Phosphorus transformations as affected by sampling date, fertilizer rate and phosphorus uptake in a soil under pasture

A phosphorus (P) fertilization study was conducted in the southeast of the Buenos Aires province in Argentina, to determine the effect of P fertilizer rate and sampling date on microbial biomass P, and of organic and inorganic P extractable with 0.5M NaHCO3, in a soil under pasture. In addition, Bray-P, dry-matter production and P uptake were measured. Soil was sampled at different times over one year, and two to three years after application of 0, 50, 100 and 200 kg P ha −1. The addition of P fertilizer significantly increased total soil P and the labile fraction of P extractable with NaHCO3, with the greatest change in the labile inorganic P form, but had no effect on microbial biomass P. Fractions of P showed different patterns of seasonal variation. Microbial biomass P had a peak in winter and a lowest value in summer, the opposite occurring with NaHCO3-extractable organic P, while NaHCO3-extractable inorganic P remained relatively constant throughout the year. The cumulative dry-matter yield after three years was 31% higher in the fertilized than in the unfertilized treatments; the highest being 27 660 kg ha−1 for 200 kg P ha−1. Concentration of Bray-P increased by 0.18 mg P kg −1 for each additional kg P ha−1 added, but remained relatively constant over the year. A significant correlation was found between available Bray-P and microbial biomass P (r = 0.53), and NaHCO3-extractable organic P (r = 0.47), suggesting that these organic fractions may contribute to plant nutrition.