Stimulation of soil organic matter fractions by maize straw return and nitrogen fertilization in the Loess Plateau of Northwest China

Soil organic matter (SOM) fractions play a key role in sustaining soil productivity; however, these fractions are relatively low in agricultural soils due to crop residue removal and tillage practices. Straw return has been reported to improve SOM fractions; however, its effects on soils with different nitrogen status is lacking. Therefore, a two-year experiment was conducted on an existing five-year different nitrogen rates (0, 75, 150 and 375 kg Nha−1 yr−1) trial field to assess the effects of nitrogen fertilizer rate and straw return on SOM fractions at different soil depths. The experiment had four nitrogen rates: 0 (CK, control), 75 (low N, LN), 150 (medium N, MN), and 375 (high N, HN) kg Nha−1 yr−1 with and without straw return (S, 9 t ha−1). The results showed that straw return significantly stimulated the SOM fractions at both 0–20 and 20–40 cm soil depths. Compared with the control, treatment MN with straw increased carbon pool management index (28.52 %), dissolved organic carbon (45.89 %), microbial biomass carbon (66.11 %), readily organic carbon (27.07 %), humic acid carbon (21.69 %), and humin carbon (10.34 %) at 0–20 cm soil depth; however, these fractions were comparable to those of HN with straw. Total organic carbon and carbon pool management index showed a positive and significant correlation with labile SOM fractions in the soil. The application of 150 kg N ha−1 after straw return stimulates SOM fractions, which in turn maintains the fertility and productivity of loess soil.