Long-Term Application of Chinese Milk Vetch Return to Soil Alters Phosphorus forms Associated with Aggregates

Chinese milk vetch (MV) as green manure is utilized to improve rice yield and soil fertility in China. However, the long-term effects of MV on the distribution and speciation of phosphorus (P) in soil aggregates are poorly understood. We conducted a 12-year field experiment in a double-rice cropping system, with four treatments: (1) no chemical fertilizer and no MV (CK); (2) 100% of chemical fertilizer (F1); (3) 70% of chemical fertilizer (F2); and (4) 70% of chemical fertilizer and MV (F2M). We analyzed the contents and forms of P in soil aggregates, and explored the mechanisms underlying the relationships among MV, chemical fertilizer, and P forms in paddy soil. The application of MV increased the proportion of > 2 mm soil aggregates and improved the stability of soil aggregates. Compared with F2, MV applied improved the P content and storage in > 2 mm aggregates and increased the available P (AP) in all soil aggregates. Application of MV significantly increased NaHCO₃-P (NaHCO₃-Pi and NaHCO₃-Po) and decreased residue-P (Res-P), and there was a negative correlation between Res-P and MV in all aggregates. Furthermore, MV influenced the P forms by affecting the amount of soil organic matter, total nitrogen and total P in soil aggregates. Although chemical fertilizer input more strongly affected P forms than did MV, the application of MV decreased Res-P. Therefore, co-incorporation of MV and chemical fertilizer improved the distribution of aggregates and increased the availability and supply of P in the double-rice-MV cropping system.