Accumulation of CO2 in the atmosphere has triggered research on topics related to causes, effects, and solutions to potential problems associated with global warming. The present research was conducted to determine if grassland plant communities can be managed to promote sequestration of carbon in the soil, potentially mitigating the effects of increasing atmospheric CO2. The effects of shrub invasion or heavy livestock grazing on peak standing crop of phytomass, root mass and soil organic carbon content were therefore studied. These studies were complimented by a study of the decomposition rates of leaves and roots of snowberry and grasses. The effects of snowberry encroachment on peak standing crop of aboveground phytomass, and soil organic carbon content (SOC) were also studied. Total aboveground phytomass in the snowberry community was more than triple that of the ecotone and was 6-times greater than that of the grassland community. Similarly, the mass of large roots was greatest in the snowberry community (1.2 kg m-2, SE= 0.19), intermediate in the ecotone (0.5 kg m-2, SE= 0.08), and least in the grassland (0.1 kg m-2, SE= 0.04). Conversely, the mass of fine and medium roots was not different (P>0.05) among the three communities, averaging 0.7 kg m-2 in all communities (SE= 0.03, 0.07, 0.49 in snowberry, ecotone and grassland, respectively). Greater aboveground phytomass did not correspond with greater SOC in the snowberry community. Soil organic carbon in the upper 50 cm averaged 8.3 (SE= 0.7), 7.9 (SE= 1.0), and 7.9 (SE= 0.7) kg m-2 in snowberry, ecotone, and grassland communities, respectively. Peak standing crop of aboveground phytomass averaged 157 g m-2 (SE= 27) and 488 g m-2 (SE= 48) in grazed and ungrazed grassland, respectively. Conversely, grazing had no affect on root mass. The mass of fine roots averaged 0.9 kg m-2 (SE= 0.04) and 0.8 kg m-2 (SE= 0.06) in grazed and ungrazed grassland, respectively, while that of medium roots averaged 0.6 kg m-2 (SE= 0.07) in both grazing treatments. Total SOC in the upper 50 cm of soil was not affected (P>0.05) by livestock grazing, averaging 5.5 kg m-2 (SE= 0.7) in grazed and 6.8 kg m-2 (SE= 0.9) in ungrazed grassland. Livestock grazing also had no effect (P>0.05) on SOC at the 0-3, 3-10, 10-20, 20-30, and 30-40 cm depths. The SOC in fine- and coarse-textured soils averaged 7.6 kg m-2 (SE= 0.8) and 5.1 kg m-2 (SE=0.7), respectively. Differences existed between decomposition of roots and leaves for graminoids and snowberry. On a monthly basis decomposition was 0.6 to 0.8 % greater in leaves than roots. The decomposition of roots and leaves ranged from 2.2 to 5.0 % month-1. Decay rate constants for leaves ranged from 0.45 yr-1 (SE= 0.03) to 0.71 yr-1 (SE= 0.02) while those of roots ranged from 0.34 yr-1 (SE= 0.03) to 0.47 yr-1 (SE= 0.04). The decomposition of roots and leaves did not correspond with macroclimatic or regional climate data nor with initial C:N content of the plant material. In summary, invasion of snowberry into grassland does not appear to conflict with goals related to maintenance of SOC in Mixed Prairie. Current grazing management regimes also appear to be consistent with goals related to maintenance of existing SOC. Soil texture had a greater effect on SOC than management of the plant community. Decomposition of leaves and roots appeared to be controlled by many interacting factors such as plant organ type, collection year, study year (climate) and physical and/or chemical characteristics of the site.

Accumulation of CO2 in the atmosphere has triggered research on topics related to causes, effects, and solutions to potential problems associated with global warming. The present research was conducted to determine if grassland plant communities can be managed to promote sequestration of carbon in the soil, potentially mitigating the effects of increasing atmospheric CO2. The effects of shrub invasion or heavy livestock grazing on peak standing crop of phytomass, root mass and soil organic carbon content were therefore studied. These studies were complimented by a study of the decomposition rates of leaves and roots of snowberry and grasses. The effects of snowberry encroachment on peak standing crop of aboveground phytomass, and soil organic carbon content (SOC) were also studied. Total aboveground phytomass in the snowberry community was more than triple that of the ecotone and was 6-times greater than that of the grassland community. Similarly, the mass of large roots was greatest in the snowberry community (1.2 kg m-2, SE= 0.19), intermediate in the ecotone (0.5 kg m-2, SE= 0.08), and least in the grassland (0.1 kg m-2, SE= 0.04). Conversely, the mass of fine and medium roots was not different (P>0.05) among the three communities, averaging 0.7 kg m-2 in all communities (SE= 0.03, 0.07, 0.49 in snowberry, ecotone and grassland, respectively). Greater aboveground phytomass did not correspond with greater SOC in the snowberry community. Soil organic carbon in the upper 50 cm averaged 8.3 (SE= 0.7), 7.9 (SE= 1.0), and 7.9 (SE= 0.7) kg m-2 in snowberry, ecotone, and grassland communities, respectively. Peak standing crop of aboveground phytomass averaged 157 g m-2 (SE= 27) and 488 g m-2 (SE= 48) in grazed and ungrazed grassland, respectively. Conversely, grazing had no affect on root mass. The mass of fine roots averaged 0.9 kg m-2 (SE= 0.04) and 0.8 kg m-2 (SE= 0.06) in grazed and ungrazed grassland, respectively, while that of medium roots averaged 0.6 kg m-2 (SE= 0.07) in both grazing treatments. Total SOC in the upper 50 cm of soil was not affected (P>0.05) by livestock grazing, averaging 5.5 kg m-2 (SE= 0.7) in grazed and 6.8 kg m-2 (SE= 0.9) in ungrazed grassland. Livestock grazing also had no effect (P>0.05) on SOC at the 0-3, 3-10, 10-20, 20-30, and 30-40 cm depths. The SOC in fine- and coarse-textured soils averaged 7.6 kg m-2 (SE= 0.8) and 5.1 kg m-2 (SE=0.7), respectively. Differences existed between decomposition of roots and leaves for graminoids and snowberry. On a monthly basis decomposition was 0.6 to 0.8 % greater in leaves than roots. The decomposition of roots and leaves ranged from 2.2 to 5.0 % month-1. Decay rate constants for leaves ranged from 0.45 yr-1 (SE= 0.03) to 0.71 yr-1 (SE= 0.02) while those of roots ranged from 0.34 yr-1 (SE= 0.03) to 0.47 yr-1 (SE= 0.04). The decomposition of roots and leaves did not correspond with macroclimatic or regional climate data nor with initial C:N content of the plant material. In summary, invasion of snowberry into grassland does not appear to conflict with goals related to maintenance of SOC in Mixed Prairie. Current grazing management regimes also appear to be consistent with goals related to maintenance of existing SOC. Soil texture had a greater effect on SOC than management of the plant community. Decomposition of leaves and roots appeared to be controlled by many interacting factors such as plant organ type, collection year, study year (climate) and physical and/or chemical characteristics of the site.

No description available.

The establishment and intensive management of forests for the production of timber can have significant effects on the soil carbon dynamics. The establishment of forest on organic soils under grasslands may lead to substantial losses in soil carbon, due to the site preparation for the planting of trees and other disturbances. This is gradually compensated by carbon accumulation in tree biomass as the forest grows until clearfelling at maturity may initiate another substantial carbon loss. This study had two main aims. The first was to investigate the long-term effects of forest establishment on natural grassland as well as clearfelling and re-growth of the forest during second rotation, by looking at the changes in soil carbon stocks and soil carbon balance in a Sitka spruce (Picea sitchsensis) in Harwood (N. E. England). Secondly, to investigate the short-term effects of forest clearfelling on the fluxes of soil CO2, N2O and CH4 and on the environmental factors (soil temperature, water content and water table depth) affecting them. The fluxes were initially measured in two mature stands (40-years old) during one growth season. One of the two stands was subsequently clearfelled while the other was kept intact and fluxes were measured for a further ten months after clearfelling. The relationships between these fluxes and the environmental factors were also examined. The study also investigated the spatial variability of soil CO2 emissions using geostatistical approaches. The soil CO2 fluxes were measured with two methods, a closed dynamic chamber and a closed static chamber, giving the opportunity to compare their relative performance. A performance further investigation on this discrepancy between the two methods took place in lab experiments and on a soil monolith, excavated from the 40-year old stand and kept under controlled conditions in the greenhouse.

Soil is the largest terrestrial carbon (C) store, containing an estimated ~1500 Gt C in the upper 1 m of soil. The long term storage of soil organic C (SOC) requires that it is somehow protected from microbial decomposition – or ‘stabilised’ – in the soil matrix. Three mechanisms are commonly identified as factors controlling the stability of SOM: chemical recalcitrance, physical protection in aggregates and adsorption to soil mineral surfaces. The stability of SOC in the soil matrix can be influenced by management practices and changes in soil structure can lead to loss of SOC and increases in greenhouse gas (GHG) emissions. It is, therefore, important to understand the impact that management practices have on SOC stability and to manage soils in such a way as to optimise the volume of SOC which is locked away for climatically significant periods of time. Two methods are generally used to estimate SOC stability: indirectly by measuring CO2 fluxes as a proxy for SOC microbial decomposition, or directly through physical fractionation of soil in to pools with different levels of physical and chemical protection. Both methods were employed in this thesis. Arable and grassland soils which represent the range of soil textures and climatic conditions of the main agricultural areas in the UK were incubated at two different moisture contents and with or without inorganic fertiliser application and GHG fluxes from them were monitored. Soil texture, mineral N concentration and soil C concentration were found to be the most important measured variables controlling GHG fluxes of the UK agricultural soils in this study. The results were generally in support of those found in the literature for a wide range of soils, conditions and locations; however, N2O emissions from the two Scottish soils appeared to be more sensitive to inorganic N fertilisation at the higher moisture content than the other soils, with the N2O emissions being exceptionally high in comparison. Although incubations of whole soils are useful in measuring the impacts of soil management practices on GHG emissions under controlled conditions they do not identify the mechanisms controlling the stability of SOC. Dividing SOM into functional pools may identify different C stabilising mechanisms and improves soil C models. A large number of operationally defined separation methods have been used to fractionate SOM into biologically meaningful pools of different stability. Direct comparisons of different fractionation methods using radiocarbon (14C) dating and spectroscopic analyses has not previously been undertaken. Average 14C ages and chemical composition of SOM fractions isolated from a grassland soil using three published and frequently applied fractionation methods were compared. (1) a density separation technique isolating three fractions (2) a combined physical and chemical separation isolating five fractions (3) a hot-water extraction method isolating two fractions. The fractions from Method 1 had the most distinct average 14C ages, the fractions from Method 2 fell into two age groups, and both Method 3 fractions were dominated by modern C. The average 14C ages of the labile fractions from Method 1 and 2 were higher than the mineral bound fractions, although they made up a relatively small proportion of the total SOC. This was a surprising result, and spectroscopic analysis confirmed that these fractions had greater relative contents of aliphatic and aromatic characteristics than the mineral bound fractions. The presence of black C in a whole soil sample and one of the labile fractions from Method 2 was confirmed by hydrogen pyrolysis. The availability of archived soils from an abandoned long term tillage treatment experiment and the ability to relocate the plots provided a unique opportunity to assess the resilience of SOC stocks to land management practices several years after the conversion from arable to grassland. SOC stability was assessed by soil fractionation of archived (1975) and freshly collected (2014) soil samples. The mass corrected SOC stocks from the four different treatments (deep plough, shallow plough, chisel plough and direct drill) were higher in 2014 than 1975 across the whole profile (0 – 36 cm). Reductions were observed at some depths for some treatments but the overall effect was an evening out of SOC stocks across all plots. The fractionations (using Method 2), revealed that there was a relative increase in the mass of the sand and aggregate fraction but a decrease in the relative proportion of SOC stored in this fraction (physically protected). There was also a significant increase in the C:N ratio of the silt and clay fraction (chemical adsorption). This suggests that reduced disturbance of agricultural soils leads to preferential physical stabilisation of fresh SOM but also increased adsorption of older material to mineral surfaces. The labile fractions were sensitive to land-use change in all tillage treatment plots, but were more sensitive in the low impact tillage plots (chisel plough and direct drill) than the inversion tillage plots (deep plough and shallow plough). It is well established that tillage disrupts aggregation. However, a direct measurement of the level of SOM physical protection in the soil matrix due to aggregation has not previously been undertaken. The soil was fractionated using Method 1 (fractions with distinctly different 14C ages) and isolated soil fractions were incubated separately, recombined and mixed in to whole soil at three different temperatures. The C respiration rate of the isolated intra-aggregate fraction was generally consistently as high as the whole soil. This supports the theory that there is a labile component of soil which is protected from decomposition by physical protection within aggregates. Therefore, the lack of any priming effect with the addition of labile fractions to the whole soil, and indeed the suppression of emissions relative to the whole soil, was unusual. Fractions and whole soils incubated at 25 and 35 °C had a wider range of Q10 (temperature sensitivity) values than those incubated at 15 and 25 °C, however, median values were surprisingly similar (range from 0.7 to 1.9). Overall, the results from this thesis highlight the importance of the soil structure in stabilising C. Disrupting aggregates leaves a proportion of otherwise stable C susceptible to loss through microbial decomposition, particularly when the entire soil matrix is disrupted. It also provided some unexpected results which warrant future investigation; in particular, further direct measurement of physical stabilisation of SOM in soils of different type, from different climates and different land uses would be useful.

Recalcitrant soil carbon is a poorly understood component of total soil organic carbon (SOC). Although the turnover rate of the recalcitrant fraction is slow, warming temperatures are expected to speed the decomposition of recalcitrant SOC resulting in an increase of atmospheric CO₂ in the future. Several studies show that the oldest SOC is associated with the smallest mineral particles (clays), making direct spectroscopic analysis of old carbon difficult. To overcome the difficulty of analyzing natural samples, we created synthetic soils to examine the association between clay surfaces and specific biomolecules based on the hypothesis that clays with higher surface charge will more strongly bond organic molecules, and also that certain molecules will be better stabilized by clay. We used kaolinite, montmorillonite, or quartz (sand) as a synthetic soil inside 12 mL septum-capped vials, added either dissolved glucose or vanillic acid to each mineral, inoculated with soil microbes, and then purged the vials with a CO₂-free atmosphere. We incubated them and measured the concentration and [delta]¹³C of CO₂ that accumulated in the vials. Respiration rates were significantly higher in experiments containing vanillic acid than in those containing glucose. Respiration rates were lowest in experiments containing montmorillonite. We repeated the experiment using dilute H₂O₂ as an oxidant, and adding vanillic acid, glucose, or glycine. Vials with montmorillonite showed lower rates of CO₂ accumulation than kaolinite, and both glycine- and glucose-containing experiments had less CO₂ than vanillic acid-experiments. We conclude that the montmorillonite protected the organic matter from oxidation better than sand or kaolinite. Both clays protected organic matter better than sand. In all experiments with clay, the respired CO₂ had lower [delta]¹³C values than bulk substrate. This carbon isotope fractionation is likely due to preferential desorption, followed by oxidation, of 12C- as opposed to 13C- bearing organic molecules. The mineral-organic interaction is a strong bond that explains the old age of labile organic compounds in soils. These results indicate that the clay fraction of soils must be considered for accurate prediction of future land-atmosphere carbon fluxes. / text

Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The research was conducted in the Kwa-Zulu Natal midlands, South Africa. The vertical distribution of soil organic carbon (SOC) stocks were successfully predicted by stochastic exponential models developed for the three main land uses in the area, which are farmlands, forestry plantations and grasslands. These models, in combination with regular surface sampling, may be used for monitoring SOC dynamics in the area and mapping SOC stocks. Bulk density measurements are needed in combination with SOC content (%wt) to calculate such SOC stocks. Considering the disadvantages of bulk density sampling and measurement, an effort was made to determine if one of the commonly-used existing stochastic models could be used to successfully predict bulk densities for soils with known texture and SOC content to replace direct measurements, taking into account that different managements might affect final results. Statistica software was used to correlate the Saxton & Rawls model predictions and associated regressions with measured values for the study area. A clear distribution trend was achieved using Statistica and the correlations were fair with r2 values close to 0.5 for individual regressions and substantially higher for area averages. However, considering the depth-stratified averages and correcting for the effects of particle density changes for soils with high soil organic matter, high correlations for 2 of the 3 studied land uses were achieved (r2 values of 0.99 and 0.81 in forests and grasslands respectively). Therefore, although Saxton and Rawls (2006) predictions of bulk density may be used, it is preferable to conduct direct bulk density determinations. The proposed models to calculate the vertical distribution of SOC would substantially reduce the cost of soil carbon inventories to 1m soil depth in the study area by limiting observations to the soil surface. Triplicate 5cm-deep soil core samples would be collected at the soil surface per observation point for determination of ρb (bulk density) and Corg (organic carbon). On average, the accuracy of the normalized depth-distribution model is rather high for grasslands and forests/forest plantations (R2 = 0.98), but somewhat lower for cultivated lands (R2 = 0.96) due to mixing of the plough layer to cultivation depth. Carbon stocks to 1m depth were calculated as an integral of the normalized exponential distribution, multiplied by the value of Corg observed at the soil surface and expressed on volume basis as carbon density (Cv, kg∙m-3). The resulting stock assessment was compared to the observed values using piece-integration for sampled depth increments to give SOC stocks on an area basis (kg∙m-2). The estimated prediction error on average was 1.2 (9%) and 3.7 kg∙m-2 (21.6%) in grasslands and forests respectively, while for cultivated lands the error was 1.3 kg.m-2 (9.5%). Further improvement to reduce these errors may be achieved by introducing the soil type as variable and grouping the functions by soil type rather than land uses. The results of this work were presented at the seminar of the department of Soil Science, Stellenbosch University (Ros et al., 2014), the combined congress of the South African Soil Science, Horticulture and Agronomy societies (Rozanov et al., 2015), the First Global Soil Map conference, France (Wiese et al., 2013), the 20th International Congress of Soil Science, Korea (Wiese et al. 2014) and were submitted for publication in Geoderma special issue dedicated to digital soil mapping of soil organic carbon following the presentation at the 20th ICSS, Korea (Wiese et al., 2014). / AFRIKAANSE OPSOMMING: Hierdie navorsing is in die Kwa-Zulu Natalse middellande van Suid-Afrika gedoen. Die vertikale verspreiding van grondorganiese koolstof (GOK) is suksesvol voorspel deur middel van stogastiese eksponensiële modelle wat vir die drie hoof landsgebruike ontwikkel is. In kombinasie met roetine monsterneming by die grondoppervlak kan hierdie modelle suksesvol aangewend word vir die monitering van GOK dinamika in die studiegebied, sowel as kartering van GOK voorraad. Bulkdigtheidsmetings word tesame met GOK inhoud (%massa) benodig om die GOK voorraad te bereken. Weens die nadele van monsterneming vir bulkdigtheidsbepalings is ‘n poging aangewend om te bepaal of een van die mees algemeen gebruikte bestaande stogastiese modelle (Saxton & Rawls 2006) gebruik kan word om die bulkdigtheid van gronde suksesvol vanaf tekstuur en GOK inhoud te voorspel en sodoende direkte metings te vervang. Statistica sagteware is gebruik om die voorspellings met behulp van die Saxton & Rawls modelle en gevolglike regressies met gemete waardes vanuit die studiegebied te korreleer en ‘n duidelike verspreidingstendens is hierdeur opgelewer. Die korrelasies vir individuele regressies was redelik met r2 waardes naby 0.5 en merkwaardig hoër waardes vir area gemiddeldes. Hoë korrelasies is egter behaal vir 2 van die 3 bestudeerde landsgebruike (r2 waardes van 0.99 en 0.81 in bosbou en grasveld onderskeidelik) wanneer die gemiddelde dieptestratifikasies gebruik en gekorrigeer word vir die verandering in deeltjiedigtheid vir gronde met hoë grondorganiese material. Alhoewel die Saxton and Rawls (2006) voorspellings van bulkdigtheid gebruik kan word, behoort bulkdigtheidsbepalings egter verkieslik direk gedoen te word. Die voorgestelde modelle vir die bepaling van vertikale GOK verspreiding tot 1m gronddiepte sou die koste van grondkoolstof opnames in die studiegebied dramaties verlaag deur grondmetings tot die grondoppervlak te beperk. Grondmonsters sal in triplikaat per waarnemingspunt met 5cm diep silinders op die grondoppervlak geneem word vir ρb (bulkdigtheid) and Corg (organiese koolstof) bepalings. Die gemiddelde akkuraatheid van die genormaliseerde diepteverspreidingsmodel is hoog vir grasveld en woude/bosbou plantasies (R2 = 0.98), maar ietwat laer vir bewerkte landerye (R2 = 0.96) as gevolg van die vermenging van die ploeglaag tot op die diepte van bewerking. Koolstof voorraad tot 1m gronddiepte is bepaal deur middel van die integraal van die genormaliseerde eksponensiele verspreiding, vermenigvuldig met die waarde van Corg op die grondoppervlak en op ‘n volume basis uitgedruk as koolstofdigtheid (Cv, kg∙m-3). Die gevolglike voorraadopname is met gemete waardes vergelyk deur middel van ‘n stuksgewyse integrasie van die gemonsterde diepteinkremente om GOK voorraad per area (kg∙m-2) te lewer. Die gemiddelde geskatte fout van voorspelling was 1.2 (9%) en 3.7 kg∙m-2 (21.6%) in grasveld and plantasies onderskeidelik en 1.3 kg.m-2 (9.5%) in bewerkte landerye. Verdere verbetering van die modelle en ‘n verlaging in hierdie foute kan verkry word deur die grondtipe inligting as veranderlike in te bring en die funksies volgens grondtipe eerder as landsgebruik te groepeer. Resultate van hierdie werk is reeds aangebied tydens ‘n seminar by die department Grondkunde, Stellenbosch Universiteit (Ros Mesa et al., 2014), die gesamentlike kongres vir die Suid-Afrikaanse Verenigings vir Grondkunde, Hortologie, Onkruidwetenskap en Gewasproduksie (Rozanov et al. 2015), die Eerste Global Soil Map konferensie, Frankryk (Wiese et al, 2013), die 20ste Internasionale Grondkunde Kongres, Korea (Wiese et al. 2014) en is ingehandig vir publikasie in ‘n spesiale uitgawe van Geoderma wat, na aanleiding van die aanbieding by die 20ste Internasionale Grondkunde Kongres, Korea (Wiese et al., 2014), fokus op digitale grondkartering van grondorganiese koolstof.

The aim of this study was to investigate the effects of biochar amendment on soil greenhouse gas (GHG) emissions and to elucidate the mechanisms behind these effects. I investigated the suppression of soil carbon dioxide (CO2) and nitrous oxide (N2O) emissions in a bioenergy and arable crop soil, at a range of temperatures and with or without wetting/drying cycles. More detailed investigation on the underlying mechanisms focused on soil N2O emissions. I tested how biochar altered soil physico-chemical properties and the subsequent effects on soil N2O emissions. In addition, 15N pool dilution techniques were used to investigate the effect of biochar on soil N transformations. Biochar amendment significantly suppressed soil GHG emissions for two years within a bioenergy soil in the field and for several months in an arable soil. I hypothesised that soil CO2 emissions were suppressed under field conditions by a combination of mechanisms: biochar induced immobilisation of soil inorganic-N (BII), increased C-use efficiency, reduced C-mineralising enzyme activity and adsorption of CO2 to the biochar surface. Soil CO2 emissions were increased for two days following wetting soil due to the remobilisation of biochar-derived labile C within the soil. Soil N2O emissions were suppressed in laboratory incubations within several months of biochar addition due to increased soil aeration, BII or increased soil pH that reduced the soil N2O: N2 ratio; effects that varied depending on soil inorganic-N concentration and moisture content. These results are significant as they consistently demonstrate that fresh hardwood biochar has the potential to reduce soil GHG emissions over a period of up to two years in bioenergy crop soil, while simultaneously sequestering C within the soil. They also contribute greatly to understanding of the mechanisms underlying the effect of biochar addition on soil N transformations and N2O emissions within bioenergy and arable soils. This study supports the hypothesis that if scaled up, biochar amendment to soil may contribute to significant reductions in global GHG emissions, contributing to climate change mitigation. Further studies are needed to ensure that these conclusions can be extrapolated over the longer term to other field sites, using other types of biochar.

Biochar is a carbon sequestration technology that has shown potential to inhibit greenhouse gas (GHG) emission and nutrient leaching from soils, however the majority of biochar research thus far has focused on arable cropland rather than livestock systems or grasslands. Livestock production is an important agricultural system, and manure generated from livestock systems is a source of GHG emission as well as nutrient loading to surface- and groundwater. The high environmental impact of livestock production in the very areas that biochar has shown potential may suggest that this would be an ideal system for biochar incorporation. However, as grassland systems in the context of livestock production often receive high nutrient inputs in the form of manure which increases the potential for nutrient leaching or runoff, the high-nutrient ash content of biochar may potentially exacerbate this problem rather than suppress nutrient loss from soils. As private companies and government-funded programmes discuss the possibility of scaling the global manufacturing and soil-incorporation of biochar to a rate of gigatonnes per year, understanding the potential of biochar for use within a livestock system could be crucial in helping to develop an appropriate deployment plan for this material. This thesis is therefore focused on the use of biochar in grassland and livestock systems. It first examines the nutrient release from biochar in a sequential leaching experiment. Phosphorus (P) release indicated that provision of soil P (though quantitatively small) may be sustained over time whilst potassium (K+) release was quantitatively large but declined rapidly following the first extraction. An incubation study was then carried out using soil columns amended with farmyard manure, liquid manure (slurry) or fertiliser (plus an unamended control), each with and without biochar, which sought to determine the impact of biochar on N2O release and N and P leaching from soils with diverse nutrient sources. N2O emission from the columns was significantly suppressed by the presence of biochar, as was the leaching of mineral N. However, the amount of PO4 3--P in leachate was increased in biocharamended columns, relative to their unamended counter-parts. A slurry incubation study was then conducted, with a control slurry and four biochar-amended treatments, which explored whether biochar could suppress GHG and NH3 emission from manure prior to land application. The resulting data indicated that biochar demonstrates potential for GHG suppression but does not demonstrate potential for NH3 suppression from slurry in storage. Finally, a one-year field-based experiment was completed which analysed the impact of biochar on CH4, N2O, and NH3 emission as well as nutrient leaching from grassland soils that had been amended with a high rate of manure application (151.4 m3 ha-1 or 409 kg N ha-1). In this study, biochar demonstrated the potential to suppress each of the three types of gaseous emissions from manure-amended soil, though the differences between mean values were not statistically significant. Extracts from ion exchange resins indicated that annual cumulative K+ leached from biochar-amended plots was significantly higher than the control, and that P and NH4 + leached from biochar-amended plots was higher than the control at the time of the first rain event following biochar and manure application. Together, the results of these component studies indicate that biochar may indeed have potential to suppress GHG emissions from livestock systems, most likely through suppression of microbial activity by organic compounds that are sorbed to the char, though (as the mechanisms of GHG suppression by biochar are thus far not well understood) the capacity of biochar to do so may vary based on the type of biochar used, the soil characteristics, and other factors. Overall, the results of these studies suggest that some types of biochar should be used with caution in systems with high rates of nutrient application, unless the ash is removed prior to soil application.

Soil carbon storage in tropical ecosystems is important in the global carbon cycle, yet consensus is lacking on how soil organic carbon stocks are altered under anthropogenic land-use change. This thesis seeks to quantify soil carbon stocks, the associated soil carbon emissions and explores the drivers of soil respiration in managed tropical Andean lands over a 2600 m elevation gradient. It investigates: grazing and burning on high altitude montane grasslands, burning in montane forests and agriculture in premontane forests. Changes among land-uses were quantified using belowground carbon stocks, the carbon distribution among density fractions, soil carbon emissions and environmental drivers of soil respiration. Soil respiration was a good proxy of soil carbon loss in premontane pastures and montane grassland soils. The total carbon stocks on some land-uses appeared to be unaffected but the distribution of carbon within the soil had changed and even when there were no net changes in soil carbon emissions, the drivers of respiration were different. The synergistic effect of burning and grazing in montane grasslands was the most detrimental to soil carbon stocks, whereas montane forests were unaffected. In the premontane elevation, soil carbon loss was dependent on the type of agricultural practice but the succession of secondary forest allowed soil carbon to recover to similar levels measured in the mature forest. These findings highlight the fact that although land-use does not always appear to have an obvious effect on total soil carbon stocks, the loss of carbon from short-term labile pools can cause higher carbon emissions and dominate soil-atmospheric feedbacks. Furthermore, the drivers of soil respiration and the synergistic relationship between soil moisture and temperature alter under different land uses. These factors should be taken into consideration with regards to predictions of regional temperature/precipitation climate change and soil carbon management policy in order to arrive at more realistic decisions.