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Lysimeter and field studies on15N in a tropical soil

II. Transformation of (NH2)2CO−15N in a tropical loam in lysimeter and field plots

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Field plots and lysimeter (micro) plots were established on St. Augustine loam in a completely randomised block design with four fertilized, mulched and unmulched treatments, replicated three times. Labelled (15N) and unlabelled (NH2)2CO fertilizers were applied at rates of 400 kg N ha−1 and CaH2PO4 and KCl were applied at rates of 100 and 150 kg ha−1 respectively. Mulch (bagasse) was applied to a depth of two cm on bare plots, maintained with regular applications of gramoxone. Soil temperature and moisture were recorded at various depths.

Within three to seven hours of fertilizer applications, hydrolysis of (NH2)2CO−15N to NH +4 15N occurred at the site, but nitrification was more gradual. Three times as much applied15N was recovered from the mulched and unmulched field plots (22.4 to 28.9 per cent) compared to the mulched and unmulched microplots (7.3 to 10.0 per cent). Mulching significantly (P=0.05) enhanced the conservation of NH +4 15N at 10 cm depth within the field plots but not in the microplots.

Significantly (P=0.05) lower soil temperatures prevailed in the field plots compared to the microplots particularly when mulched and levels of NO 3 15N and moisture were significantly (P=0.05) higher. The potential for downward movement of soil moisture and15N ions was low during the periods of evaluation and it is highly probable that the role of lateral movement of soil moisture and15N ions is more significant than previously recognised.

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Nkrumah, M., Griffith, S.M. & Ahmad, N. Lysimeter and field studies on15N in a tropical soil. Plant Soil 114, 13–18 (1989).

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