Carbon mineralization and lignin content of eroded sediments from a grazed watershed of South-Africa

Despite the increasing interest in the role of soil erosion on the global carbon cycle, few studies were carried out to evaluate the fate of eroded organic carbon (OC) at the landscape level. Although eroded C may constitute an atmospheric C source upon mineralization or a C sink if entrapped, it is largely unknown how landscape position and molecular composition influence the fate of terrestrial C mobilized during erosion-induced storm events. The objective of this study was to assess the impact of soil water erosion on potential carbon mineralization and to interpret these results using additional information on organic matter composition in both soils and sediments. The study was conducted in a 23 ha grassland catchment of the South-African Drakensberg foothill showing numerous features of land degradation. The elemental and stable isotope composition and the lignin component of soils from different landscape positions were compared to those of sediments eroded from 1 × 1 m microplots. In addition, and to assess changes in OC availability during soil erosion, CO2 emissions from soil and eroded sediments were measured continuously under laboratory conditions over a 20 day period. The lignin component of the samples was characterized by CuO oxidation. Our results showed that eroded sediments were enriched in organic carbon by factor 1.8 and in lignin by factor 3.7. Lignin characterization indicated that it was less decomposed in eroded sediments than in the original soils, but did not control carbon mineralization. A negative trend was observed between the C/N ratio of eroded sediments and soil and CO2 production, suggesting that microbial biomass could be N limited. Carbon mineralization in eroded sediments greatly exceeded that of soils and was described by a single pool first order model, whereas mineralization of soil C followed a sigmoidal pattern. We conclude that organic matter present in eroded sediments consists of relatively undecomposed plant material and is mineralized to a greater extent than soil organic matter regardless of landscape position.

► Mineralisation potential of detached carbon at various landscape positions.
► Different models to explain carbon mineralisation of soils and eroded sediments.
► Molecular characterisation of lignin contribution to eroded sediments.