecosystem functions. This study examines the effect of riparian rehabilitation via tree planting along a first-order creek in Southern Ontario, 25 years after rehabilitation. Litter input, soil quality parameters and soil CO2 production rates were determined for the rehabilitated riparian zone, a grass-forb riparian zone and a natural forest riparian zone. Total litter input was 480, 580 and 295 g m-2 y-1 for the rehabilitated riparian zone, grass riparian zone and forest riparian zone, respectively. Soil bulk density was higher and hydraulic conductivity was lower for the rehabilitated riparian zone compared to the grass riparian zone and forest riparian zone. The concentration and soil stock of organic carbon and total nitrogen was lowest for the rehabilitated riparian zone compared to the grass riparian zone and forest riparian zone which were similar. The effect of riparian zone on soil CO2 production rates varied over the season. From spring to mid-summer, rates were 167, 224 and 104 mg C m-2 h-1 for the rehabilitated riparian zone, grass riparian zone and forest riparian zone, respectively. Soil CO2 production rates did not differ significantly (p < 0.05) between riparian zones for late summer and fall sampling dates. Soil CO2 production rates were significantly negatively correlated with soil C/N and positively correlated with soil pH and litter input. Soil CO2 production rates were positively correlated with soil temperature (r = 0.32) and negatively correlated with soil moisture (r = -0.48). Of the three riparian zones, the natural forest riparian zone exhibited the least amount of seasonal fluctuation for soil CO2 production rates, soil moisture and temperature. Results from this research indicated that more time is needed before soil quality and soil CO2 production rates of the rehabilitated riparian zone reach values similar to the natural forest riparian zone.

<p>Afforestation, the conversion of historically treeless areas into forests, is a rapidly spreading land-use change with the potential to sequester carbon. Afforested plantations typically feature fast growing exotic tree species that give landowners rapid returns. The efficient growth of plantations compared to less intensively managed forests also can provide greater timber yields in a smaller area. This increased efficiency in turn could require fewer acres to meet global forest product demands and could also reduce the need to log intact primary forests. Reduced primary forest harvest and high primary productivity make afforestation a highly efficient carbon sequestration tool.</p><p> However, the rapid growth and planting disturbance due to afforestation can have deleterious effects on soils and hydrology that undermine its benefits in some locations. The effects on hydrology include depletion of groundwater and reduced or complete elimination of surface water flow. Additionally, groundwater use can lead to increased concentrations of salts and trace metals in soil that could be deleterious for future plant productivity. Plantations have also been shown to acidify surface soils and stream water and to reduce soil carbon and nitrogen.</p><p> Despite the known effects of afforestation on soils, there has been little research on the mechanisms controlling these effects. For instance, there have been few studies on the effects of afforestation on soil microbes which mediate most biogeochemical processes. There is also little knowledge on what controls the effects of afforestation on soil carbon and nitrogen, vital indexes of soil quality, across regions with high levels of afforestation. The overarching goal of this dissertation is to examine the effects of afforestation on soils, microbes, and biogeochemical processes across local, regional and global scales. Understanding the mechanisms by which afforestation alters soils and biogeochemical cycling and how these mechanisms change across different scales will aid in evaluating the true costs and benefits of afforestation. These results will be useful in determining if the benefits of afforestation will continue to outweigh its costs in the long-term.</p><p> The goal of Chapter 1 is to evaluate how afforestation across the globe affects mineral soil quality, including pH, sodium, exchangeable cations, organic carbon, and nitrogen, and to examine the magnitude of these changes in regions where afforestation rates are high. To control for different initial soil conditions across the globe, I examined paired sites of afforested plantations and controls. Controls included land-use types that are frequently afforested, such as grasslands, shrublands, and pastures. I also examined potential mechanisms to reduce the impacts of afforestation on soils and to maintain long-term productivity. Across diverse plantation types (153 sites) to a depth of 30cm of mineral soil, I observed significant decreases in nutrient cations (Ca, K, Mg), increases in sodium (Na), or both with afforestation. For the global dataset, afforestation reduced soil concentrations of the macronutrient Ca by 29% on average compared with native controls (p<0.05). Afforestation by Pinus alone decreased soil K by 23% (p<0.05). Overall, plantations of all genera also led to an average 71% increase of soil Na (p<0.05). Average pH decreased 0.3 units (p<0.05) with afforestation. Afforestation caused a 6.7% and 15% (p<0.05) decrease in soil C and N content respectively, though the effect was driven principally by Pinus plantations (15% and 20% decrease, p<0.05). Carbon to nitrogen ratios in soils under plantations were 5.7-11.6% higher (p<0.05). The major implication of these results are that in several regions with high rates of afforestation, cumulative losses of C, N, Ca, and Mg are likely in the range of tens of millions of metric tons. The decreases indicate that trees take up considerable amounts of nutrients from soils; harvesting this biomass repeatedly could impair long-term soil fertility and productivity in some locations. Based on this study and a review of other literature, I suggest that proper site preparation and sustainable harvest practices, such as avoiding the removal or burning of harvest residue, could minimize the impact of afforestation on soils. These sustainable practices could in turn slow erosion, organic matter loss, and soil compaction from harvesting equipment, maintaining soil fertility to the greatest extent possible. </p><p> Soil microbes are highly diverse and control most soil biogeochemical reactions. Given the observed changes in Chapter 1, in Chapters 2 and 3 I examined how microbial functional genes and biogeochemical pools responded to the altered chemical inputs accompanying afforestation. I examined paired native grasslands and adjacent Eucalyptus plantations (previously grasslands) in Uruguay, a region that lacked forests before European settlement. Along with measurements of soil carbon, nitrogen, and bacterial diversity, I analyzed functional genes using the GeoChip 2.0 microarray that simultaneously quantified several thousand genes involved in soil carbon and nitrogen cycling. Plantations and grasslands differed significantly in functional gene profiles, bacterial diversity, and biogeochemical pool sizes. Afforestation decreased both bacterial diversity and richness compared to grasslands, though diversity remained relatively high. Most grassland functional gene profiles were similar, but plantation profiles generally differed from grasslands due to differences in functional gene abundance across many microbial groups. Eucalypts decreased ammonification and N-fixation functional genes by 11% and 7.9% (p<0.01) which correlated with decreased microbial biomass N and more NH4+ in plantation soils. Chitinase, an important carbon polymer degrading enzyme, decreased in functional gene abundance 7.8% in plantations compared to grasslands (p=0.017), and C polymer degrading genes decreased by 1.5% overall (p<0.05), which likely contributed to 54% (p<0.05) more C in undecomposed extractable soil pools and 27% less microbial C (p<0.01) in plantation soils. In general, afforestation altered the abundance of many microbial functional genes corresponding with changes in soil biogeochemistry. These changes were driven by shifts in the whole community functional gene profile, not just one or two constituent microbial taxa. Such changes in microbial functional genes correspond with altered C and N storage and have implications for long-term productivity in these soils.</p><p> The area studied in Chapters 2 and 3 lies near the middle of a precipitation gradient that stretches across the Rio de la Plata grasslands. In Chapter 4 I studied if the effects of afforestation on soil C and N from Chapters 2 and 3 varied with different precipitation levels. The effect of afforestation on soil C has been shown to depend on mean annual precipitation (MAP), with drier sites gaining C and wetter sites losing C with afforestation. This precipitation dependence of soil C changes with afforestation may be controlled by changes in soil nitrogen (N) cycling. In particular, loss of N due to leaching after afforestation could lead to soil C losses. However, the link between C and N changes due to afforestation has primarily been suggested by models and to my knowledge has never been explicitly tested across a precipitation gradient. The goal of this study was to test how precipitation affects changes in labile and bulk pools of soil C and N across a precipitation gradient, which will provide novel insight into the linkage between land-use change, different pools of soil C and N, and precipitation. I conducted this study across a gradient of precipitation in the Rio de la Plata grasslands of Argentina and Uruguay which ranged from 600mm to 1500mm of precipitation per year. The sites were all former grasslands that had been planted with Eucalyptus. I found that changes in bulk soil C and N were related to MAP with drier sites gaining and wetter sites losing C and N (R2=0.59, p=0.003), which supports the idea that N losses are strongly linked to C losses with afforestation. C and N in microbial biomass and extractable pools followed similar patterns to bulk soil C and N. Interestingly, losses of C and N decreased as the plantations aged, suggesting that longer rotation times for plantations could reduce potential soil carbon and nitrogen losses. These results indicate that afforestation is still be a valuable tool for carbon sequestration, but calculations of the benefits of afforestation must take into account site factors such as age and precipitation to accurately calculate total sequestration benefit and ensure continued high productivity and carbon sequestration.</p><p> In conclusion, afforestation could be an effective tool for carbon sequestration. However, its benefits need to be carefully weighed against its costs for soil such as reduced microbial diversity, decreased soil microbial functional capacity, losses of soil organic matter, and nutrient depletion. Careful management and consideration of afforestation is needed to ensure the greatest benefits with the least long-term damage to soils.</p> / Dissertation