Applicability of biochar for limiting interrill erosion and organic carbon export of sloping cropland in a semi-arid area of China

As a recalcitrant carbon source, the residence time of biochar is estimated to be hundreds to thousands of years. Since biochar is now being applied as a soil amendment in many areas of the world, the effects of long-term stationary biochar and its applicability for limiting soil erosion and nutrient export must be considered. To this end, this study investigated the effects of biochar on interrill erosion processes and total organic carbon (TOC) export by sediment and runoff at five slope gradient levels (5°, 10°, 15°, 20°, and 25°). A silt loam soil was incubated with or without wheat straw-derived biochar for 3 years in the field before carrying out a simulated rainfall experiment, in which a rainfall intensity of 60 mm h–1 was applied for 90 min. Compared with the non-biochar control, biochar-amended soils showed decreased interrill erosion potential by up to 21.34%, due to improved soil aggregate stability and organic carbon (OC) content. No substantial differences in runoff rate were detected between the control and biochar treatments. Light soil particles with a higher OC content were transported preferentially on gentle sloping cropland (≤15°) and there was a negative correlation between the enrichment ratio of OC and sediment loss rate. The OC in sediment was the primary source of TOC export from sloping cropland, while a limiting effect of biochar for reducing this TOC export occurred between 15° and 20° slope gradients. Biochar amendment reduced the relative TOC export and protected sloping cropland from soil degradation. Although the biochar application enhanced soil erosion resistance, it was deemed more appropriate to use biochar as a soil amendment to ≤15°-sloping cropland given its pronounced effect on TOC export.