Nitrogen economy of a Leucaena/sorghum agroforestry system

Martha E Avery, Purdue University

Abstract

Agroforestry, the mixture of trees and crops, and/or livestock, is suggested as an alternative cropping system for maintaining site productivity with minimal inputs on agricultural and forest lands. The inclusion of N-fixing plants, primarily legumes, as a component of mixed cropping, traditional bush fallow and agroforestry systems has led to the belief that N fixed in the root nodules may be used by the associated crop. Direct evidence of a significant N contribution by the N-fixing plant to the associated crop through below-ground processes is limited, especially in agroforestry systems. This research provides data on the growth characteristics and total N content of the woody legume, Leucaena leucocephala, and the companion crop Sorghum bicolor grown in the greenhouse with N-limiting (NH$\sb4\sp+$ concentration = 447 nM) nutrient medium in sole and mixed cropping. Plant height, dry weight, and total N content increased significantly in sorghum grown intercropped with nodulated Leucaena over the control, sole sorghum. The gain in N content of intercropped sorghum, however, represented less than 1% of the N budget of N-fixing Leucaena, and was inadequate to sustain normal physiological development of sorghum. The nitrogenous solutes of Leucaena tissues and xylem sap were characterized to discriminate between root/nodule turnover or excreted nitrogenous compounds as transfer mechanisms. Mass spectral analyses of Leucaena tissues showed large accumulations of L-mimosine and other potentially toxic (to mammals and plants) amino acids such as pipecolic acid and hydroxypipecolic acid. A new compound, dihydroxypipecolic acid betaine was identified albeit at relatively low levels ($<$1 $\mu$mol $\cdot$ gfw$\sp{-1}$). The principal nitrogenous solutes in the xylem sap of nodulated plants were asparagine (37%) followed by the nonprotein amino acid pipecolic acid. Ureides were barely detectable (0.014 $\mu$mol $\cdot$ ml$\sp{-1}$). Hydroxypipecolic acid (65%) and glutamate (30%) predominated in the xylem sap of nonnodulated Leucaena plants. Preliminary analyses of soil leachates and root exudates from sole cropped Leucaena revealed significant quantities of free NH$\sb4\sp+$. This suggests that the Leucaena roots and/or nodules may actively excrete fixed nitrogenous compounds to the soil which may then be assimilated by sorghum. The low levels of L-mimosine, pipecolic acid and hydroxypipecolic acid in soil leachates from nodulated Leucaena plants suggests root turnover is not the major transfer mechanism, during the first year of establishment.

Degree

Ph.D.

Advisors

Rhodes, Purdue University.

Subject Area

Forestry

COinS