Effect of Fertilizer P and Farmyard Manure on Bioavailable P as Influenced by Rhizosphere Microbial Activities in Soybean-Wheat Rotation

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ABSTRACT The efficiency of use of applied P may be improved through an understanding of soil P dynamics in relation to management practices in a cropping system. The objective of this study was to examine the influence of farmyard manure (FYM) and fertilizer P application on the activities of native arbuscular mycorrhizal fungi (AMF), inorganic P-solubilizing and organic-P mineralizing organisms, and selected enzyme activities in a soybean (Glycine max merril L.)-wheat (Triticum aestivum L.) rotation of five years duration. The results showed that the greatest microbial activities were measured when maximum crop growth occurred and that their activities increased significantly with the application of manure. AMF root colonization was twice higher in soybean than in wheat. The application of fertilizer P (44 kg P ha−1) reduced AMF colonization in roots, which was 15% lower than in control plots in both soybean and wheat. The combined application of fertilizer P and manure significantly improved the inorganic-P solubilizing activity of rhizosphere soil, which ranged from 0.87 to 1.53 μg TCP g−1 soil d−1 and organic-P mineralizing rate ranged from 0.07 to 0.31 μg inosine-triphosphate g−1 soil d−1. Root acid phosphatase was maximal at the early-growth stage of the crops and significantly increased with both manure and P application, but decreased as crops matured. However, rhizosphere soil alkaline phosphatase remained higher even after maturity than at vegetative stage. It was observed that after six cropping cycles of soybean-wheat; the labile organic phosphorus (Po) (NaHCO3-P) and moderately labile Po (NaOH-P) were relatively higher than stable Po (HCl-P). The bioavailable P was significantly correlated with the rates of inorganic-P solubilization and organic-P mineralization and with the activities of root phosphatase and rhizosphere soil alkaline phosphatase. These results demonstrate that qualitative P solubilizing/mineralizing organisms associated with AMF, and activity of enzymes mediated through microbes are a major factor in controlling organic and inorganic P in this management system.