Synergistic effects of fertilizer reduction and fulvic acid application on decreasing NaCl content and N, P availability of salinized soil

Objectives Salinization restricts the sustainable development of agriculture in the Yellow River Delta. The combined effects of fulvic acid and chemical fertilizer reduction on soil desalination and winter wheat yield were studied.

Methods A pot experiment was conducted in a moderately saline-alkaline soil (pH 7.73, EC 1.18 dS/m). The treatments were composed of conventional N and P fertilizer rate (N100P100), and reduced N or P rate as N85P100, N70P100, P85N100, and P70N100, and the treatments combined with addition of 10 kg/hm² of fulvic acid (H). At the seedling and harvest stages of winter wheat, soil samples at 0–10 cm and 10–20 cm depths were collected to determine the contents of available P, NO₃^–-N, EC, pH, and ion composition. The wheat yield, N and P contents were determined at harvest.

Results Chemical fertilizer reduction combined with fulvic acid treatments decreased 0–10 cm soil NO₃^–-N content at seedling and harvest stages, but increased 10–20 cm soil NO₃^–-N content at harvest stage. Soil available P decreased with increasing nitrogen application rate at 0–10 cm soil depth. With increased rate of N application, soil salt, Na⁺, Ca²⁺, and Cl^– contents increased, while Mg²⁺ content decreased. Compared with N100P100, N85P100, N70P100, P85N100 and P70N100 treatments decreased soil Na⁺ contents by 29.74%, 55.84%, 28.62% and 43.25%, decreased Cl^– content by 37.68%, 43.81%, 26.11% and 14.53%. Compared with the same chemical N and P rate treatments, N100P100+H, N85P100+H, N70P100+H, P85N100+H and P70N100+H decreased Na⁺ contents by 64.63%, 31.20%, 5.14%, 32.66%, and 30.59%, decreased Cl^– contents by 66.74%, 55.07%, 35.93%, 53.56%, and 70.44%. The yield increase in N70P100+H, P85N100+H were significantly higher than those in N70P100, P85N100. Compared with N100P100, the N uptake efficiency of N85P100 and N70P100 increased by 11.22% and 29.37%, chemical fertilizer reduction with fulvic acid treatments improved uptake efficiency and partial factor productivity of N and P. Through correlation analysis, soil salinity was the direct reason of yield decrease. Na⁺, Cl^– and EC were significantly and negatively correlated with uptake efficiency and partial factor productivity of N and P.

Conclusions The main salt is NaCl in moderately salinized soil of Yellow River Delta. The Na⁺, Cl^– contents determine the EC value of soil, and negatively correlated with nutrient efficiency and partial productivity. Reducing N and P application rate could significantly decrease soil salt content, and increase the available P content in 0–10 cm soil. Their combination with application of fulvic acid further decrease the Na⁺ and Cl^– contents, alleviate salt stress to crop, increase N and P uptake by winter wheat. Reducing 30% of nitrogen leads to higher soil available P and lower soil EC value than normal and 15% less of nitrogen rate.