Effects of phosphorus application on changes in soil phosphorus under wheat/maize/soybean strip relay intercropping system

In Southwest China, one of the most densely populated agricultural regions, intercropping has been practiced in major grain production systems for a long period. Wheat/maize/soybean strip relay intercropping (W/M/S) system is one of the main planting patterns in Sichuan Province. In the system, wheat was sowed in autumn of the last year, maize transplanted around half month before wheat harvest, and soybean sowed after wheat harvest. This system is very important for achieving optimal crop yield, promoting system productivity and simultaneously decreasing phosphorus (P) losses through optimizing soil P management in the system. A three-year field experiment (20112013) was conducted with 5 P application rates on maize strip: 0 kg·hm^-2, 37.5 kg(P₂O₅)·hm^-2, 75 kg(P2O5)hm^-2, 112.5 kg(P₂O₅)·hm2 and 150 kg(P₂O₅)·hm^-2; on wheat strip: 0 kg(P₂O₅)·hm^-2, 45 kg(P₂O₅)·hm^-2, 90 kg(P₂O₅)·hm^-2, 135 kg(P₂O₅)·hm^-2 and 180 kg(P₂O₅)·hm^-2; soybean was not fertilized to determine the changes in soil Olsen-P, total P, CaCl2-P and annual variability of available P in the cropping system. The results showed that P application rate at 165 kg(P₂O₅)·hm^-2 (75 kg·hm^-2 on maize strip and 90 kg·hm^-2 on wheat-soybean strip, P₂ treatment) met the demand for P in W/M/S system. There was an apparent balance between P input and P output with soil Olsen-P content maintained at 20 mg·kg^-1. The linear-plateau model well described the correlation between Olsen-P and crop yield, with the change-point showing that the critical levels of soil Olsen-P for maximum wheat, maize and soybean yields were 12.6 mg·kg^-1, 16.5 mg·kg^-1 and 8.8 mg·kg^-1, respectively. From 2011 to 2013, soil Olsen-P in the top 020 cm soil layer under P₀, P₁, P₂, P₃ and P₄ treatments (P application rates of the W/M/S system were the total of wheat and maize application rates, respectively) changed by 1.2 mg·kg^-1·a^-1, 0.9 mg·kg^-1·a^-1, 0.2 mg·kg^-1·a^-1, 2.0 mg·kg^-1·a^-1 and 2.7 mg·kg^-1·a^-1, respectively, while total P changed by 0.024 g·kg^-1·a^-1, 0.016 g·kg^-1·a^-1, 0.016 g·kg^-1·a^-1, 0.11 g·kg^-1·a^-1 and 0.15 g·kg^-1·a^-1, respectively. Soil Olsen-P increased 1.70 mg·kg^-1 and 6.49 mg·kg^-1 when soil total P was below 0.55 g·kg^-1 and above 0.55 g·kg^-1, respectively, with increasing soil total P per 0.1 g·kg^-1. Soil CaCl2-P increased by 0.017 mg·kg^-1 for per 1 mg·kg1 increase in Olsen-P when soil Olsen-P content was lower than 40 mg·kg^-1. In short, simultaneously maintaining soil fertility and increasing crop yield or productivity in W/M/S system required keeping soil total P content under 0.55 g·kg1 and holding soil Olsen-P at 20 mg·kg^-1.