Modeling of soil available phosphorus surplus in an intensive wheat-maize rotation production area of the North China Plain

The excessive application of phosphorus fertilizer has caused available phosphorus (AP) surplus in cultivated soils. By using the Soil and Water Assessment Tool (SWAT), the soil AP surplus and its relationship with fertilization and environmental changes was estimated for the wheat-maize rotation production area in the Baiyangdian basin, which is a typical arid region in North China Plain. Due to the little runoff, the model was validated by the measured AP concentration in deep soil profiles of the cropland. The results indicated that with an evident overuse of phosphorus fertilizer, the average rate of AP surplus in soils in the basin was about 10.3% of the applied fertilizer. A large part of the applied phosphorus (57.2%) was accumulated in active mineral forms. The phosphorus fertilizer recovery efficiency averaged 41.4%, ranging between 29.3% and 51.7%. This lower efficiency was expected to be the main reason for the serious AP surplus in cultivated soils. As a result, it was predicted that for every 100 kg ha−1 of P fertilizer added, the AP of the plough layer (0-20 cm) will be increased by an average of 3.35 mg kg−1 (around 10 kg P ha−1) in the study area. Furthermore, the soil AP surplus was predicted to reach a higher level under the condition of lower precipitation because of less runoff-induced erosion and leaching to deeper layers. From 1993-2010, the soil AP accumulation was 445.7 kg ha−1 in the study area, with an average accumulation rate of 24.8 kg ha−1yr−1, which belongs to the highest level worldwide. Through combining the measured data with the simulated results, it was suggested that excessive phosphorus fertilizer application along with low phosphorus fertilizer recovery efficiency will increase the risk of P leaching, which threatens the security of groundwater.