Mineralization of N, S, and P in soils amended with poultry litter or inorganic fertilizers
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Estimating soil N, S, and P mineralization (conversion from organic to plant-available, inorganic nutrients) potential is necessary to improve the determination of fertilizer rates for crop production. The purpose of this study was to measure the N, S, and P mineralization potentials of soils amended with inorganic fertilizer (IF) or poultry litter (PL), and to evaluate different methods to estimate mineralization. To this end, soil samples from four long-term (> 10 yr) field trials with PL- and IF-amended plots were analyzed. The N mineralization potential (No) measured by aerobic incubations (24 wk) was 1.8 to 233.1 g N kg-1 greater in PL- than in IF-amended soils, depending on the type of soil. Different chemical and biological methods to estimate No were evaluated, but only the 7-d anaerobic incubation (Nan) presented a strong association with No, which was similar for both fertilization treatments (r2= 0.96). The S mineralization potential (So) was 0 to 21.2 g S kg-1 greater in PL-amended soils than in IF-amended soils, depending on the type of soil. The 7-d aerobic incubation, the extraction with NaHCO3, and Nan were good estimators of So (r2=0.85, 0.85 and 0.99, respectively). The increase in No and So observed in PL- relative to IF-amended soils was closely associated with the soil clay content (r2> 0.90). The cumulative organic P mineralized in a 13-d incubation experiment, measured by the isotopic dilution method, varied between 9.7 and 90.7 mg P kg-1, and was greater or equal in PL- than in IF-amended soils, depending on the type of soil. The mineralized P was strongly associated with the organic P content in the coarse fraction of the soil (> 53 µm) (r2=0.92), but not with any other measured labile P pool in the soil. Additionally a model was developed to predict P mineralization based on soil respiration during an aerobic incubation and the C:P ratio in the coarse fraction of the soil. 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.