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

Diagnosis of river basins as CO2 sources or sinks subject to sediment movement

Sediment movement during erosion, transport and deposition greatly affects the ecosystem of river basins. However, there is presently no consensus as to whether particular river basins act as carbon dioxide (CO2) sources or sinks related to these processes. This paper introduces a rule-of-thumb coordinate system based on sediment delivery ratio (SDR) and soil humin content (SHC) in order to evaluate the net effect of soil erosion, sediment transport and deposition on CO2 flux in river basins. The SDRSHC system delineates CO2 source and sink areas, and further divides the sink into strong and weak areas according to the world-average line. The Yellow River Basin, most severely suffering soil erosion in the world, only appears to be a weak erosion-induced CO2 sink in this system. The average annual CO2 sequestration is similar to 0.235 Mt from 1960 to 2008, a relatively small value considering its 3.1% contribution to the World's sediment discharge. The temporal analysis shows that the Yellow River Basin was once a source in the 1960s, but changed its role to become a weak sink in the past 40?years due to both anthropogenic and climatic influences. The spatial analysis identifies the middle sub-basin as the main source region, and the lower as the main sink. For comparison, sediment-movement-related CO2 fluxes of eight other major basins in four continents are examined. It is found that the six basins considered in the Northern Hemisphere appear to be sinks, while the other two in the Southern Hemisphere act as sources. Copyright (C) 2012 John Wiley & Sons, Ltd.http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000309446900005&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=8e1609b174ce4e31116a60747a720701Geography, PhysicalGeosciences, MultidisciplinarySCI(E)EI2ARTICLE131398-14063

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

Model based analysis of lateral and vertical soil carbon fluxes induced by soil redistribution processes in a small agricultural catchment

Soil redistribution on arable land significantly affects lateral and vertical soil carbon (C) fluxes (caused by C formation and mineralization) and soil organic carbon (SOC) stocks. Whether this serves as a (C) sink or source to the atmosphere is a controversial issue. In this study, the SPEROS-C model was modified to analyse erosion induced lateral and vertical soil C fluxes and their effects upon SOC stocks in a small agricultural catchment (4·2ha). The model was applied for the period between 1950 and 2007 covering 30years of conventional tillage (1950-1979) followed by 28years of conservation tillage (1980-2007). In general, modelled and measured SOC stocks are in good agreement for three observed soil layers. The overall balance (1950-2007) of erosion induced lateral and vertical C fluxes results in a C loss of -4·4gCm -2 a -1 at our test site. Land management has a significant impact on the erosion induced C fluxes, leading to a predominance of lateral C export under conventional and of vertical C exchange between soil and atmosphere under conservation agriculture. Overall, the application of the soil conservation practices, with enhanced C inputs by cover crops and decreased erosion, significantly reduced the modelled erosion induced C loss of the test site. Increasing C inputs alone, without a reduction of erosion rates, did not result in a reduction of erosion induced C losses. Moreover, our results show that the potential erosion induced C loss is very sensitive to the representation of erosion rates (long-term steady state versus event driven). A first estimate suggests that C losses are very sensitive to magnitude and frequency of erosion events. If long-term averages are dominated by large magnitude events modelled erosion induced C losses in the catchment were significantly reduced. © 2011 John Wiley & Sons, Ltd