Phosphorus in Field Crop Rotations of the Pampas of Argentina: Dynamics of Bray P, P balance and soil P fractions.

A better understanding of the phosphorus (P) cycle is necessary for improved management strategies that enhance P bioavailability to crops. Our objective was to evaluate the effect of continuous P fertilization on 1) P balance, 2) Bray P 1 dynamics, and 3) soil P fractionation, in two crop rotations: corn-double cropped wheat/soybeans (C-W/S; two sites), and corn-full season soybeans-double cropped wheat/soybeans (C-S-W/S; two sites). P balances and Bray P 1 dynamics were determined in four soils, after seven years with or without continuous P application. Soil P was fractionated into Resin-Pi, NaHCO₃-Pi and Po, NaOH-Pi and Po, HCl-Pi, Residue-P and P associated to particulate organic carbon (POC-P). Bray P 1 declined with time in the non-fertilized plots and increased in fertilized plots with differences between crop rotations. The changes in P balance between rotations were explained by differences in P removal. In the fertilized treatment, the P budget surplus was converted onto different fractions. All P fractions responded positively to continuous P application. Without considering Residue-P, HCl-Pi appeared to be the most important Pi fraction, in absolute (85 mg P kg^-1) and relative (17% of total P) terms. POC-P increased in fertilized P plots and the greatest differences appeared at the superficial layer, 0-5 cm. The ratio of POC to P-POC declined towards the lower soil layers (0-5 cm>5-10 cm>10-20 cm), and the lowest values were obtained in the fertilized treatment. These results were related with an increase in labil P fractions that could be explained by a higher phosphatase activity and, consequently, an enhancement of the rate of P mineralization. At a turn, it would explain the higher values found for NaHCO₃-Po and, mainly, for NaOH-Po fractions observed in plots with continuous P application.