Phosphorus in the coarse soil fraction is related to soil organic phosphorus mineralization measured by isotopic dilution

•We analyzed soil P pools and fluxes in three long-term field trials in the USA.•Organic P (Po) mineralization was compared to Po in the coarse fraction (CF, >53 μm).•Po in the CF was related to gross and net Po mineralization during 6 and 13 days.•Po mineralization could be predicted from respiration and the C:Po ratio of the CF.

A sound prediction of soil organic phosphorus (Po) mineralization would be helpful to improve fertilizer recommendations. However, in most mineral soils expensive and time consuming isotopic dilution experiments are required to assess Po mineralization. A proposed method to estimate Po mineralization is the quantification of P in the coarse fraction (CF, >53 μm) of the soil. The aim of this study was to evaluate if P in the CF can effectively predict Po mineralization. Soil samples from three North American long-term arable field experiments (>10 years) with different textures (sand content 15–82%) and different fertilization treatments (non-P-fertilized, mineral P fertilizer, poultry litter) were analyzed. Soil samples were physically fractioned into CF and fine fraction (FF, <53 μm). Total P (Pt), Po, and total carbon (Ct) were determined in each fraction. Gross and net Po mineralization rates as well as soil respiration were determined in a 13-day-incubation experiment. The cumulative gross Po mineralization over 13 days ranged from 8.3 to 38.6 mg P kg−1, while Pt in CF varied between 9.7 and 90.7 mg P kg−1 and Po in CF between 6.1 and 17.7 mg kg−1. No association between Pt in CF and gross or net mineralized Po was observed. However, a significant linear relationship between Po in CF and net (R2:0.68, p < 0.01) and gross mineralized Po (R2:0.72, p < 0.01) was observed after 6 days and 13 days of incubation. Additionally, gross and net mineralized Po could also be predicted from C respiration, and these models were significantly improved by incorporating the Ct:Po ratio of the CF. Our results suggest that the quantification of Po in the CF as well as the measurement of respiration corrected by the Ct:Po ratio of the CF are promising non-isotopic indicators of Po mineralization rates.

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