Woodlot and natural woodland systems in the semi-arid regions in Tanzania are believed to have a high potential to sequester carbon (C) in their biomass and the soil which may qualify for C credits under the current voluntary C market schemes like, the REDD program. However, our understanding of the processes influencing storage and dynamics of C in soils under semi-arid agroforestry systems such as these woodlot systems is limited. This study evaluated C pools in soil and tree biomass in woodlot species of Albizia lebbeck, Leucaena leucocephala, Melia azedarach, and Gmelina arborea; and in farmland and ngitili systems. Synchrotron-based C K-edge x-ray absorption near-edge structure (XANES) spectroscopy was also used to study the influence of these land use systems on the soil organic matter (SOM) chemistry to understand the mechanisms of soil C changes. Soil samples were collected to 1 m depth and subsamples for each land use system to 0.4 m depth were fractionated into macroaggregates (2000-250 μm), microaggregates (250-53 μm), and silt and clay-sized aggregates (<53 μm) to provide information of C dynamics and stabilization in various land uses. SOC was analyzed in whole and soil aggregates and biomass C was estimated using developed biomass models from the literatures. Aboveground biomass carbon in the woodlots from the Kahama district ranged from 11.76 Mg C ha-1 to 24.40 Mg C ha-1. Based on the age of woodlots and the rate of carbon sequestration potential (CSP), Gmelina arborea had the highest rate of aboveground C sequestration (3.59 Mg C ha-1 year-1). The SOC stocks in whole soil for the land use systems from the two districts ranged from 43-67 Mg C ha-1. The degraded ngitili did not show a reduction in SOC stocks despite reducing aboveground biomass C stocks by 15.11 Mg C ha-1. SOC in the woodlots were found to be associated more with the micro and silt-and clay-sized aggregates than the macroaggregates, reflecting high stability of SOC in the woodlot systems. The XANES C K-edge spectra revealed the stabilization of recalcitrant aromatic C compounds in the silt and clay-sized aggregates. This study demonstrates the significant contributions of woodlots in biomass C accumulation as well as long-term SOC stabilization in soil fractions. Thus, these agroforestry practices hold promise to meet household energy needs while contributing to climate change mitigation and adaption.

The urgent issue with a changing climate has shed a light on the agricultural system and related climate mitigatory opportunities. One natural climate solution that has reached a lot of attention during the later years is carbon sequestrations in soils. The pedosphere, the outermost layer of the earth, constitute a great carbon pool and does therefore possibly provide big opportunities to adjust carbon levels in the atmosphere. Several measures can be utilized to enhance carbon levels in soils but could possibly imply additional negative impacts on other sustainability aspects. One of these are biodiversity, which are a trait strongly connected to semi-natural grasslands. This study therefore presents an overview of the knowledge regarding carbon sequestration on semi-natural grasslands, framed in a Swedish context. Conflicts and synergies between the climate aspect and other values of semi-natural grasslands are here investigated. Firstly, a lack of reliable Swedish data was identified and problems with extrapolating data from international studies regarding soil carbon sequestration were pointed out, especially in the context of semi-natural grasslands and biodiversity. The report shows that Swedish semi-natural grasslands have a low level of carbon sequestration and moreover a low opportunity to act as a climate mitigator. Biodiversity is often a more prioritized factor for these lands and often interpreted as a hinder for climate mitigatory measures. There are, however, possible opportunities to improve the climatic impact these lands have, depending on several aspects, but do often require a broad system perspective. Synergies between climate and biodiversity were difficult to identify except the questioned statement that grazers possibly could enhance soil carbon sequestration and at the same time provide biodiversity benefits. Conflicts were based on difficulties in implementation of sequestration enhancing measures since they often, locally, implied impoverished biodiversity. / Klimatförändringar är ett aktuellt problem som även satt fokus på jordbrukets roll och dess möjligheter att minska dess koldioxidutsläpp och dämpa den globala uppvärmningen. En av de möjliga naturliga klimatlösningarna är att lagra kol i jorden. Pedosfären, jordskorpans yttersta skikt, är en stor kol-pool och utgör därför potentiellt en god möjlighet till att minska koldioxidhalten i atmosfären. Det finns flertalet möjligheter att öka kolinlagringen i jorden men dessa kan dock innebära negativa konsekvenser av andra hållbarhetsaspekter. En av dessa är biologisk mångfald, som är något som naturbetesmarker är starkt kopplade till. Denna studie presenterar en översikt över kolinlagring i naturbetesmarker i en svensk kontext. Konflikter och synergier mellan kolinlagring och andra värden av naturbetesmarker tas även upp. Först kan det nämnas att det finns osäkerheter i data från svenska naturbetesmarker och att data av kolinlagring från internationella studier ofta är svåra att extrapolera till svenska marker. Det fanns även få studier som behandlade både klimatperspektivet och biologisk mångfald på naturbetesmarker. Rapporten visar även att svenska naturbetesmarker har en låg nivå av kolinlagring och därmed en låg möjlighet att agera som en mildrande faktor gällande ett förändrat klimat, där en anledning är den begränsande totala ytan som finns definierad som naturbetesmark. Däremot finns det möjligheter att förändra klimatavtrycket dessa marker genererar. I en sådan ansats bör man inkludera en rad olika faktorer och använda ett brett systemperspektiv för vidare analys. Synergier är svåra att identifiera, bortsett från en ifrågasatt åsikt om att betande djur potentiellt båda kan bidra till biologisk mångfald och öka kolinlagringen på den betande marken. Konflikter utgörs ofta av svårigheten att många potentiellt kolinlagrande åtgärder hade direkt negativa konsekvenser för den biologiska mångfalden lokalt.

O estudo foi dividido em quatro partes. Na primeira parte analisou-se o impacto do clima na estocagem de carbono em solos de canaviais sem a queima da palhada. Foram escolhidos três regimes climáticos contrastantes do centro-sul do Brasil. O potencial de seqüestro de carbono orgânica no solo (COS) foi determinado, em cada regime climático, de acordo com a taxa anual de carbono remanescente no solo proveniente da palhada depositada. Para obter esta taxa, compararam-se os estoques de COS em canaviais com e sem queima da palhada. Esse ganho anual foi comparado com a entrada anual de carbono via palhada depositada. O ganho anual de COS (0-30 cm) em canaviais sem queima da palhada não resultou em diferenças significativas por clima: 1, 97 Mg ha-1 (clima norte), 2,00 Mg ha-1 (clima centro) e 1,70 Mg ha-1 (clima sul). Os regimes climáticos estudados também não revelaram diferenças significativas entre suas temperaturas e precipitações médias anuais. Porém, o aumento anual de COS um pouco menor no clima sul levou à conclusão que o potencial de seqüestro de carbono diminui ligeiramente em latitudes mais altas, devido à maior precipitação no centro-sul do Brasil. Na segunda parte avaliou-se o impacto da palhada sobre a estabilidade de agregados dos solos, que foi calculada pelo método de fracionamento proposto por Six et al. (2000a). A estabilidade de agregados no solo do canavial sem a queima da palhada aumentou, em média, 15,3% por ano na profundidade 0-30 cm. Na terceira parte, avaliou-se a diferença da estabilidade de agregados entre canaviais e mata nativa. O solo da mata nativa mostrou uma estabilidade de agregados significativamente maior (7,2 vezes). Supõe-se que a estabilidade de agregados seja maior na mata nativa como resultado da maior presença de matéria orgânica e biota no solo. Na quarta parte avaliou-se a origem do carbono nos agregados estáveis em água de fluxo contínuo para determinar que forma de agregação possui a melhor proteção contra a decomposição de carbono. Houve uma diferença significativa de \'delta\'13C entre os macroagregados e microagregados na camada superior, com 10% mais \'delta\'13C nos macroagregados. Isto indica mais carbono derivado da cobertura vegetal atual (C4), ou seja, mais incorporação da palhada nos macroagregados. O \'delta\'13C da fração de partículas livres ou agregados não estáveis em água de fluxo contínuo é menor em todas as profundidades, indicando que a matéria orgânica recente (C4) encontra-se no solo principalmente de forma agregada estável em água, evidenciando seu papel fundamental na estabilidade dos agregados. Constatou-se também, que quanto mais novo o macroagregado, maior o seu teor em carbono. Nos microagregados verificou-se o efeito contrário. Quanto mais velho o microagregado, maior o seu teor em carbono. Isso indica que o microagregado possui uma melhor proteção contra a decomposição de carbono, e também a capacidade de um aumento no teor de carbono, no decorrer do tempo, em virtude da assimilação de carbono mais novo. Conclui-se, que em termos de seqüestro de carbono de longa duração, é propício avaliar mecanismos de proteção do carbono recalcitrante dentro dos microagregados e estudar como técnicas agrícolas podem proteger melhor esta fração / The study was divided in four parts. The first part investigated climate impact on soil carbon stocks in sugar cane fields cultivated without green trash burning. For this purpose, three contrasting climates were chosen in Central and Southern Brazil. The sequestration potential of soil organic carbon (SOC) was determined for each climate, calculating how much of the carbon derived from deposited green trash remains in the soil every year. To obtain this rate, SOC stocks of sugar cane fields cultivated with and without green trash burning were compared. The annual difference (0-30 cm) did not differ significantly between climates: 1, 97 Mg ha-1 (Northern Climate), 2,00 Mg ha-1 (Central Climate) and 1,70 Mg ha-1 (Southern Climate). The climates did not show significant differences between their average annual temperatures and their annual precipitation. Nevertheless, a slightly smaller gain of SOC stocks in the Southern Climate leads to the conclusion that higher latitudes tend to stock slightly less COS due to higher annual precipitation. The second part evaluated the impact of green trash deposition on soil aggregate stability, which was calculated according to Six et al. (2000a). Soil aggregate stability under sugar cane fields cultivated with green trash burning had an average increase of 15,3% for soil depth 0-30 cm. The third part studied soil aggregate stability between sugar cane and natural forest vegetation. The forest soil revealed significantly higher (7,2 times) aggregate stability, supposedly due to higher organic matter content and more soil biota. The forth part examined the origin of carbon inside water-stable aggregates to determine which aggregation form provides better protection against carbon decomposition. The top soil layer showed a significant difference in \'delta\'13C between macro-aggregates (10% more \'delta\'13C) and microaggregates which means that more carbon derived from green trash (C4) was incorporated in macro-aggregates. \'delta\'13C of free soil particles or water-unstable aggregates is smaller for all soil layers indicating that fresh organic matter (green trash) is predominantly encountered in a water stable aggregated form in the soil. Furthermore, a correlation analysis revealed that more recent macro-aggregates had higher C content. The opposite was observed for micro-aggregates: the older the micro-aggregate, the higher its carbon content, leading to the conclusion that microaggregates protect carbon better against decomposition and are also capable to enrich their carbon pool through C assimilation. Given its long-term soil carbon sequestration potential, it is recommended to investigate carbon protection mechanisms for the recalcitrant C pool in micro-aggregates and also to study how agricultural techniques could improve the protection of the recalcitrant C pool

<p>This study seeks to investigate the dynamics of dead plant carbon over fifty years of old-field forest development at the Calhoun Long Term Soil-Ecosystem Experiment (LTSE) in South Carolina, USA. Emphasis is on the transition phase of the forest, which is less well studied than the establishment and early thinning phase or the steady state phase. At the Calhoun LTSE, the biogeochemical and ecosystem changes associated with old field forest development have been documented through repeated tree measurements and deep soil sampling, and archiving of those soils, which now allow us to examine changes that have occurred over the course of forest development to date.</p><p> In this dissertation, I first quantify the accumulation of woody detritus on the surface of the soil as well as in the soil profile over fifty years, and estimate the mean residence times of that detrital carbon storage. Knowing that large accumulations of C-rich organic matter have piled onto the soil surface, the latter chapters of my dissertation investigate how that forest-derived organic carbon has been incorporated into mineral soils. I do this first by examining concentrations of dissolved organic carbon and other constituents in soil solutions throughout the ecosystem profile and then by quantifying changes in solid state soil carbon quantity and quality, both in bulk soils and in soil fractions that are thought to have different C sources, stabilities, and residence times. To conclude this dissertation, I present the 50-year C budget of the Calhoun LTSE, including live and dead plant carbon pools, to quantify the increasing importance of detrital C to the ecosystem over time.</p><p>This exceptional long term soil ecosystem study shows that 50 years of pine forest development on a former cotton field have not increased mineral soil carbon storage. Tree biomass accumulated rapidly from the time seedlings were planted through the establishment phase, followed by accumulations of leaf litter and woody detritus. Large quantities of dissolved organic carbon leached from the O-horizons into mineral soils. The response of mineral soil C stocks to this flood of C inputs varied by depth. The most surficial soil (0-7.5cm), saw a large, but lagged, increase in soil organic carbon (SOC) concentration over time, an accumulation almost entirely due to an increase of light fraction, particulate organic matter. Yet in the deepest soils sampled, soil carbon content declined over time, and in fact the loss of SOC in deep soils was sufficient to negate all of the C gains in shallower soils. This deep soil organic matter was apparently lost from a poorly understood, exchangeable pool of SOM. This loss of deep SOC, and lack of change in total SOC, flies in the face of the general understanding of field to forest conversions resulting in net increases in soil carbon. These long term observations provide evidence that the loss of soil carbon was due to priming of SOM decomposition by enhanced transpiration, C inputs, and N demand by the growing trees. These results suggest that large accumulations of carbon aboveground do not guarantee similar changes below.</p> / Dissertation

O estudo foi dividido em quatro partes. Na primeira parte analisou-se o impacto do clima na estocagem de carbono em solos de canaviais sem a queima da palhada. Foram escolhidos três regimes climáticos contrastantes do centro-sul do Brasil. O potencial de seqüestro de carbono orgânica no solo (COS) foi determinado, em cada regime climático, de acordo com a taxa anual de carbono remanescente no solo proveniente da palhada depositada. Para obter esta taxa, compararam-se os estoques de COS em canaviais com e sem queima da palhada. Esse ganho anual foi comparado com a entrada anual de carbono via palhada depositada. O ganho anual de COS (0-30 cm) em canaviais sem queima da palhada não resultou em diferenças significativas por clima: 1, 97 Mg ha-1 (clima norte), 2,00 Mg ha-1 (clima centro) e 1,70 Mg ha-1 (clima sul). Os regimes climáticos estudados também não revelaram diferenças significativas entre suas temperaturas e precipitações médias anuais. Porém, o aumento anual de COS um pouco menor no clima sul levou à conclusão que o potencial de seqüestro de carbono diminui ligeiramente em latitudes mais altas, devido à maior precipitação no centro-sul do Brasil. Na segunda parte avaliou-se o impacto da palhada sobre a estabilidade de agregados dos solos, que foi calculada pelo método de fracionamento proposto por Six et al. (2000a). A estabilidade de agregados no solo do canavial sem a queima da palhada aumentou, em média, 15,3% por ano na profundidade 0-30 cm. Na terceira parte, avaliou-se a diferença da estabilidade de agregados entre canaviais e mata nativa. O solo da mata nativa mostrou uma estabilidade de agregados significativamente maior (7,2 vezes). Supõe-se que a estabilidade de agregados seja maior na mata nativa como resultado da maior presença de matéria orgânica e biota no solo. Na quarta parte avaliou-se a origem do carbono nos agregados estáveis em água de fluxo contínuo para determinar que forma de agregação possui a melhor proteção contra a decomposição de carbono. Houve uma diferença significativa de \'delta\'13C entre os macroagregados e microagregados na camada superior, com 10% mais \'delta\'13C nos macroagregados. Isto indica mais carbono derivado da cobertura vegetal atual (C4), ou seja, mais incorporação da palhada nos macroagregados. O \'delta\'13C da fração de partículas livres ou agregados não estáveis em água de fluxo contínuo é menor em todas as profundidades, indicando que a matéria orgânica recente (C4) encontra-se no solo principalmente de forma agregada estável em água, evidenciando seu papel fundamental na estabilidade dos agregados. Constatou-se também, que quanto mais novo o macroagregado, maior o seu teor em carbono. Nos microagregados verificou-se o efeito contrário. Quanto mais velho o microagregado, maior o seu teor em carbono. Isso indica que o microagregado possui uma melhor proteção contra a decomposição de carbono, e também a capacidade de um aumento no teor de carbono, no decorrer do tempo, em virtude da assimilação de carbono mais novo. Conclui-se, que em termos de seqüestro de carbono de longa duração, é propício avaliar mecanismos de proteção do carbono recalcitrante dentro dos microagregados e estudar como técnicas agrícolas podem proteger melhor esta fração / The study was divided in four parts. The first part investigated climate impact on soil carbon stocks in sugar cane fields cultivated without green trash burning. For this purpose, three contrasting climates were chosen in Central and Southern Brazil. The sequestration potential of soil organic carbon (SOC) was determined for each climate, calculating how much of the carbon derived from deposited green trash remains in the soil every year. To obtain this rate, SOC stocks of sugar cane fields cultivated with and without green trash burning were compared. The annual difference (0-30 cm) did not differ significantly between climates: 1, 97 Mg ha-1 (Northern Climate), 2,00 Mg ha-1 (Central Climate) and 1,70 Mg ha-1 (Southern Climate). The climates did not show significant differences between their average annual temperatures and their annual precipitation. Nevertheless, a slightly smaller gain of SOC stocks in the Southern Climate leads to the conclusion that higher latitudes tend to stock slightly less COS due to higher annual precipitation. The second part evaluated the impact of green trash deposition on soil aggregate stability, which was calculated according to Six et al. (2000a). Soil aggregate stability under sugar cane fields cultivated with green trash burning had an average increase of 15,3% for soil depth 0-30 cm. The third part studied soil aggregate stability between sugar cane and natural forest vegetation. The forest soil revealed significantly higher (7,2 times) aggregate stability, supposedly due to higher organic matter content and more soil biota. The forth part examined the origin of carbon inside water-stable aggregates to determine which aggregation form provides better protection against carbon decomposition. The top soil layer showed a significant difference in \'delta\'13C between macro-aggregates (10% more \'delta\'13C) and microaggregates which means that more carbon derived from green trash (C4) was incorporated in macro-aggregates. \'delta\'13C of free soil particles or water-unstable aggregates is smaller for all soil layers indicating that fresh organic matter (green trash) is predominantly encountered in a water stable aggregated form in the soil. Furthermore, a correlation analysis revealed that more recent macro-aggregates had higher C content. The opposite was observed for micro-aggregates: the older the micro-aggregate, the higher its carbon content, leading to the conclusion that microaggregates protect carbon better against decomposition and are also capable to enrich their carbon pool through C assimilation. Given its long-term soil carbon sequestration potential, it is recommended to investigate carbon protection mechanisms for the recalcitrant C pool in micro-aggregates and also to study how agricultural techniques could improve the protection of the recalcitrant C pool

<p dir="ltr">Globally, soils hold more carbon than both the atmosphere and aboveground terrestrial biosphere combined. Changes in land use and land cover have the potential to alter soil carbon cycling throughout the soil profile, from the surface to meters deep, yet most studies focus only on the near surface impact (< 25 cm deep). This research bias toward shallow soil carbon cycling has ramifications for understanding the full impacts of agricultural and restoration management practices on soil organic and inorganic carbon dynamics. The primary objective of my thesis research is to evaluate the factors controlling the impact of deep-rooting perennial grass on soil carbon cycling during prairie restoration of soil following long term, row crop agriculture. Paired soil pits were established to compare the effects of restoration on soil C dynamics in a corn-soy cropping system (minimal tillage) and restored prairie sites in Nebraska and Illinois. At each site, soil organic carbon (SOC) and inorganic carbon (SIC) content, stock, and stable carbon isotope analysis were preformed to ~2 m depth to assess long term integrated C dynamics. Estimating the contribution of prairie carbon inputs to the SOC in the soil profile was examined using stable carbon isotopic signatures in the SOC in relation to the above ground vegetation changes in C₃ and C₄ photosynthetic pathway plant community composition. Comparative analysis of edaphic properties and soil carbon suggests that deep loess deposits in Nebraska permit enhanced water infiltration and SOC deposition to depths of ~100 cm in 60 years of prairie restoration. In Illinois, poorly drained, clay/lime rich soils on glacial till and a younger restored prairie age (15 years) restricted the influence of prairie restoration to the upper 30 cm. Comparing the δ¹³C values of SOC and SIC in each system demonstrated that SIC at each site is likely of lithogenic origin. This work indicates that the magnitude of influence of restoration management is dependent on edaphic properties inherited from geological and geomorphological controls. Future work should quantify root structures and redox properties to better understand the influence of rooting depth on soil carbon concentrations. Fast-cycling C dynamics can be assessed using continuous, in-situ CO₂ and O₂ soil gas concentration changes. The secondary objective of my thesis was to determine if manual, low temporal resolution gas sampling and analysis are a low cost and effective means of measuring soil O₂ and CO₂, by comparing it with data from in-situ continuous (hourly) sensors. Manual analysis of soil CO₂ and O₂ from field replicates of buried gas collection cups resulted in measurement differences from the continuous sensors. Measuring CO2 concentration with manual methods often resulted in higher concentrations than hourly, continuous measurements across all sites. Additionally, O₂ concentrations measured by manual methods were higher than hourly values in the restored prairie and less in agricultural sites. A variety of spatial variability, pressure perturbations, calibration offsets, and system leakage influences on both analysis methods could cause the discrepancy.</p>

<p>Current watershed-scale, nonpoint source (NPS) pollution models do not represent the processes and impacts of agricultural best management practices (BMP) on water quality with sufficient detail. To begin addressing this gap, a novel process-based, watershed-scale, water quality model (WEPP-WQ) was developed based on the Water Erosion Prediction Project (WEPP) and the Soil and Water Assessment Tool (SWAT) models. The proposed model was validated at both hillslope and watershed scales for runoff, sediment, and both soluble and particulate forms of nitrogen and phosphorus. WEPP-WQ is now one of only two models which simulates BMP impacts on water quality in ‘high’ detail, and it is the only one not based on USLE sediment predictions. Model validations indicated that particulate nutrient predictions were better than soluble nutrient predictions for both nitrogen and phosphorus. Predictions of uniform conditions outperformed nonuniform conditions, and calibrated model simulations performed better than uncalibrated model simulations. Applications of these kinds of models in real-world, historical simulations are often limited by a lack of field-scale agricultural management inputs. Therefore, a prototype tool was developed to derive management inputs for hydrologic models from remotely sensed imagery at field-scale resolution. At present, only predictions of crop, cover crop, and tillage practice inference are supported and were validated at annual and average annual time intervals based on data availability for the various management endpoints. Extraction model training and validation were substantially limited by relatively small field areas in the observed management dataset. Both of these efforts contribute to computational modeling research and applications pertaining to agricultural systems and their impacts on the environment.</p>