Different groups of nitrite-reducers and N2O-reducers have distinct ecological niches and functional roles in West African cultivated soils

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Increasing attention has been paid to microorganisms able to produce nitrous oxide (N2O), a potent greenhouse gas, or reduce it to harmless N2. Based on previous studies, niche differentiation could exist between nirK- and nirS-nitrite reducers and nosZI- and nosZII-N2O reducers, and nosZII-bacteria would have a key role for N2O reduction in soils. Most previous studies have been performed for agricultural systems but never in the moist savanna zone which covers half a million km2 in West Africa and whose soils are among the poorest in nitrogen (N) on earth. Here, we quantified potential gross and net N2O production rates along with the abundances of nirK-, nirS-, nosZI- and nosZII-harbouring bacteria for soils under six agricultural practices with maize rotations (slash-and-burn, chemical fertilization, mulching with or without inclusion of crop legumes, and without any input) after 4 and 5 crop cycles at nine sites in Ivory Coast. Sites and practices influenced denitrifier abundances and activities, the ratio of total abundances of nitrite-to-N2O reducers being highest and lowest for the mulching + green soya and slash-and-burn practices, respectively. Using structural equation modelling, we showed that nirS- and nosZI-bacteria both strongly depended on nitrate availability whereas nirK- and nosZII-bacteria were related to soil organic carbon and pH. Furthermore, potential gross and net N2O production rates depended strongly and only on the abundances of nirS- and nosZI-bacteria. Our results support the view of a clear niche differentiation between these four microbial groups but invalidate the assumption of a prominent functional role of soil nosZII-N2O reducers.