Abstract:
Laboratory studies were conducted to investigate the effect of phosphorus concentration in residues of cowpea (Vigna
unguiculata, L. Walp) and stylo (Stylosanthes hamata, L., cv Verano) on their rate of nitrogen mineralisation when
incubated in a soil whose P status was deficient for plant growth. Residues with a range of P concentrations were
obtained by applying varying rates of P to soil in which the plants were grown 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 concentrations). Both lignin and polyphenol concentrations were
higher in the field-grown than in the glasshouse-grown residues. Lignin concentration was greater in cowpea than in
stylo, but polyphenols were higher in stylo. Cowpea residues mineralised N less rapidly than stylo. N mineralisation
from residues with low P concentration was consistently less than from those of higher P concentration; reduced
mineralisation was observed for P concentration in the residues below 1.6 g kg1.
When inorganic P was added
to the residue-soil systems, N mineralisation from the residues was increased, though no interaction between the
effects of adding inorganic P and P concentration in the residues was observed.
