Influence of cultivation and fertilization on total organic carbon and carbon fractions in soils from the Loess Plateau of China

To evaluate the degradation of soil quality and find ways to maintain soil fertility on the Loess Plateau of China, the effects of cultivation time on total organic carbon (TOC), light fraction of organic carbon (LFOC), and microbial biomass carbon (MB-C) in two soil chronosequences comprised of Huangmian (Calcaric Cambisols, FAO) and Huihe (Haplic Greyxems, FAO) soils were investigated. The effects of fertilization on the TOC and its fractions were also studied using samples from a long-term experiment on Heilu soil (Calcic Kastanozems, FAO). Upon cultivation, Huangmian soil (0–20 cm) lost 77% of TOC within 5 years, at a reduction rate of 2.15 Mg C ha−1 per year. The Huihe soil (0–20 cm) lost 70% of TOC at a rate of 0.96–1.06 Mg C ha−1 per year over 42 years. In the Huangmian soil, water and tillage erosion are likely the main reasons for organic carbon decline, while organic matter decomposition and water erosion appear to be dominant factors in the Huihe soil. The LFOC decreased by 73 and 90% for the Huangmian and Huihe soil for the corresponding period. Changes in microbial biomass carbon (MB-C) showed the same trend as TOC and LFOC. The results of the long-term experiment on the Heilu soil indicated that manure alone and manure plus nitrogen and phosphorus fertilizer treatments restored TOC and MB-C to the level of the native sod, indicating the importance of manure addition in maintaining soil fertility over the long term (20 years). The straw return plus nitrogen and phosphorus fertilizer treatment had a significantly higher TOC than nitrogen plus phosphorus fertilizer alone. Organic matter additions in the form of manure or straw, either alone or in combination with chemical fertilizers, appears to be more effective in maintaining or restoring organic matter in Heilu soil on the Loess Plateau than chemical fertilizer alone.