Decomposition of woody legume nodules in two tree/grass associations undercontrasting environmental conditions

Turnover of nitrogen-rich root nodules follows the pruning of legume trees, forming a potentially important yet little studied way of N release to the soil. The effects of soil moisture, herbivory by soil mesofauna and microb ial decomposition on the disappearance rate of woody legume nodules was stu died in two tree/grass forage production associations. Litter bags containi ng nodules of Erythrina variegata L. (Papilionoideae: Phaseoleae) were incu bated for four weeks in grass-covered alleys between Gliricidia sepium (Jac q.) Walp. (Papilionoideae: Robinieae) hedgerows, established on a deep allu vial Oxisol under a humid tropical climate and on a shallow Vertisol under a subhumid tropical climate in Guadeloupe, French Antilles. Soil moisture w as regulated by irrigating or covering small plots from natural rainfall. F ine nylon mesh bags were used to study the rate of microbial decomposition, and open-ended perforated cylinders were used to estimate nodule herbivory . The chemical traits, especially the lignin: nitrogen ratio, of E. variega ta and G. sepium nodules were similar (lignin: N 1.70 and 1.55, respectivel y), and suggest that the results are probably also applicable to the G. sep ium nodules in the associations. Both soil moisture and decomposing agent ( microbes or mesofauna) had a significant effect on the nodule disappearance rate, but soil type did not have any apparent effect. The nodule half-life varied from three to seven days under different treatments. The N release rate from the nodules was high, with N half-life varying from three to five days. Herbivory accounted for ca. 10% of total mass and N loss from nodule s during the four-week field incubation period, but its importance increase d towards the end of the incubation, especially in Vertisol, after the most easily decomposable part of the nodules had decayed. After pruning, the no dule N is released to soil more rapidly than from mulch.