Phosphorus Fractions and Fate of Phosphorus‐33 in Soils under Plowing and No‐Tillage

Minimum tillage may alter soil P fractions through the application of P fertilizers and the deposition of organic matter on the surface rather than being incorporated into the soil. This study was conducted to determine whether no‐tillage (NT) systems affected soil organic and inorganic P fractions and the transformation of P from residues applied to soils. Surface soils (0–2 cm) under NT and conventional tillage (CT) were sampled from three long‐term research sites. Inorganic and organic P was measured in the NaHCO 3 , microbial, NaOH, NaOH after sonication, HCl, and residual fractions extracted sequentially. Soybean ( Glycine max L.) residues labeled with 33 P were added to soils, incubated, and extracted periodically, and 33 P was counted in the different P fractions. Levels of 31 P in NT were higher in some of the fractions compared with CT; however, there was no consistency in 31 P fractionation across soil types due to tillage in any of the inorganic and organic fractions. At the start of incubation, 56 to 82% of the applied 33 P was extracted in the resin fraction in the three soils. Resin‐ 33 P followed a three‐parameter single exponential decay model with a corresponding increase in other pools depending on soil. The increase in these pools followed a quadratic model in the three soils. By the end of the incubation period, the NaOH fraction accounted for the majority of the 33 P released from the labile resin pool. An increase in the calcium phosphate pool occurred in the calcareous soil. Tillage had no effect on the fate of 33 P released from soybean residues during the incubation period.