Recovery of N applied as ¹⁵N-manure or ¹⁵N-gliricidia biomass by maize, cotton and cowpea

Proper management of N applied in fertilizers is important to optimize crop production and to avoid negative environmental impacts. The best way to study N dynamics in the soil plant system is to use fertilizers labeled with ¹⁵N. Recoveries of nitrogen following fertilization with ¹⁵N-labeled goat (Capra hircus L.) manure and gliricidia (Gliricidia sepium Jacq. Walp) biomass were evaluated in a greenhouse experiment with three successive planting cycles of three crops: maize (Zea mays L.), cotton (Gossypium hirsutum L.), and cowpea (Vigna unguiculata (L.) Walp.). Each 1 kg soil pot received 8 g (equivalent to 20 Mg ha⁻¹) of either manure (12.3 mg g⁻¹ of N) or gliricidia (37.8 mg g⁻¹ of N). Plants were harvested 50 days after germination and real (¹⁵N) and apparent recoveries of the applied N were determined. Biomass and N amounts in the cotton and maize crops in all three cycles were higher with gliricidia application than with manure, except for cotton in the first cycle. The biomass of cowpea was also higher with gliricídia in the first and second cycles but the amount of N was significantly higher only in the second cycle. In the first cycle, the largest recoveries of ¹⁵N were obtained with gliricidia, for all three crops, but in the second and third cycles recoveries were greater with manure, so that the real recoveries of gliricidia and manure were similar (cotton, 35 and 37 %; maize, 27 and 26 %; and cowpea, 41 and 38 % of the applied N, respectively). Estimates of apparent recoveries were different from the real ones and therefore inadequate for cotton and cowpea. The fast release of N from gliricidia prunings and, on the other hand, the strong residual effect of goat manure-N to subsequent cropping cycles should be considered by farmers in their fertilization strategies.