Growth and Nitrogen Fixation in Soybean as Affected by Phosphorus Fertilizer and Sheep Manure using15N 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 (N2) fixation of soybean (Glycine max L.). Sorghum bicolor L. was employed as a reference crop to evaluate N2 fixation using the 15N-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 N2 fixation. A tangible increase in the amounts of N2 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 N2 fixation. However, considering the high cost of sheep manure, P2M1 was the optimal treatment for improving growth and N2 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 N2 fixation through root growth and soil property improvements besides being a source of P and other nutrients that are essential for N2-fixation process.