Responses of wheat-maize yield, phosphorus budget and phosphorus fractions to phosphorus application rates in calcareous brown soil

Objectives This study investigated the effects of phosphorus (P) application rates on the annual grain yield of wheat-maize rotation systems and soil phosphorus budget, and the relationships among phosphorus budget, soil total phosphorus (TP), available phosphorus (Olsen-P), phosphorus activation coefficient (PAC), phosphorus fractions, and phosphorus pools. The findings will provide a theoretical support for improving phosphorus fertilizer efficiency in calcareous cinnamon soils.

Methods A long-term field experiment was initiated since 2008 (the no-P treatment started since 2015) under a winter wheat-summer maize rotation system in Shijiazhuang, Hebei Province, China. Five P₂O₅ application rates were setup: 0, 75, 120, 165, and 210 kg/hm², denoted as P0, P75, P120, P165, and P210, respectively. Annual crop above-ground yields and phosphorus uptake were assessed to calculate soil phosphorus budget from 2016 to 2022. Soil samples (0−20 cm depth) were collected after maize harvest in 2016, 2019, and 2022 for analysis of soil TP, Olsen-P, PAC, phosphorus fractions (using the Hedley sequential extraction method), and phosphorus pools.

Results Compared to P75, the P120, P165, and P210 treatments significantly increased annual grain yields by 9.54%, 11.81%, and 12.46%, and grain P uptake by 16.64%, 30.09%, and 39.58%, respectively. A linear-plus-plateau model analysis indicated the optimal annual P₂O₅ application rate of 112 kg/hm² for stable yields. The average annual P surpluses for P0, P75, P120, P165, and P210 treatment were −18.54, −0.84, 13.22, 27.93, and 43.91 kg/hm², respectively, with significant differences among treatments. A model fitting analysis indicated that soil phosphorus began to accumulate when the P₂O₅ application rate exceeded 76.64 kg/hm². Cumulative P surplus increased over the experimental period. For every 100 kg/hm² of soil P surplus, soil TP, Olsen-P and PAC increased by 29.70 mg/kg, 3.88 mg/kg and 0.31%, respectively; the contents of HCl-Pi, NaHCO₃-Pi, NaOH-Pi, H₂O-Pi, NaOH-Po, NaHCO₃-Po increased by 20.90, 4.09, 1.53, 0.57, 0.53 and 0.44 mg/kg, respectively; Labile P, moderately labile P and stable P increased by 5.37, 2.58 and 21.61 mg/kg, respectively; inorganic and organic P increased by 26.96 and 1.03 mg/kg, respectively. Soil phosphorus surplus accumulated primarily in the surface layer, accounting for 77.22% of the total-P.

Conclusions In a wheat-maize rotation system on calcareous cinnamon soil, soil P began to accumulate when the P₂O₅ application rate exceeded 76.64 kg/hm². Subsequently, the contents of total soil P, available P, P activation coefficient, and various forms of P (excluding Residual-P) increased, with HCl-Pi (a form of inorganic phosphorus) showing the most pronounced rise. The response of soil phosphorus pools to P surplus demonstrated that stable phosphorus > active phosphorus > moderately active phosphorus, and inorganic phosphorus > organic phosphorus. Considering both yield effects and environmental risks, the recommended reasonable range for P₂O₅ application is 112–135 kg/hm², which not only ensures stable annual crop yields but also keeps P surplus within environmentally safe limits.