Introduction Agriculture is the main source of terrestrial N2O emissions, a potent greenhouse gas and the main cause of ozone depletion ((Hu et al., 2015). The reduction of N2O into N2 by microorganisms carrying the nitrous oxide reductase gene (nosZ) is the only known biological process eliminating this greenhouse gas. Recent studies showed that a previously unknown clade of N2O-reducers (nosZII) was related to the potential capacity of the soil to act as a N2O sink (see Hallin et al., 2017 and references therein). However little is known about how this group responds to different agricultural practices. Here, we investigated how N2O-producers and N2O-reducers were affected by agricultural practices across a range of cropping systems in order to evaluate the consequences for N2O emissions Materials and Methods Soil samples were collected in spring 2014 from 4 experimental sites in France, which undergo a large range of agricultural practices. The abundance of both ammonia oxidizers and denitrifiers was quantified by real-time qPCR, and the diversity of both nosZ clades was determined by 454 pyrosequencing. Denitrification and nitrification potential activities as well as in situ N2O emissions were also assessed. The physical and chemical soil characteristics were measured for all samples (INRA Laboratory of Soil Analysis, Arras, France) Results and Discussion Overall, greatest differences in microbial activity, diversity and abundance were observed between sites rather than between agricultural practices at each site. To better understand the contribution of abiotic and biotic factors to the in situ N2O emissions, we subdivided more than 59.000 field measurements into fractions from low to high rates. We found that the low N2O emission rates were mainly explained by variation in soil properties (up to 59%), while the high rates were explained by variation in abundance and diversity of microbial communities (up to 68%). Notably, the diversity of the nosZII clade but not of the nosZI clade was important to explain the variation of in situ N2O emissions fractions (Figure 1). I. Landscape Functioning – Microbes in the Landscape 61 Figure 1. Variation partitioning of in situ N2O emissions. (a) Variance of in situ N2O emissions was partitioned into soil physicochemical properties (S), abundance of N2O-producers and abundance of N2O-reducers (A), diversity of N2O-reducers (D), and by combinations of predictors. Geometric areas are proportional to the respective percentages of explained variation. The edges of the triangles depict the variation explained by each factor alone, while percentages of variation explained by interactions of two or all factors are indicated on the sides and in the middle of the triangles, respectively. (b) Variance partitioning of basal in situ N2O emissions (25% fraction), (c) variance partitioning of median in situ N2O emissions (50% fraction), (d), (e), (f) and (g) correspond to the variation partitioning of high N2O emissions of 75%, 90%, 95% and 99%, respectively. All numbers represent percentages. Only variance fractions ≥ 0.05% are shown. Conclusions Our results highlight the higher sensitivity of the nosZII- than nosZI-community to environmental factors. However, despite significant variations in the nosZII community across the sites examined, only a few of the studied agricultural practices resulted in shifts on the diversity of this community. Nevertheless, comparison of all plots across the different sites showed for the first time that a higher diversity of the nosZII community was concomitant with lower in situ fluxes. Moreover, our work also indicates that microbial communities were more important for explaining variations in high than in low N2O emissions. This work emphasizes the consideration that the N2O-reducing community should have when addressing process-related N2O fluxes, particularly in studies aiming at mitigating emissions.