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.