Agroforestry has been identified as a sustainable land use system for degraded lands in the humid tropics. Therefore, nutrient dynamics and soil improvements potential of agroforestry through alley cropping systems in situations where land degradation is the highest, need detail investigation. This study was initiated to assess and quantify nutrient dynamics in Gliricidia-maize alley-cropping systems through the injection of labeled ¹⁵N fertilizer to Gliricidia trees and subsequent cropping with maize. The experiment was conducted at the University of Peradeniya, Sri Lanka.

Three-year-old isolated gliricidia trees were injected with 98% ¹⁵N-enriched ammonium sulphate (1.415 g/tree) fertilizer. Four weeks after injection, trees were killed and above-ground biomass was harvested and the ¹⁵N labeled residues of these trees and non injected residues obtained from control trees were applied to three compartmentalized areas in 2 m radius of the injected tree base to form three treatments; T1 - ¹⁵N labeled roots not removed and ¹⁵N labeled above-ground gliricidia residues applied; T2 - ¹⁵N labeled roots not removed + non-labeled gliricidia residues applied and T3; ¹⁵N labeled roots removed and non-labeled gliricidia residues applied. Four weeks after incorporation of residues, a maize crop was planted and managed as recommended by the Department of Agriculture. ¹⁵N cycling and corn productivity were recorded.

Distribution of ¹⁵ N among different plant parts of gliricidia resulted in a greater recovery of injected ¹⁵ N. Approximately 71% and 15% were recovered from the stems and leaves. The % ¹⁵ N excess in roots were 18% less than in the stems and leaves. From the amount of %¹⁵ N injected (294.49 mg per tree) %¹⁵ N overall recovery was around 87% by the gliricidia trees. ¹⁵ N recoveries by the subsequent maize crop were around 13%. Of this quantum, approximately 10% of N was recovered from leaves and stems of injected Gliricidia residues, while around 2% was recovered from Gliricidia roots. Soil organic matter (C) and N contents increased with the addition of gliridicidia residues up to 4-6 weeks and gradually declined with time. The C and N contents were significantly lower in deeper soil layers. Soil C derived from gliricidia residues range of 21-36%. The injection of tracer N provides a relatively easy ways to determine the transfer of ¹⁵ N into trees and it promises to be an effective way to evaluate N transformation and cycling in mixed cropping systems.

Keywords: Tree injection, ¹⁵ N fertilizer, Alley cropping, Residues.