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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]

In large areas of sub-Saharan Africa crop production must cope with low soil fertility. To increase soil fertility, the application of biochar (charred biomass) has been suggested. In urban areas, untreated waste water is widely used for irrigation because it is a nutrient-rich year-round water source. Uncertainty exists regarding the interactions between soil properties, biochar, waste water and fertilization over time. The aims of this study were to determine these interactions in two typical sandy, soil organic carbon (SOC) and nutrient depleted soils under urban vegetable production in Tamale (Ghana) and Ouagadougou (Burkina Faso) over two years. The addition of biochar at 2 kg m -2 made from rice husks and corn cobs initially doubled SOC stocks but SOC losses of 35% occurred thereafter. Both biochar types had no effect on soil pH, phosphorous availability and effective cation exchange capacity (CEC) but rice husk biochar retained nitrogen (N). Irrigation with domestic waste water increased soil pH and exchangeable sodium over time. Inorganic fertilization alone acidified soils, increased available phosphorous and decreased base saturation. Organic fertilization increased SOC, N and CEC. The results from both locations demonstrate that the effects of biochar and waste water were less pronounced than reported elsewhere.