The nitrogen mineralization rate of legume residues in soil as influenced by their polyphenol, lignin, and nitrogen contents

TLDR: The (lignin + polyphenol):N ratio appears to be a good predictor of N mineralization rates of incorporated legumes, but the method for analyzing plant poly phenol needs to be standardized.

Abstract: 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 15N-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 15N 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 sampling dates at the 005 level and at the 001 level at 6 weeks (r2=0866) Legume N, lignin, or polyphenol concentrations or the lignin:N ratio were not significantly correlated with N mineralization at any time The polyphenol:N ratio was only significantly correlated with N mineralization after 9 weeks (r2=0692) 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 15N recovery methods were significantly different at all sampling dates for both 15N-labeled legumes After 12 weeks, estimates of legume-N mineralization averaged 20% more with the difference method than with the 15N recovery method This finding suggests that estimates of legume N available to subsequent crops should not be based solely on results from 15N recovery experiments