Influence of past soil erosion and burial on the reactivity of deep soil carbon
Abstract
The stability of deep soil organic carbon (SOC) pools, down to 2 meters, is typically assumed to be controlled by microbial accessibility, through mineral or aggregate protection, or mediated by nutrient and oxidant availability, through water table fluctuation, deep root penetration, or bioturbation. Land management also plays a critical role, and quantifying the stocks and reactivity of SOC buried by past agricultural practices is difficult but necessary, as it can represent a significant proportion of some watersheds' SOC stocks. This is especially important when erosion and deposition is a net sink or source for atmospheric C. Past erosion and burial are rarely studied as primary controls on the reactivity of deep SOC, despite their potential impacts on SOC composition. Floodplains within the Intensively Managed Landscapes Critical Zone Observatory sites in Clear Creek Iowa exhibit up to one meter of post settlement alluvium (PSA) with eroded and buried soil overlying floodplain surface soil profiles. We assessed if PSA held distinct reactive and chemical properties from underlying SOC. Transects from toeslope to stream edge exhibit profiles with greater stocks of SOC from 40-80 cm than the 0-40 cm PSA. Reactivity measures such as basal respiration, denitrification potential, and thermal stability and energetics all showed that mobilization imparts significant reactivity and chemical changes. For the same SOC concentration PSA exhibits greater (> 50%) denitrification potential, reduced energy density (mJ/mg C) and temperature ranges in thermal stability, and differences in yield of pyrophosphate extractable Fe, Al and organic carbon. Comparisons of d13C values of water extractable OC and respired CO2 among depth increments indicate differing microbial accessibility to water extractable and tightly- bound fractions that may be controlled by past disturbance. Additionally, 14C profiles indicate that mobilized profiles, once presumably contemporaneous with adjacent and/or deeper buried material, are heavily influenced by modern C inputs through dynamic replacement. Thus, erosion and burial can potentially impart long lasting changes to chemistry and accessibility in buried carbon, significantly influencing SOC reactivity, and possibly sequestration under future climate and land use changes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.B41D..02F
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCESDE: 0428 Carbon cycling;
- BIOGEOSCIENCESDE: 0460 Marine systems;
- BIOGEOSCIENCESDE: 0486 Soils/pedology;
- BIOGEOSCIENCES