DGT technique to assess P mobilization from greenhouse vegetable soils in China: A novel approach.

Intensive phosphorus (P) inputs to plastic-covered greenhouse vegetable production (PGVP) in China has led to excessive soil P accumulation increasing the potential for leaching to surface waters. This study examined the mobility and hence the potential risk of P losses through correlations between soil solution P (PSol) and soil extractable P as determined by conventional soil P test methods (STPs) including degree of P saturations (DPSs), and diffusive gradient in thin-films (DGT P) technique. A total of 75 topsoil samples were chosen from five representative Chinese PGVPs covering a wide range of physiochemical soil properties and cultivation history. Total P and Olsen P contents varied from 260 to 4900, and 5 to 740mgkg-1, respectively, while PSol concentrations were between 0.01 and 10.8mgL-1 reflecting the large differences in vegetation history, fertilization schemes, and soil types. Overall, DGT P provided the best correlation with PSol (r2=0.97) demonstrating that DGT P is a versatile measure of P mobility regardless of soil type. Among the DPSs tested, oxalate extractable Al (DPSOx-Al) had the best correlation with PSol (r2=0.87). In the STP versus PSol relationships, STP break-points above which P mobilization increases steeply were 513μgL-1 and 190mgkg-1 for DGT P or Olsen P, respectively, corresponding to PSol concentration of 0.88mgL-1. However, for PSol concentration of 0.1mgL-1 that initiates eutrophication, the corresponding DGT P and Olsen P values were 27μgL-1 and 22mgkg-1, respectively. Over 80% of the investigated soils had DGT P and Olsen P above these values, and thus are at risk of P mobilization threatening receiving waters by eutrophication. This paper demonstrates that the DGT extracted P is a powerful measure for soluble P and hence for assessment of P mobility from a broad range of soil types.