Vegetation and microbial determinants of soil carbon isotopic composition

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dc.contributor.advisorCramer, Michael Den_ZA
dc.contributor.authorNel, Jacques Aen_ZA
dc.date.accessioned2018-05-14T12:30:52Z
dc.date.available2018-05-14T12:30:52Z
dc.date.issued2018en_ZA
dc.description.abstractTypically, soil δ¹³C and δ¹⁵N values tend to increase with depth across a wide range of ecosystems. Changes in δ¹³C with depth have been attributed to vegetation changes (i.e. C₃/C₄ shifts), but the similarity in δ¹³C and δ¹⁵N profiles suggest that microbial decomposition may play an important role. The determinants of soil δ¹³C and δ¹⁵N, however, are complex and microbial decomposition and vegetation shifts are not the only mechanisms that drive the fractionation of the isotopes with depth. We explored the utility of using δ¹³C as a proxy for vegetation change by considering alternative mechanisms for the changes in soil δ¹³C with depth. These alternate mechanisms may weaken the interpretation of soil δ¹³C as an indicator of vegetation change if the measured δ¹³C changes are small. We hypothesized that: (1) if soil-related processes such as mineralization and dark CO₂-fixation by microbes and roots contribute significantly to the δ¹³C signature of bulk soil at depth, one cannot simply determine whether the δ¹³C value of the soil at depth is indicative of a past vegetation assemblies (i.e. C₃/C₄ transitions); (2) changes in soil δ¹³C and δ¹⁵N values are linked through common microbially mediated decomposition-related processes; (3) anaplerotic CO₂ fixation by microbes and roots may contribute significantly to soil δ¹³C values, while N₂-fixation may contribute to soil δ¹⁵N values with depth. Microbial processing of SOM during decomposition leads to ¹³C-enrichement of SOM with depth and has been modelled using a Rayleigh distillation process. Anaplerotic fixation of soil CO₂ is, however, known to occur in microbes and roots and we suggest that this has a role in determining soil SOM δ¹³C values through cumulative incorporation of bulk atmosphere CO₂ into SOM. These processes vary greatly between soils and environments. The correspondence between soil δ¹³C and δ¹⁵N was assessed by compiling data from soil depth profiles from widely distributed sites and conducting an analysis of global δ¹³C and δ¹⁵N variations in surface soils in order to determine relationships between soil isotopes and with climate and soil properties. Strong positive correlations between δ¹³C and δ¹⁵N values through soil profiles were found at a number of sites and were found to be independent of vegetation type. Globally, soil δ¹³C and δ¹⁵N values were also found to be significantly positively correlated across a wide range of climates and biomes. The global correspondences between δ¹³C and δ¹⁵N values may suggest a mechanistic link between δ¹³C and δ¹⁵N through the process of SOM decomposition and microbial processing. Anaplerotic CO₂ fixation by soil microbes and roots was assessed using soils from 10 sites across South Africa differing in soil properties and incubated in the dark for 3 d under continuous exposure to ¹³CO₂- and ¹⁵N₂-enriched atmospheres with varying soil moisture (10, 50 and 100% of field capacity) and temperature (4, 25, 40°C). There was no evidence of significant N₂ fixation in any treatment. Significant soil anaplerotic CO₂ fixation, however, occurred in all soils. Highest rates of anaplerotic CO₂ fixation occurred in soils at 50% field capacity and 25°C, suggesting a link with microbial biotic activity. Soils with low C and N concentrations and low C:N ratios exhibited the highest rates of CO₂ fixation in soils, indicating a link between anaplerotic CO₂ fixation rates and soil nutrient status. The higher rates of CO₂ fixation in soils with low nutrients may indicate that soil microbes rely increasingly on anaplerotic fixation as SOM-N declines, forcing greater reliance on de novo amino acid synthesis, and thus anaplerotic CO₂ fixation. The ubiquitous occurrence of anaplerotic ¹³CO₂ fixation in these soils indicates that anaplerotic fixation is likely important in contributing to determining soil δ¹³C values. Diffusion of low δ¹³C bulk atmospheric CO₂ (ca. -10‰) into the soil atmosphere (<< -10‰) will drive soil CO₂ δ¹³C towards ca. -10‰, and constant anaplerotic CO₂ fixation will result in SOM δ¹³C also tending towards 10‰ in more highly processed SOM deeper in the soil. The consequences of decomposition and the linked anaplerotic activity for soil δ¹³C values may be erroneously interpreted as evidence for C₄ vegetation being invaded by C₃ vegetation, potentially leading to incorrect conservation decisions. We argue that δ¹³C should only be used as a proxy for vegetation change where decomposition rates and anaplerotic CO₂ fixation are low and/or their effect on soil δ¹³C values can be accounted for.en_ZA
dc.identifier.apacitationNel, J. A. (2018). <i>Vegetation and microbial determinants of soil carbon isotopic composition</i>. (Thesis). University of Cape Town ,Faculty of Science ,Department of Biological Sciences. Retrieved from http://hdl.handle.net/11427/28072en_ZA
dc.identifier.chicagocitationNel, Jacques A. <i>"Vegetation and microbial determinants of soil carbon isotopic composition."</i> Thesis., University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2018. http://hdl.handle.net/11427/28072en_ZA
dc.identifier.citationNel, J. 2018. Vegetation and microbial determinants of soil carbon isotopic composition. University of Cape Town.en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Nel, Jacques A AB - Typically, soil δ¹³C and δ¹⁵N values tend to increase with depth across a wide range of ecosystems. Changes in δ¹³C with depth have been attributed to vegetation changes (i.e. C₃/C₄ shifts), but the similarity in δ¹³C and δ¹⁵N profiles suggest that microbial decomposition may play an important role. The determinants of soil δ¹³C and δ¹⁵N, however, are complex and microbial decomposition and vegetation shifts are not the only mechanisms that drive the fractionation of the isotopes with depth. We explored the utility of using δ¹³C as a proxy for vegetation change by considering alternative mechanisms for the changes in soil δ¹³C with depth. These alternate mechanisms may weaken the interpretation of soil δ¹³C as an indicator of vegetation change if the measured δ¹³C changes are small. We hypothesized that: (1) if soil-related processes such as mineralization and dark CO₂-fixation by microbes and roots contribute significantly to the δ¹³C signature of bulk soil at depth, one cannot simply determine whether the δ¹³C value of the soil at depth is indicative of a past vegetation assemblies (i.e. C₃/C₄ transitions); (2) changes in soil δ¹³C and δ¹⁵N values are linked through common microbially mediated decomposition-related processes; (3) anaplerotic CO₂ fixation by microbes and roots may contribute significantly to soil δ¹³C values, while N₂-fixation may contribute to soil δ¹⁵N values with depth. Microbial processing of SOM during decomposition leads to ¹³C-enrichement of SOM with depth and has been modelled using a Rayleigh distillation process. Anaplerotic fixation of soil CO₂ is, however, known to occur in microbes and roots and we suggest that this has a role in determining soil SOM δ¹³C values through cumulative incorporation of bulk atmosphere CO₂ into SOM. These processes vary greatly between soils and environments. The correspondence between soil δ¹³C and δ¹⁵N was assessed by compiling data from soil depth profiles from widely distributed sites and conducting an analysis of global δ¹³C and δ¹⁵N variations in surface soils in order to determine relationships between soil isotopes and with climate and soil properties. Strong positive correlations between δ¹³C and δ¹⁵N values through soil profiles were found at a number of sites and were found to be independent of vegetation type. Globally, soil δ¹³C and δ¹⁵N values were also found to be significantly positively correlated across a wide range of climates and biomes. The global correspondences between δ¹³C and δ¹⁵N values may suggest a mechanistic link between δ¹³C and δ¹⁵N through the process of SOM decomposition and microbial processing. Anaplerotic CO₂ fixation by soil microbes and roots was assessed using soils from 10 sites across South Africa differing in soil properties and incubated in the dark for 3 d under continuous exposure to ¹³CO₂- and ¹⁵N₂-enriched atmospheres with varying soil moisture (10, 50 and 100% of field capacity) and temperature (4, 25, 40°C). There was no evidence of significant N₂ fixation in any treatment. Significant soil anaplerotic CO₂ fixation, however, occurred in all soils. Highest rates of anaplerotic CO₂ fixation occurred in soils at 50% field capacity and 25°C, suggesting a link with microbial biotic activity. Soils with low C and N concentrations and low C:N ratios exhibited the highest rates of CO₂ fixation in soils, indicating a link between anaplerotic CO₂ fixation rates and soil nutrient status. The higher rates of CO₂ fixation in soils with low nutrients may indicate that soil microbes rely increasingly on anaplerotic fixation as SOM-N declines, forcing greater reliance on de novo amino acid synthesis, and thus anaplerotic CO₂ fixation. The ubiquitous occurrence of anaplerotic ¹³CO₂ fixation in these soils indicates that anaplerotic fixation is likely important in contributing to determining soil δ¹³C values. Diffusion of low δ¹³C bulk atmospheric CO₂ (ca. -10‰) into the soil atmosphere (<< -10‰) will drive soil CO₂ δ¹³C towards ca. -10‰, and constant anaplerotic CO₂ fixation will result in SOM δ¹³C also tending towards 10‰ in more highly processed SOM deeper in the soil. The consequences of decomposition and the linked anaplerotic activity for soil δ¹³C values may be erroneously interpreted as evidence for C₄ vegetation being invaded by C₃ vegetation, potentially leading to incorrect conservation decisions. We argue that δ¹³C should only be used as a proxy for vegetation change where decomposition rates and anaplerotic CO₂ fixation are low and/or their effect on soil δ¹³C values can be accounted for. DA - 2018 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 2018 T1 - Vegetation and microbial determinants of soil carbon isotopic composition TI - Vegetation and microbial determinants of soil carbon isotopic composition UR - http://hdl.handle.net/11427/28072 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/28072
dc.identifier.vancouvercitationNel JA. Vegetation and microbial determinants of soil carbon isotopic composition. [Thesis]. University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2018 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/28072en_ZA
dc.language.isoengen_ZA
dc.publisher.departmentDepartment of Biological Sciencesen_ZA
dc.publisher.facultyFaculty of Scienceen_ZA
dc.publisher.institutionUniversity of Cape Town
dc.subject.otherBiological Sciencesen_ZA
dc.titleVegetation and microbial determinants of soil carbon isotopic compositionen_ZA
dc.typeMaster Thesis
dc.type.qualificationlevelMasters
dc.type.qualificationnameMScen_ZA
uct.type.filetypeText
uct.type.filetypeImage
uct.type.publicationResearchen_ZA
uct.type.resourceThesisen_ZA
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