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 (P_Sol) 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 P_Sol 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 P_Sol (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 (DPS_Ox-Al) had the best correlation with P_Sol (r²=0.87). In the STP versus P_Sol 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 P_Sol concentration of 0.88mgL^-1. However, for P_Sol 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.