Risk assessment in soil phosphorus environment of Guanxi pummelo orchard in Pinghe County, Fujian Province

Risk assessment in soil phosphorus environment is important for rational fertilization and management of orchards, and reduce non-point source pollution of orchards. The aim of this study was to determine the sensitive critical value of soil phosphorus environments in Guanxi pummelo orchards in Pinghe County of Fujian Province. In the study, 030 cm soil layer was sampled both in mountain (red soil) and arable (paddy soil) pummelo orchards, and 0100 soil profiles were sampled in 19 towns of Pinhe County with different soil and water conservation measures. The laboratory soil culture method was used to determine soil phosphorus contents. And risk of soil phosphorus environment was classified according to Garden Soil Nutrient Abundant-Deficiency Index (Trial), and the sensitive critical value of soil phosphorus environment was determined by using correlation between soil available phosphorus and soluble phosphorus. The results showed that the sensitive critical value of soil phosphorus environment in Guanxi pummelo orchard in mountain regions was 96.3 mgkg¹ and that in arable was 62.3 mgkg¹. Base on phosphorus content, 3 grades of risk of soil phosphorus environments in Guanxi pummelo orchards were classified — high, medium and low. Based on the risk classification criteria, the available phosphorus contents in the 030 cm and 0100 cm soil layers in orchard fields were used to evaluate the risk of soil phosphorus environments. The evaluation results indicated that 50.5% of soil phosphorus environments in orchard fields were in high risk condition (48.4% mountain and 52.7% arable), 12.2% in medium risk condition (10.6% mountain and 13.8% arable) and then 37.3% in low risk condition (41.0% mountain and 33.4% arable). Some 42.1% of the orchard soil in the 040 cm soil layer was in high risk condition and even part of the 0100 cm soil layer was at risk of phosphorus loss. In order to reduce non-point source pollution risk, it is workable to adopt different phosphate fertilizer application strategies in different risk conditions in Guanxi pummelo orchards. It was recommended that in low risk orchard of phosphorus environments, farmers apply more phosphate fertilizer; while in medium risk phosphorus environments, farmers reduce phosphate fertilization dose; and then in high risk phosphorus environments, farmers stop phosphate fertilization. Excessive application of phosphate fertilizer caused non-point source pollution.