DECOMPOSITION AND NUTRIENT RELEASE DYNAMICS OF 2 TROPICAL LEGUME COVER CROPS

Shifting agriculture is a main contributor to deforestation in the hum id tropics. Substitution of managed falloffs using cover crops is a po ssible alternative to natural fallow that may contribute to more susta inable farming systems via more efficient nutrient cycling. Our object ive was to determine the rate of decomposition and nutrient (N, P, K, Ca, Mg) release patterns resulting from two managed fallow periods (12 or 18 mo) for Desmodium adscendens (Sw.) DC. (syn. D. ovalifolium Gui llemin & Perrottet) and Pueraria phaseoloides (Roxb.) Benth, (tropical kudzu) grown on Typic and Aquic Dystropept soils at two locations in the Bolivian Amazon. Dry matter disappearance and nutrient release rat es from decomposing cover crop residues were monitored using 1-mm mesh nylon bags. The longer managed fallow (18 mo) reduced the decompositi on rate of both cover crops. After 30 wk, residues remaining from 12-m o managed fallows averaged 24% for desmodium and 16% for pueraria, whe reas residues from 18-mo managed fallows averaged 53% for desmodium an d 32% for pueraria. Similar to dry matter disappearance, cover crops g rown for 12 mo released nutrients faster than those grown for 18 mo. N utrient release patterns between sites were similar. By 30 wk, nutrien t percentages remaining in desmodium vs. pueraria grown for 12 mo at S ite 1 were 46 vs. 17% N, 17 vs. 9% P, 4 vs. 2% K, 36 vs. 20% Ca, and 3 0 vs. 21% Mg. Similar proportional differences in release patterns wer e observed for all nutrients except Ca for 18-mo managed falloffs. The se results indicate a greater potential for asynchrony between crop nu trient demand and cover crop residue nutrient release for 12- vs. 18-m o managed falloffs, and more so for pueraria than desmodium.