Nitrogen economy of a Leucaena/sorghum agroforestry system

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 ($\u3c$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