Nitrogen and Carbon Mineralization in Cultivated Acrisols and Vertisols in a Sub-Humid Tropical Climate
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In the southern part of the Republic of Benin (West Africa), investigations were carried out on two contrasting sites (Ferrali-Haplic Acrisols and Stagni-Eutric Vertisols), with the aim of assessing the effect of soil type and micro-climate on carbon and nitrogen turnover under sub-humid tropical conditions. The Acrisols in Southern Benin are characterized by a sandy topsoil with high water permeability. In contrast, the Vertisols have a clayey topsoil and show stagnic properties. Maize was planted during two years i.e. four consecutive cropping seasons (no-till, no organic or mineral fertilizer, weeding by hoe) on a total of fourteen Acrisol and nine Vertisol plots. Fresh organic material like fallow vegetation or crop residues were systematically removed. In spite of their higher clay content the Vertisols showed significantly higher carbon mineralization rates (12.0%) under field conditions than Acrisols (5.2% of C(org) per year). The net nitrogen mineralization rates were, however, similar on both sites (13.8% and 13.0% of N(t) per year respectively). Differences between Vertisols and Acrisols were due to the drier microclimate in the Acrisols. Incubation studies with Acrisol topsoil samples under controlled conditions demonstrated that decreasing soil matrix potential and temperature fluctuations, as they occur in the field, reduce CO2 evolution more than the net nitrogen mineralization, which was not the case in Vertisol topsoil samples. Carbon mineralization is, therefore, under tropical sub-humid conditions highly related to soil microclimate, hence site specific. Carbon and nitrogen mineralization may differ considerably from each other depending on site conditions.
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