NITROGEN RELEASE FROM THE LEAVES OF SOME TROPICAL LEGUMES AS AFFECTED BY THEIR LIGNIN AND POLYPHENOLIC CONTENTS

Leguminous plant materials used as mulches, green manures and cover crops are generally assumed to provide a readily-available source of N to crops. However, little is known about the chemical composition and N release patterns of the variety of legumes being used in tropical agroecosystems. N release patterns from the leaflets of 10 tropical legumes and rice straw were determined in a laboratory experiment. Ground leaf material was allowed to decompose in an acid soil (pH 4.5) for 8 weeks and the soil was analyzed periodically for extractable NH4+-N and NO3(-)-N. N release in the soil plus plant material were compared to that of the soil without plant material added and related to the N, lignin and polyphenolic concentrations of the leaflets. Three patterns of net N mineralization emerged during the 8-weeks. One pattern exhibited by the control soil, rice straw and leaves of two of the leguminous plants was a low, positive net mineralization. Another pattern showed much higher rates of mineralization than the control soil and the third pattern showed initial net immobilization followed by low but positive net mineralization rates. The amount of N mineralized during the 8 weeks as compared to the control soil ranged from +46 to -20% of the N added in plant material. Net mineralization was not correlated to % N or % lignin in the leaf material but was found to be negatively correlated to the polyphenolic concentration, r = -0.63, or the polyphenolic-to-N ratio, r = -0.75. Mineralization in excess of the control soil was found only for materials with a polyphenolic-to-N ratio < 0.5. Mechanisms to explain the low mineralization by materials high in polyphenolics include the formation of stable polymers between polyphenolics and amino groups, and nitrosation, a chemical reaction of nitrite (NO2-) with polyphenolics. Our results show that leguminous plant material with a high polyphenolic content or polyphenolic-to-N ratio may not be a readily-available source of N.