Changes in soil organic carbon and nitrogen after 26 years of farmland management on the Loess Plateau of China

Soil carbon (C) and nitrogen (N) play a crucial role in determining the soil and environmental quality. In this study, we investigated the effects of 26 years (from 1984 to 2010) of farmland management on soil organic carbon (SOC) and soil N in abandoned, wheat (Triticum aestivum L.) non-fertilized, wheat fertilized (mineral fertilizer and organic manure) and alfalfa (Medicago Sativa L.) non-fertilized treatments in a semi-arid region of the Loess Plateau, China. Our results showed that SOC and soil total N contents in the 0–20 cm soil layer increased by 4.29 (24.4%) and 1.39 Mg/hm2 (100%), respectively, after the conversion of farmland to alfalfa land. Compared to the wheat non-fertilized treatment, SOC and soil total N contents in the 0–20 cm soil layer increased by 4.64 (26.4%) and 1.18 Mg/hm2 (85.5%), respectively, in the wheat fertilized treatment. In addition, we found that the extents of changes in SOC, soil total N and mineral N depended on soil depth were greater in the upper soil layer (0–30 cm) than in the deeper soil layer (30–100 cm) in the alfalfa land or fertilizer-applied wheat land. Fertilizer applied to winter wheat could increase the accumulation rates of SOC and soil total N. SOC concentration had a significant positive correlation with soil total N concentration. Therefore, this study suggested that farmland management, e.g. the conversion of farmland to alfalfa forage land and fertilizer application, could promote the sequestrations of C and N in soils in semi-arid regions.