Induced decline of alkali and sodium content in the rhizosphere by applying ammonium containing fertilizer can increase the adaptation and yield of crops to salt-affected soil

Enhancing the green productivity of saline-alkali soil is of considerable importance by changing the cultivation mode to stimulate the potential of crop resistance to salt and alkali and to achieve increasing yields and efficiency of crops by making corp adaptation to saline soil through cultivation. This experiment used a winter wheat-summer maize rotation system in coastal saline-alkali land as the research object. The field positioning experiment included six treatments: AS0 (100% urea, phosphorus fertilizer reduction), AS50 (50% urea +50% ammonium sulfate, phosphorus fertilizer reduction), AS75 (25% urea +75% ammonium sulfate, phosphorus fertilizer reduction), AS50 (100% ammonium sulfate, phosphorus fertilizer reduction), FP (farmers over fertilization, 100% urea, control 1) and P0 (no phosphorus fertilizer, 100% urea, control 2). Changes in the yield, nutrient uptake, nitrogen and phosphorus fertilizer utilization efficiencies, as well as rhizosphere soil pH, salinity, and sodium adsorption ratio (SAR) of winter wheat and summer maize were analyzed under different ammonium sulfate replacement urea ratio treatments. The results of the 2-year field experiment showed that the yields of wheat and maize treated with P0 were the lowest, indicating that the soil lacked phosphorus. No significant difference was observed in yields between the phosphorus reduction and FP treatments. Under iso-nitrogen and iso-phosphorus conditions, the AS50 treatment resulted in an average yield increase of 5.4% for winter wheat and 1.5% for summer maize, compared to AS0. Compared with AS0 treatment, increased application of ammonium sulfate (AS50, AS75, and AS100) reduced the pH of wheat rhizosphere soil by 0.08 to 0.24 units and corn rhizosphere soil by 0.06 to 0.35 units. With an increase in the ammonium sulfate ratio, the available phosphorus content in the rhizosphere soil first increased and then decreased; the available phosphorus content in the AS50 treatment was the highest. The contents of Ca²⁺ and Na⁺, and SAR in the rhizosphere soil decreased. Compared with the FP treatment, the rhizosphere pH, SAR, and Na⁺ content of the AS50 and AS75 treatments were significantly reduced, and nitrogen and phosphorus fertilizer utilization efficiencies were significantly improved. Moreover, the treatment effect was more significant during the maize season than during the wheat season. In coastal saline-alkali-cultivated lands, substituting 50% of urea with ammonium sulfate under reduced phosphorus application can promote the reduction of SAR and Na⁺ content and the increase in available phosphorus and Ca²⁺ content by inducing a decrease in rhizosphere pH. This creates a more suitable rhizosphere environment with lower salt, lower alkali, and higher nutrient content for crops, thereby enhancing yield and fertilizer utilization efficiency. Based on the above understanding, the technical principle of adding ammonium, reducing alkali, mobilizing phosphorus, and increasing calcium to induce the separation of fertilizer and salt distribution at the root-soil interface was proposed to achieve an increase in crop yield and efficiency in light-to-medium saline-alkali soil.