Winter wheat yield and soil critical phosphorus value response to yearly rainfall and P fertilization on the Loess Plateau of China

The critical value of soil available phosphorus (AP) plays a key role in crop P fertilization, and this value is based on a soil test, but the guidelines for P fertilization of winter wheat based on soil AP in northern China do not yet consider different rainfall patterns, such as above- or below-average annual rainfall or wheat-yearly rainfall (rainfall from June (when crop harvest was normally complete) until the following May). To determine the responses of winter wheat yield to wheat-yearly rainfall and P fertilization, calculate the soil critical P value in different rainfall patterns and then provide some suggestions on how to manage the P fertilization of winter wheat. Based on 781 pairs of datasets from field experiments with (+P) and without (−P) P fertilizer input on the Loess Plateau of China in 2005 − 2018, wheat yield, P uptake by wheat shoots were measured. Soil Olsen P (0 − 20 cm, sampled at harvest of previous season wheat) and wheat-yearly rainfall from 781 wheat fields were also determined. The wheat grain yield followed a piecewise linear model with the amount of wheat-yearly rainfall, with a threshold rainfall of 527 and 550 mm for the −P and +P treatments, respectively; the P uptake by wheat was also fitted to a piecewise model with wheat-yearly rainfall. When each site-year was classified into dry, intermediate, and wet rainfall years, a P uptake response to Olsen P was evident only in dry and intermediate years. Olsen P at harvest was 18.9% lower in dry/wet years than in intermediate years. Across all years, the critical Olsen P values were higher for −P (16.2 mg kg-1) than for +P (15.3 mg kg−1) and were lower in dry and intermediate years than in wet years. In addition, an empirical model is proposed to manage P fertilization, where P application rates are calculated from the expected wheat P uptake and the Olsen P level in the soil based on wheat-yearly rainfall or summer-season rainfall. Wheat-yearly rainfall is the main factor affecting wheat yield, and the wheat-yearly rainfall and P input strongly affected the soil critical Olsen P values in the drylands of the Loess Plateau. A soil critical Olsen P value of 14 − 18 mg kg−1 was determined for dryland wheat production. This research implied that the management of P fertilization for green wheat production should consider the rainfall and soil critical Olsen P value in dryland grain production systems.