Abstract:

Phosphorus （P） is an essential nutrient for crops， and is a key factor that is monitored in the control of non-point pollution. It is important to clarify the characteristics of the soil P pool to devise strategies to manage P levels and reduce P-related non-point pollution in intensively cultivated vegetable fields. In this study， we determined the characteristics of the soil P pool in the perennial vegetable fields in the latosolic red soil zone of the Pearl River Delta. A total of 89 soil samples were collected from the surface layer （0-20 cm） of vegetable fields located in the suburbs of Guangzhou， Jiangmen， Zhaoqing， and Huizhou. The composition of the soil P pool in the samples was determined using a modified Hedley method. Correlation analyses were conducted to reveal relationships between soil properties and the P pool. The results show that the proportions of inorganic P （Pi）， organic P （Po）， and residual P in the soil P pool were 88%， 8.1% and 3.9%， respectively， indicating that Pi was the dominant form in the soil P pool. The concentrations of these forms were as follows： Pi （1176.78 mg·kg^-1）， Po （109.03 mg·kg^-1）， and residual P （52.19 mg·kg^-1）. The Pi pool consisted of several fractions， namely H₂O-Pi， NaHCO₃-Pi， NaOH-Pi， Dli HCl-Pi， and Con HCl-Pi， with concentrations of 46.35 mg·kg^-1， 264.64 mg·kg^-1， 427.45 mg·kg^-1， 274.82 mg·kg^-1， 163.52 mg·kg^-1， respectively； and proportions of 3.32%， 20.74%， 31.29%， 16.32%， and 14.13%， respectively， in the total P pool. The fractions in the Po pool were NaHCO₃-Po， NaOH-Po， and Con HCl-Po， with concentrations of 27.24 mg·kg^-1， 62.35 mg·kg^-1， and 19.44 mg·kg^-1， respectively； and proportions of 2.33%， 5.70%， and 1.56%， respectively， in the total P pool. The dominant form in both the Po and Pi pools was NaOH-P， specifically NaOH-Pi in the Pi pool and NaOH-Po in the Po pool. In terms of soil P availability， the concentrations of labile P， moderately labile P， and recalcitrant P were 338.23 mg·kg^-1， 764.62 mg·kg^-1， and 235.15 mg·kg^-1， respectively. The proportions of labile P， moderately labile P， and recalcitrant P in the total P pool were 25.3%， 57.1%， and 17.6%， respectively. The sum of labile P and moderately labile P accounted for more than half of the total P pool in almost all the 89 soil samples， suggesting that labile P and moderately labile P were the main P types in the soil. On the whole， the soil in the perennial vegetable fields in the latosolic red soil zone was rich in P in highly available forms. Significant positive correlations and concentration effects were detected between soil properties （organic matter， total nitrogen， available nitrogen， available potassium， cation exchange capacity） and labile P and moderately labile P contents. The soil P pool accumulates via anthropogenic activities i.e. fertilization and cultivation， and is also affected by the soil’s intrinsic properties. Considering the climate characteristics in the Pearl River Delta， including heavy rainfall， the application of P nutrient management strategies is suggested to reduce the risks of P pollution in vegetable field soil.

Key words: vegetable field in latosolic red soil zone, Hedley phosphorus fractionation, inorganic phosphorus fractions, organic phosphorus fractions, phosphorus availability