SOIL TEXTURE AND RESIDUE ADDITION EFFECTS ON SOIL-PHOSPHORUS TRANSFORMATIONS

The effect of soil texture on P transformation and the extent and timi ng of crop residue decomposition affects both P availability and the f ate of the P added in the residue. Our objectives were to examine P tr ansformation in sand, sandy loam, and loam soils during surface or inc orporated wheat (Triticum aestivum L.) straw decomposition with and wi thout N and P addition. Packed soil cores (4.8 cm diameter by 2.5 cm d eep) were incubated in the dark for 10, 30, 60, and 90 d at 25 degrees C and -0.033 MPa initial soil water pressure. We determined CO2 respi ration, labile inorganic P (Pi), labile organic P(Po), microbial P, Na OH-extractable Pi and Po, and HCl-extractable Pi. Soil texture had a g reater effect on P transformation than did residue placement. Microbia l P was greatest at 10 d and was 13.0, 9.8, and 6.5 mg kg(-1) for the loam, sandy loam, and sand, respectively. Labile Pi was lowest at 10 d and was 22.9, 18.0, and 11.5 mg kg(-1) for the loam, sandy loam, and sand, respectively. More of the nutrient P added remained as labile Pi when the residue was surface applied. Residue placement did not affec t microbial P, indicating that the effect of placement was indirect. P hosphorus transformations were closely linked to microbial activity an d C dynamics.