Sunday, 9 July 2006

Soil N Enrichment Using Biomass of Gliricidia Sepium Injected with Labelled 15n Fertilizer and Subsequent Recovery by Zea Mays in an Alley-Cropping System.

S. P. Nissanka and U. R. Sangakkara. Univ of Peradeniya, Dept of Crop Science, Peradeniya, Sri Lanka

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.

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