The effects of water potential on some active forms of phosphorus in a calcareous soil amended with sewage sludge

Immobilization and mobilization reactions of soil phosphorus depend on biological properties of soil and these soil properties strongly depend on the soil water potential. the objective of this study was to test the effects of water potential on some active forms of soil p. a semiarid soil classified as calcic haploxerept was treated with raw sewage sludge at a rate of 20 g kg^-1. water potentials established for soil incubation were: saturation (sa, 0 bar), field capacity (fc, -0.3 bar), and permanent wilting point (pwp, -15 bar). an irrigation treatment was drying-rewetting cycle (dwc) between -0.3 to -15 bars. after 0, 20, 60 and 90 days of incubation soils were sampled for analysis. the addition of sewage sludge increased soil total p, organic p, available p, microbial p, soluble and easily soluble p contents. the increase of soluble p was relatively higher. the effects of soil moisture, incubation time and their interaction on all active forms of soil p were significant. during 20 days of incubation, available p and soluble p decreased, whereas microbial p, easily soluble p and dicalcium phosphate increased significantly. after that, available p and easily soluble p increased continuously, but microbial p, soluble p and dicalcium phosphate fluctuated during incubation. microbial p had negative and significant correlations with available p and easily soluble p. soils incubated in dwc and fc compared to soils incubated in sa and pwp had significantly higher available p, soluble p and easily soluble p contents. however microbial p and dicalcium phosphate were significantly higher in soils incubated in higher water potential.