Effects of land use on soil total and light fraction organic, and microbial biomass C and N in a semi-arid ecosystem of northwest China

There is limited information about the influence of farming systems in soil organic carbon (C), total nitrogen (N) and microbial biomass in drought-prone environments. This study aimed at comparing responses of these soil parameters to alfalfa grassland, alfalfa-crop rotations (9-year alfalfa followed by 3-year annual crops), and conventional farming (long-term annual crops) in the semiarid Loess Plateau region of northwest China. It was found that the farming systems had little effect on soil organic C (SOC), but alfalfa-crop rotations decreased soil total N (STN) by 5.9–21.7% and thus increased the SOC/STN ratio from 7.85 in alfalfa grassland to 9.63 over a period of 3 years. By comparison, a SOC/STN ratio of 11 in conventional farming was largely due to its low STN. Light fraction organic C and N in alfalfa grassland were significantly lower than in conventional farming and alfalfa-crop rotation systems. Microbial biomass C (MBC) and MBC/SOC ratio did not differ between alfalfa grassland and alfalfa-crop rotation systems, but microbial biomass N (MBN) and MBN/STN ratio were increased in alfalfa grassland. Conventional farming had lowest MBC, MBN, MBC/SOC and MBN/STN ratios. Alfalfa-crop rotations increased moisture content in the dry layers of soil profile that often occur in alfalfa grassland. As alfalfa grassland had low SOC/STN ratio and alfalfa-crop rotations increased the ratio mainly through reducing STN, a long-term practice of these two farming systems like conventional farming may not be sustainable in the dry and fragile agro-ecosystem, with annual rainfall of about 300 mm, on the semiarid Loess Plateau of northwest China.