Land use and topographic position control soil organic C and N accumulation in eroded hilly watershed of the Loess Plateau

Land use and topography strongly influence soil organic C (SOC) and N accumulation in eroded hilly regions. However, their combined effects and the underlying mechanism remain unclear. In this study, five land uses and three topographic positions across an eroded hilly watershed of the Loess Plateau were selected to investigate their effects on SOC and N accumulation. The restored grassland, shrubland and woodland (25 to 30 years) increased SOC and total N by 32% to 119% in the slope and 17% to 81% in the gully, respectively compared with the cropland. These restored vegetation increased soil dissolved organic C (DOC) and microbial biomass C and N (MBC and MBN) by 1.1- to 3.0-fold in the slope and 30% to 108% in the gully, respectively. Similar increases were observed in soil aggregates, MBN/total N, soil C:N and microbial C:N ratios. These improvements were higher in the shrubland than in the other land uses. The SOC significantly decreased from the gully to the slope, and the magnitudes decreased in a sequence of cropland, grassland, shrubland and woodland. Soil DOC, total N, and MBN also decreased from the gully to the slope in the cropland, but remained unchanged in the grassland, shrubland, and woodland. Land use (P < 0.05), topographic position (P < 0.05), and their interaction (P < 0.1) influenced SOC, total N, DOC, MBC, MBN, soil C:N and microbial C:N ratios. There were close relationships among soil aggregates, SOC and total N, and DOC, MBC and MBN. Therefore, land use, topographic position (erosion and deposition), and their interactions regulate SOC and N accumulation and their labile fractions in the eroded hilly region at a watershed scale. Our results suggested that converting cropland to shrubland is an initial strategy to restore degraded ecosystems and increase soil C sequestration in eroded hilly region of the Loess Plateau.