Impacts of Soil Redistribution Processes on Soil Organic Carbon Stocks and Fluxes in a Small Agricultural Catchment

Soils play a major role in the global carbon cycle and have a huge potential for either sequestering or releasing carbon (C) to the atmosphere. Globally, large amounts of soil organic carbon (SOC) are laterally redistributed on sloped arable land by soil erosion. Besides the lateral SOC fluxes, soil erosion also indirectly alters the vertical C fluxes between terrestrial and aquatic ecosystems and the atmosphere. Whether this results in a net source or sink of atmospheric CO2 is unclear. Global estimates range from a source of ~1 Pg C per year to a sink of the same magnitude. Against this background, this study investigates impacts of soil redistribution processes on SOC stocks and fluxes and the corresponding C source or sink function in a small agricultural catchment (4.2 ha) in Germany. Spatial patterns of SOC stocks were studied by analysis of soil samples of the plough layer and two subsoil layers (up to a depth of 0.9 m) taken in a 17 x 17 m raster. Results revealed a substantial accumulation and stabilisation of SOC in the subsoil at depositional sites, stressing the importance of subsoil C for budgets on agricultural land prone to erosion. In situ measurements of soil respiration were carried out in the most dynamic area with respect to soil redistribution in three consecutive growing periods (2007-2009). No universal relation to modelled patterns of soil redistribution or to other parameters (soil properties, plant parameters, and terrain attributes) was found for the three measurement periods, underlining the large variability of soil respiration. However in two years, heterotrophic soil respiration was significantly linearly related to total erosion (including water and tillage erosion). Hence, a possible C sink at erosion sites might partly be compensated by enhanced mineralisation at depositional sites. Besides bulk SOC, depth distributions of particle-size SOC fractions at erosional, depositional, and reference (without erosion or deposition) sites were compared to improve the understanding of the involved processes. Results showed that the labile SOC pool was depleted in the topsoil of eroding profiles, indicating that the reduced decomposition not only resulted from a general depletion of SOC but also from a relative higher abundance of more passive SOC pools. As the labile SOC pool was also depleted in the topsoil at the depositional profile, this fraction was either mineralised during transport or rapidly after deposition, or it was exported out of the test site. On the one hand, this supports an enhanced mineralisation at depositional sites, and on the other hand, this suggests an enrichment of the labile pool in the exported soil. For a spatially integrated analysis of the impact of soil redistribution on SOC dynamics at the catchment scale, the combined soil erosion and SOC model SPEROS-C was modified and applied at the test site. It was applied for the period from 1950 to 2007, covering first a period of conventional tillage followed by a period of conservation tillage. Measured SOC stocks in the three soil layers were satisfactorily reproduced, indicating a good process representation within the model. Deposition was successfully validated by two depth profiles of 14C measured by accelerator mass spectrometry. The erosion induced net vertical C flux showed substantial spatial variation within the test site. Depositional areas acted as a C source due to the mineralisation of buried C while erosional sites acted as a C sink due to (partial) dynamic C replacement. As erosion sites constituted two thirds of the catchment area, the erosion induced mean net vertical C flux was a sink for atmospheric CO2 at the test site. However, a substantial amount of C was lost laterally due to the export by water erosion. This resulted in an overall negative C balance for the test site. Land management had a profound effect on the lateral and vertical C fluxes. The change from conventional to conservation agriculture reduced the lateral C export and enhanced the vertical C fluxes, leading to a reduced negative C balance. Overall, this study substantially improves the knowledge about the impacts of soil redistribution on SOC stocks and fluxes at the small catchment scale and provides improved and new methods applicable in future research

Erosion-induced carbon redistribution, burial and mineralisation — Is the episodic nature of erosion processes important?

There is still an ongoing scientific discussion regarding the importance of erosion-induced lateral soil organic carbon (SOC) redistribution for the burial and/or mineralisation of carbon and the resulting long-term C balance at the catchment scale. Especially the effects of the event driven nature of water erosion and the potentially associated enrichment of SOC in sediment delivery are still unclear. In general, two processes lead to enrichment of SOC: (i) enrichment due to selective interrill erosion at erosion sites, and (ii) enrichment due to selective depletion at deposition sites. In this study, the conceptual soil erosion and SOC turnover model SPEROS-C was adapted to integrate these processes and applied in a small arable catchment (4.2ha) in Germany for a 57-year period. A total number of 901 model runs were performed with different realisations of frequency and magnitude of water erosion as well as realisations of enrichment and depletion ratios taken from literature and compared to a reference model run representing mean annual erosion without enrichment processes. In general, our modelling study indicates that ignoring temporal variability and enrichment processes may lead to a substantial misinterpretation of erosion-induced C fluxes. Especially the vertical C flux (difference between C inputs from plant assimilates and organic fertilizer and SOC mineralisation) at deposition sites strongly depends on the model parameterisation ranging from a maximum C source of -336gCm-2 to a maximum C sink of 44gCm-2. In combination with a substantially higher C export due to enrichment processes, the overall C balance of the catchment potentially turns into a maximum C source of -44gCm-2 at the end of the simulation period compared to a C source of -1gCm-2 for the reference run