Long-term application of Chinese milk vetch return altered the distribution and species of phosphorus associated with soil aggregates in double-rice cropping system

Abstract Purpose Chinese milk vetch (MV) as green manure is utilized to improve rice yield and paddy soil fertility in China. However, the long-term effects of MV on the distribution and species of phosphorus (P) in soil aggregates is poorly understood. Methods There were four treatments including with (1) no chemical fertilizer and no MV (CK); (2) chemical fertilizer alone (F1); (3) 70% of chemical fertilizer alone (F2); and (4) 70% of chemical fertilizer and MV (F2M). The content and fractions of P in soil aggregates, and the mechanisms underlying relationships among MV, chemical fertilizer, and P species in paddy soil were investigated. Results The proportion in > 2 mm aggregates positively responded to MV, also enhancing soil aggregate stability. MV applied improved P content and storage in > 2 mm aggregates and increased the available P (AP) in all soil aggregates compared with F2. The MV applied increased significantly NaHCO3-P (NaHCO3-Pi and NaHCO3-Po) and reduced the residue-P (Res-P), and there were negative correlation between Res-P and MV in all aggregates. Furthermore, MV influenced the P species by influencing soil organic matter, total nitrogen and total P in aggregates. Conclusions Although the input of chemical fertilizer played more important role on the P species, yet the reduction of Res-P was attributed to MV applied. Therefore, the co-incorporation of MV and chemical fertilizer improved the aggregate distribution and the availability and supply of P effectively in the double rice-MV cropping system.