EFFECT OF TISSUE PHOSPHORUS CONCENTRATION ON THE MINERALIZATION OF NITROGEN FROM STYLO AND COWPEA RESIDUES

Laboratory studies were conducted to investigate the effect of phospho rus concentration in residues of cowpea (Vigna unguiculata, L. Walp) a nd style (Stylosanthes hamata, L., cv Verano) on their rate of nitroge n mineralisation when incubated in a soil whose P status was deficient for plant growth. Residues with a range of P concentrations were obta ined by applying varying rates of P to soil in which the plants were g rown in the field or the glasshouse. Variations in P concentration of field- or glasshouse-grown residues were not accompanied by variations in other chemical components (C:N ratio, lignin and polyphenol concen trations). Both lignin and polyphenol concentrations were higher in th e field-grown than in the glasshouse-grown residues. Lignin concentrat ion was greater in cowpea than in style, but polyphenols were higher i n style. Cowpea residues mineralised N less rapidly than style. N mine ralisation from residues with low P concentration was consistently les s than from those of higher P concentration; reduced mineralisation wa s observed for P concentration in the residues below 1.6 g kg(-1). Whe n inorganic P was added to the residue-soil systems, N mineralisation from the residues was increased, though no interaction between the eff ects of adding inorganic P and P concentration in the residues was obs erved.