Phosphorus accumulation and leaching risk of greenhouse vegetable soils in Southeast China

Over-fertilization has caused significant phosphorus (P) accumulation in Chinese greenhouse vegetable production (GVP) soils. This study, for the first time, quantified profile P accumulation directly from soil P measurements, as well as subsoil P immobilization, in three alkaline coarse-textured GVP soil profiles with 5 (S5), 15 (S15), and 30 (S30) years of cultivation in Tongshan, Southeast China. For each profile, soil samples were collected at depths of 0-10 (topsoil), 10-20, 20-40, 40-60, 60-80, and 80-100 cm. Phosphorus accumulation was estimated from the difference in P contents between topsoil and parent material (60-100 cm subsoil). Phosphorus mobility was assessed from measurements of water-soluble P concentration (P-Sol). Finally, P sorption isotherms were produced using a batch sorption experiment and fitted using a modified Langmuir model. High total P contents of 1 980 (S5), 3 190 (S15), and 2 330 (S30) mg kg(-1) were measured in the topsoils versus lower total P content of approximately 600 mg kg(-1) in the 80-100 cm subsoils. Likewise, topsoil P-Sol values were very high, varying from 6.4 to 17.0 mg L-1. The estimated annual P accumulations in the topsoils were 397 (S5), 212 (S15), and 78 (S30) kg ha(-1) year(-1). Sorption isotherms demonstrated the dominance of P desorption in highly P-saturated topsoils, whereas the amount of adsorbed P increased in the 80-100 cm subsoils with slightly larger P adsorption capacity. The total P adsorption capacity of the 80-100 cm subsoils at a solution P concentration of 0.5 mg L-1 was 15.7 (S5), 8.7 (S15), and 6.5 (S30) kg ha(-1), demonstrating that subsoils were unable to secure P concentrations in leaching water below 0.5 mg L-1 because of their insufficient P-binding capacity.