Is P in the Coarse Soil Fraction Related to Soil Organic P Mineralization Measured By Isotopic Dilution?.

Measuring soil organic phosphorus (P_o) mineralization would be helpful to improve fertilizer recommendations. However, the only reliable method to measure P_o mineralization is based on isotopic dilution experiments, which are expensive and time consuming. A proposed method to estimate P mineralization is the quantification of P in the coarse fraction of the soil (CF). The aim of this study was to evaluate if P in the CF can effectively predict P_o mineralization. To this end, soil samples from three North American long-term field experiments with different fertilization treatments (non-fertilized, mineral P fertilizer, poultry litter) for more than 10 years were analyzed. Soil samples were physically fractioned into the CF and the fine fraction (FF), and total P (P_t), P_o, and total carbon (C_t) were determined in each fraction. A 13-day-incubation experiment was also conducted in which soil respiration as well as gross and net Po mineralization rates were determined. The cumulative gross P_o mineralized over 13 days ranged from 7.3 to 38.6 mg P kg^-1, while P_t in the CF varied between 9.7 and 90.7 mg P kg^-1 and P_o in the CF between 7.2 and 17.7 mg kg^-1. No association between P_t in CF and mineralized P_o was observed. However, a significant linear relationship between P_o in the CF and mineralized P_o was observed after 6 days (R²:0.79) and a quadratic one after 13 days of incubation(R²:0.90). Additionally, mineralized P_o could also be predicted from C respiration (R²:0.52), and this model was significantly improved by incorporating the C_t:P_o ratio of the CF (R²:0.77), but not by the C_t:P_o of the FF (R²:0.54). Our results suggest that the quantification of P_o in the CF is a promising non-isotopic alternative to estimate basal P_o mineralization.