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A hot-spot approach applied to nitrification in tropical acid soils

Abstract : Several authors have reported that nitrification in acid soils may be restricted to microsites having a more favorable pH. The aim of this study was to propose a conceptual model of the functioning of nitrification in hot-spots, and to test it with the experimental data obtained in laboratory conditions using twelve tropical unamended and NH4+-amended soils with a wide range of pH (from 4.2 to 6.9). Nitrification was also measured in two selected soils where the pH was adjusted from 3.5 to 6.2. The model characterizes the relationship between the nitrification rates in unamended and NH4-amended soils as a function of pH. It is based upon the assumption that nitrification of the NH4+ coming from N mineralization occurs in the hot-spot (RNh), and the nitrification of the added NH4+ occurs in the hot-spot and also in its adjacent surrounding region (RNs). The experimental design was chosen to be able to estimate both nitrification rates. Soil acidity limited nitrification more in NH4+ -amended soils than in unamended ones. From our approach, this is due to less favorable conditions for nitrification in the region surrounding the hot-spot. The effect of self-induced acidity on nitrification was not noticeable neither in unamended nor in NH4+ amended treatments. The model described well three observations made in the experiments: (i) the minimum pH for nitrification to occur was lower for RNh (pH < 4.2) than for RNs (pH < 4.7), (ii) the RNh/RNS ratio increased with the decrease of pH (from 1.5 at pH 6 to 8.5 at pH 4), and (iii) for a given pH, the RNh/RNS ratio increased with the decrease of the initial pH of the soil. Among the soil parameters determined in this study (i.e. exchangeable Al, EDTA-extractable Cu and Zn, total C and N), only pH was related to nitrification. However, for a given pH, nitrification varied 3-fold among soils, depending upon their initial pH. This suggests that soil pH as determined on bulk soil is not suitable to predict nitrification in each individual soil, because it is not representative of the acidity level within the hot-spot.
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Jorge Sierra. A hot-spot approach applied to nitrification in tropical acid soils. Soil Biology and Biochemistry, Elsevier, 2006, 38 (4), pp.644-652. ⟨10.1016/j.soilbio.2005.06.016⟩. ⟨hal-02658586⟩

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