Effects of nitrogen and phosphorus fertilization on inorganic phosphorus forms of typical farmland soil in the dry farming area of the Loess Plateau

Objectives The transformation characteristics of soil inorganic phosphorus forms under different nitrogen and phosphorus application conditions and the factors affecting the conversion of inorganic phosphorus forms were discussed. It provides a reference for the efficient use of phosphorus in farmland and soil nutrient balance in the dry farming area of Longzhong Loess Plateau.

Methods Based on the long-term positioning experiment of different nitrogen and phosphorus combined application of spring wheat set in Mazichuan Village, Lijiabao Town, Dingxi City, Longzhong Loess Plateau in 2017, nitrogen (N) and phosphorus (P₂O₅) were set at 4 levels, respectively, 0, 75.0, 115.0, 190.0 kg/hm², a total of 16 treatments in pairs. Using Gu Yichu-Jiang Bofan method to determine the content of each form of inorganic phosphorus forms in the soil after harvest (0–20 cm), as well as environmental factors (soil organic carbon, total nitrogen, total phosphorus, Olsen-P, pH, grain yield, biomass,apparent recovery efficiency of applied phosphorus, microbial biomass carbon, nitrogen, phosphorus and alkaline phosphatase).

Results The distribution order of inorganic phosphorus forms in soil is Ca₁₀-P > Ca₈-P > O-P > Fe-P ≈ Al-P > Ca₂-P. The content of inorganic phosphorus is mainly Ca-P, and the other three forms of Al-P, Fe-P and O-P account for about 20% of the total inorganic phosphorus. Phosphorus application significantly increased the phosphorus content of each inorganic phosphorus form in the soil. Nitrogen application significantly reduced the phosphorus content of other inorganic phosphorus components except O-P and Ca₈-P, and O-P increased significantly. Nitrogen application has little effect on the change of the proportion of each inorganic phosphorus form. The proportion of Ca₂-P and Ca₈-P in the total inorganic phosphorus increased with the increase of phosphorus application, and the proportion of Ca₁₀-P and O-P showed a significant downward trend with the increase of phosphorus application. The proportion of Fe-P in inorganic phosphorus is basically unchanged with the increase of phosphorus application. In this study, soil available phosphorus was significantly positively correlated with Ca₂-P, Ca₈-P, Fe-P, O-P (P < 0.01), and significantly positively correlated with Al-P (P < 0.05), while correlated with Ca₁₀-P did not significantly (P > 0.05). Path analysis results show that the direct contribution of various forms of inorganic phosphorus to available phosphorus is Ca₂-P > O-P > Al-P > Ca₁₀-P > Fe-P > Ca₈-P. In this area, Ca₂-P is the main source of available phosphorus in the soil, and Ca₈-P and Fe-P are potential sources of phosphorus. Nitrogen application significantly increased soil organic carbon, total nitrogen, grain yield, biomass, microbial biomass carbon (MBC), microbial biomass N (MBN),microbial biomass P (MBP), alkaline phosphatase activity (ALP), apparent recovery efficiency of applied phosphorus, and reduced total phosphorus, available phosphorus, and pH. Phosphorus application significantly increased total nitrogen, total phosphorus, available phosphorus, grain yield, biomass, MBC, MBN, MBP, ALP, and reduced organic carbon. Redundancy analysis results show that soil organic carbon is a key factor affecting the change of soil inorganic phosphorus forms in the farmland layer of spring wheat farmland in the Longzhong Loess Plateau; Ca₈-P with total nitrogen, Al-P with apparent recovery efficiency of applied phosphorus, O-P with grain yield, Fe-P with biomass, alkaline phosphatase and microbial biomass nitrogen is significantly and positively correlated, and soil organic carbon is negatively correlated with each inorganic phosphorus form.

Conclusions The combined application of nitrogen and phosphorus can promote the activation of soil phosphorus, increase the ratio of Ca₂-P available for plant direct use and Ca₈-P, Al-P with slow-acting effect, and reduce the insoluble Ca₁₀-P in soil. The ratio of O-P improves the potential phosphorus supply capacity of the soil. Soil organic carbon is a key factor regulating the conversion of soil phosphorus forms in the plow layer.