THE NITROGEN MINERALIZATION RATE OF LEGUME RESIDUES IN SOIL AS INFLUENCED BY THEIR POLYPHENOL, LIGNIN, AND NITROGEN CONTENTS

A 12-week greenhouse experiment was conducted to determine the effect of the polyphenol, lignin and N contents of six legumes on their N mineralization rate in soil and to compare estimates of legume-N release by the difference and N-15-recovery methods. Mature tops of alfalfa (Medicago sativa L.), round leaf cassia (Cassia rotundifolia Pers., var. Wynn), leucaena (Leucaena leucocephala Lam., deWit), Fitzroy stylo (Stylosanthes scabra Vog., var Fitzroy), snail medic (Medicago scutellata L.), and vigna (Vigna trilobata L., var verde) were incorporated in soil at the rate of 100 mg legume N kg-1 soil. The medic and vigna were labeled with N-15. Sorghum-sudan hybrid (Sorghum bicolor, L. Moench) was used as the test crop. A non-amended treatment was used as a control. Net N mineralization after 12 weeks ranged from 11% of added N with cassia to 47% of added N for alfalfa. With the two legumes that contained less than 20 g kg-1 of N, stylo and cassia, there was net N immobilization for the first 6 weeks of the experiment. The legume (lignin + polyphenol):N ratio was significantly correlated with N mineralization at all samplings dates at the 0.05 level and at the 0.01 level at 6 weeks (r2 = 0.866). Legume N, lignin, or polyphenol concentration or the lignin:N ratio was not significantly correlated with N mineralization at any time. The polyphenol:N ratio was only significantly correlated with N mineralization after 9 weeks (r2 = 0.692). The (lignin + polyphenol):N ratio appears to be a good predictor of N mineralization rates of incorporated legumes, but the method for analyzing plant polyphenol needs to be standardized. Estimates of legume-N mineralization by the difference and N-15 recovery methods were significantly different at all sampling dates for both N-15-labeled legumes. After 12 weeks, estimates of legume-N mineralization averaged 20% more with the difference method than with the N-15 recovery method. This finding suggests that estimates of legume N available to subsequent crops should not be based solely on results from N-15 recovery experiments.