Effects of Different Fertilization and Fallowing Practices on Soil Carbon and Nitrogen Mineralization in a Dryland Soil with Low Organic Matter

Soil carbon (C) and nitrogen (N) mineralization plays an important role in crop growth and nutrition cycling. Based on a 10-year field experiment, an aerobic incubation experiment (105 days) was conducted to determine soil mineralized C and mineralized N at two depths (0–20 and 20–40 cm) under four cropping treatments (winter wheat with no fertilizer (Ctr), N fertilizer (N), phosphorus fertilizer (P), and N and P fertilizers (NP)) and two fallowing treatments (natural-F and bare-F). Results showed that compared with Ctr, N and NP significantly increased the wheat grain and straw yield. N, NP, and natural-F significantly increased the soil organic N concentration by 10.4%, 15.2%, and 22.0%, respectively, at 0–20-cm soil depth, and the natural-F significantly increased the soil organic C concentration by 16.8%. Additionally, bare-F led to lower soil organic N at the two soil depths and soil organic C at 0–20-cm depth when compared with NP and natural-F. The highest net mineralized N and C were observed in soils with the highest organic N and C contents (i.e., natural-F). Meanwhile, the lowest net mineralized N and C were observed in the treatments with the lowest soil organic N and C contents (i.e., bare-F). Therefore, in the Loess Plateau, the management of NP application is critical for increasing grain yield, maintaining soil C and N levels and supplying adequate N to the plants. By contrast, the application of N alone, P alone, or bare fallow is not a useful method for maintaining soil fertility.