Water-saving and nitrogen-reducing regulation of bacterial nitrogen metabolism pathways in the salt-affected soil to control nitrogen loss

Water-saving and nitrogen-reducing practices can regulate the bacterial nitrogen metabolism pathways in the rhizosphere. This study aims to control nitrogen loss in the salt-affected soil. A field plot experiment was conducted in the irrigation district of the Tumuchuan Plain from 2023 to 2024. The local conventional water-nitrogen management (nitrogen application of 200 kg/hm² with irrigation of 450 m³/hm² at the bud stage) was used as the control. Five treatments were established to combine the deficit irrigation (300 m³/hm² mild deficit and 150 m³/hm² moderate deficit) with reduced nitrogen application by 25% (150 kg/hm², medium nitrogen) and 50% (100 kg/hm², low nitrogen) at the bud stage. Results indicated that both the mild water deficit (W1) and the 25% nitrogen reduction (N1) treatments improved the rhizosphere soil environment and root growth, compared with the CK. The W1 and N1 treatments significantly increased oxalate and citrate content in the rhizosphere at both the bud stage and maturation, while also increasing root length density (RLD). Furthermore, the W1 treatment increased K⁺ content during the bud stage and Ca²⁺ content at the maturation, while Na⁺ content was reduced during the anthesis. Moderate water deficit (W2) combined with 50% nitrogen reduction (N2) significantly decreased Cl- and SO₄2^- content in the rhizosphere during the bud stage. But there was no variation in the root surface area density (RSAD). In bacterial community structure, the W1 treatment significantly increased the Chao1, Observed_species, and Ace indices of nitrogen-metabolising bacterial communities in the rhizosphere soil during the seedling stage, bud stage, Anthesis, and Maturation (P<0.05). The W2 treatment significantly reduced the Chao1 and Observed_species indices during the bud stage. In nitrogen metabolism pathways, the N1 treatment also reduced the relative abundance of rhizosphere bacteria norB, nirK, and napA during the anthesis, while increasing the relative abundance of nxrA. A ‘high nitrification-low denitrification’ pattern also increased the risk of nitrogen loss from the rhizosphere soil. Water-saving and nitrogen-reducing treatments (W1, N1, and N2) significantly reduced the relative abundance of rhizosphere bacteria nxrA, norB, nirK, and napA during the bud stage; The W1 treatment also significantly increased the relative abundance of rhizosphere bacterial nosZ (11.16%) during the bud stage. The nitrogen loss from rhizosphere soil was promoted by N₂ reduction, thereby reducing the potential for N₂O greenhouse gas emissions. In summary, a mild water deficit effectively reduced the risk of nitrogen loss from sunflower rhizosphere soils in saline-affected farmland during the bud stage. The rhizosphere ionic environment promoted the plant root growth in agricultural water conservation and environmental protection.