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Soil erosion by water and tillage are major drivers for soil degradation in the mountainous regions of NW Vietnam. Data on cumulative and recent erosion of bulk soil and soil organic carbon (SOC) are needed for carbon budgeting and to evaluate e.g. the impact of erosion on climate change, environmental services and subsequent socioeconomic consequences. Thus, the aims of the present study were (1) to quantify cumulative erosion and recent erosion rates of bulk soil (2) to quantify cumulative eroded SOC and (3) to estimate the proportions of water and tillage induced soil erosion after land use change from primary forest to continuous maize cultivation for up to 21 years. Soil erosion rates were determined by 137Cs and ranged from 12 to 89 t ha- 1 a- 1. A large part of the variation of cumulative bulk soil (R² = 0.79) and SOC loss (R² = 0.67) between the sites was simply but effectively explained by the ratio of site specific cumulative RUSLE LS factors to clay contents. The newly developed CIDE approach, which attributes SOC changes on steep slopes to either carbon input, decomposition or erosion, delivered 20% higher (up to 0.7 kg m- 2) and more reliable estimates on cumulative SOC erosion than a traditional approach, because CIDE considered the effects of soil profile truncation, decomposition and humification, which all affected SOC simultaneously to soil erosion. Tillage induced soil flux accounted for 38 ± 3 kg m- 1 per tillage pass, which was lower than found in similar studies. Soil erosion by water was higher than tillage induced erosion in middle and foot slope positions, accounting for 86 to 89% of total soil erosion. To prevent further soil degradation, erosion protection measures should be implemented. [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]