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The assessment of soil organic carbon ( SOC) loss and input after land-use change is based largely on comparisons between disturbed sites and an undisturbed reference site. At sites affected by soil erosion this comparison neglects the interactions between soil erosion, decomposition and SOC input. Our aim was to develop a stable carbon isotope-based method ( CIDE; carbon input, decomposition and erosion) to characterize SOC dynamics on sites affected by soil erosion. Using the CIDE approach, rates of (i) SOC loss by erosion, (ii) decomposition of forest-derived SOC and (iii) maize-derived net SOC input were determined. The approach was tested on a steep slope maize cultivation chronosequence of up to 20 years following deforestation of primary forest. An uncertainty analysis showed that even small erosion rates (0.3 kg soil m−2 year−1) underestimated SOC loss (sum of erosion and decomposition) and overestimated real SOC input using unadjusted measurement data. The misrepresentation was particularly large under conditions of severe erosion, small changes in SOC concentration with depth, large changes in δ13C with depth, and small changes in SOC stocks or δ13C with time. Twenty years after land-use change CIDE-derived SOC loss was 89% and 52% larger than the unadjusted SOC loss estimates based on the RUSLE (revised universal soil loss equation) erosion model and the RothC carbon turnover model, respectively. The unadjusted maize-derived SOC input was 36% and 14% larger when derived with the RUSLE and RothC model, respectively, 20 years after land-use change. This study demonstrates the care that must be taken when assessing SOC dynamics on erosion prone sites and provides a framework to address these challenges. [ABSTRACT FROM AUTHOR]

Abstract: Soil organic carbon decline after land use change from forest to maize usually leads to soil degradation and elevated CO2 emissions. However, limited knowledge is available on the interactions between rates of SOC change and soil erosion and how SOC dynamics vary with soil depth and clay contents. The 13C isotope based CIDE approach (Carbon Input, Decomposition and Erosion) was developed to determine SOC dynamics on erosion prone slopes. The aims of the present study were: (1) to test the applicability of the CIDE approach to determine rates of decomposition and SOC input under particular considerations of concurrent erosion events on three soil types (Alisol, Luvisol, Vertisol), (2) to adapt the CIDE approach to deeper soil layers (10–20 and 20–30 cm) and (3) to determine the variation of decomposition and SOC input with soil depth and soil texture. SOC dynamics were determined for bulk soil and physically separated SOC fractions along three chronosequences after land use change from forest to maize (up to 21 years). Consideration of the effects of soil erosion on SOC dynamics by the CIDE approach yielded a higher total SOC loss (6–32%), a lower decomposition (13–40%) and a lower SOC input (14–31%) relative to the values derived from a commonly applied 13C isotope based mass balance approach. Comparison of decomposition between depth layers revealed that tillage accelerated decomposition in the plough layer (0–10 cm), accounting for 3–34% of total decomposition. With increasing clay contents SOC input and mass of sand sized stable aggregates increased. In addition, decomposition increased with increasing clay contents, too, being attributed to decomposition of labile SOC which was attached to clay particles in the sand sized stable aggregate fraction. In conclusion, this study suggests that in situ SOC dynamics on erosion prone slopes are commonly misrepresented by erosion unadjusted approaches leading to an overestimation of SOC input and underestimation of SOC loss. [Copyright &y& Elsevier]