Poly-γ-glutamic acid improved biological nitrogen fixation, water-nitrogen productivity, and nitrate residue in cotton/soybean intercropping.

Abstract

Poly-γ-glutamic acid (γ-PGA) can efficiently promote crop growth and improve soil properties. However, the optimal application rate of γ-PGA is still unclear in the legume/non-legume intercropping system. A potted experiment was conducted to investigate the effects of five γ-PGA rates (0%, 0.1%, 0.2%, 0.3%, and 0.4%, represented by CK, P1, P2, P3, and P4, respectively) on biological nitrogen (N) fixation (BNF), water-N productivity, and nitrate (NO- 3-N) distribution in cotton/soybean intercropping system. The results showed that the growth indicators (plant height, stem diameter, leaf area index, root dry weight, root length) of cotton and soybean increased first and then decreased with increasing γ-PGA rates, and all growth indicators of cotton and soybean showed peaks in P3 and P2 treatments respectively. The stable 15 N isotope method indicated that γ-PGA promoted the BNF capacity of soybean and soil, especially the percentage of N derived from the atmosphere (Ndfa) in soybean reached 61.94% in P2 treatment. The γ-PGA improved water-N productivity, and the total N partial factor productivity (NPFP) and water productivity (WP) in P3 treatment increased by 23.80% and 43.86% compared with the CK. In addition, the γ-PGA mitigation of potential NO- 3-N residue was decreased first and then increased with increasing γ-PGA rates. Multivariate regression analysis showed that 0.22% of the optimal γ-PGA application rate could simultaneously obtain a higher yield and water-N productivity in cotton/soybean intercropping system. This article is protected by copyright. All rights reserved.