Growth and Nitrogen Fixation in Soybean as Affected by Phosphorus Fertilizer and Sheep Manure using 15N Isotopic Dilution

A field experiment was conducted to study the effect of adding different phosphorus (P) fertilizer levels [0, 40, and 80 kg phosphorus pentoxide (P2O5) ha(-1) (abbreviated as P0, P1, and P2, respectively)] and rates of sheep manure (M) [0, 20, and 40 ton ha(-1) (abbreviated as M0, M1, and M2, respectively)] on growth and nitrogen (N-2) fixation of soybean (Glycine max L.). Sorghum bicolor L. was employed as a reference crop to evaluate N-2 fixation using the N-15-isotpic dilution technique. Results showed that addition of P fertilizer or sheep manure had positive effects on dry-matter production, N accumulation, and seed yield. Such effects were more pronounced when adding sheep manure and P together than adding separately. Solely P fertilizer had a small impact on N-2 fixation. A tangible increase in the amounts of N-2 fixed due to manure addition occurred. The efficient use of N fertilizer (%NUE) increased significantly as the result of adding a high level of P fertilizer. However, a drastic decrease in %NUE was observed when sheep manure was added solely or in combination with P fertilizer. From productivity and ecological standpoints, P2M1 and P2M2 surpassed the other treatments in showing greater grain yield and greater N-2 fixation. However, considering the high cost of sheep manure, P2M1 was the optimal treatment for improving growth and N-2 fixation in soybean plants with minimal manure consumption. In conclusion, the integrated use of manure and P fertilizer could be considered a useful agricultural practice for improving the performance of soybean plants grown in an Aridisol. Their beneficial effects were mainly attributed to the enhancement of N-2 fixation through root growth and soil property improvements besides being a source of P and other nutrients that are essential for N-2-fixation process.