There is ample evidence that microbial processes can exhibit large variations in activity on a field scale. However, very little is known about the spatial distribution of the microbial communities mediating these processes. Here we used geostatistical modelling to explore spatial patterns of size and activity of the denitrifying community, a functional guild involved in N-cycling, in a grassland field subjected to different cattle grazing regimes. We observed a non-random distribution pattern of the size of the denitrifier community estimated by quantification of the denitrification genes copy numbers with a macro-scale spatial dependence (6-16 m) and mapped the distribution of this functional guild in the field. The spatial patterns of soil properties, which were strongly affected by presence of cattle, imposed significant control on potential denitrification activity, potential N(2)O production and relative abundance of some denitrification genes but not on the size of the denitrifier community. Absolute abundance of most denitrification genes was not correlated with the distribution patterns of potential denitrification activity or potential N(2)O production. However, the relative abundance of bacteria possessing the nosZ gene encoding the N(2)O reductase in the total bacterial community was a strong predictor of the N(2)O/(N(2) + N(2)O) ratio, which provides evidence for a relationship between bacterial community composition based on the relative abundance of denitrifiers in the total bacterial community and ecosystem processes. More generally, the presented geostatistical approach allows integrated mapping of microbial communities, and hence can facilitate our understanding of relationships between the ecology of microbial communities and microbial processes along environmental gradients.

The objective of this study was to investigate how changes in soil pH affect the N 2 O and N 2 emissions, denitrification activity, and size of a denitrifier community. We established a field experiment, situated in a grassland area, which consisted of three treatments which were repeatedly amended with a KOH solution (alkaline soil), an H 2 SO 4 solution (acidic soil), or water (natural pH soil) over 10 months. At the site, we determined field N 2 O and N 2 emissions using the 15 N gas flux method and collected soil samples for the measurement of potential denitrification activity and quantification of the size of the denitrifying community by quantitative PCR of the narG, napA, nirS, nirK, and nosZ denitrification genes. Overall, our results indicate that soil pH is of importance in determining the nature of denitrification end products. Thus, we found that the N 2 O/(N 2 O ؉ N 2 ) ratio increased with decreasing pH due to changes in the total denitrification activity, while no changes in N 2 O production were observed. Denitrification activity and N 2 O emissions measured under laboratory conditions were correlated with N fluxes in situ and therefore reflected treatment differences in the field. The size of the denitrifying community was uncoupled from in situ N fluxes, but potential denitrification was correlated with the count of NirS denitrifiers. Significant relationships were observed between nirS, napA, and narG gene copy numbers and the N 2 O/(N 2 O ؉ N 2 ) ratio, which are difficult to explain. However, this highlights the need for further studies combining analysis of denitrifier ecology and quantification of denitrification end products for a comprehensive understanding of the regulation of N fluxes by denitrification.Denitrification is the microbial reduction of NO 3 Ϫ via NO 2 Ϫ to gaseous NO, N 2 O, and N 2 , which are then lost into the atmosphere (36). It therefore results in considerable loss of nitrogen, one of the most limiting nutrients for crop production in agriculture (20). Denitrification is also of environmental concern since, together with nitrification, it is the main biological process responsible for N 2 O emissions (7). N 2 O is a potent greenhouse gas which has a global warming potential about 320 times greater than that of CO 2 and has a lifetime of approximately 120 years (32). In the stratosphere, N 2 O can also react with O 2 to produce NO, which induces the destruction of stratospheric ozone (8). N 2 O can be released into the atmosphere by incomplete denitrification due to the effect of environmental conditions on the regulation of the different denitrification reductases (14, 41, 51), but it has recently been suggested that it could also be due to lack of nitrous oxide reductase in some denitrifiers (19,41). Since N 2 O is an intermediate in the denitrification pathway, both the amount of N 2 O produced and the N 2 O/(N 2 O ϩ N 2 ) ratio are important in understanding and predicting N 2 O fluxes from soils. The main environmental factors known to influence the N 2 O/(N 2 O...

Whether bacteria display spatial patterns of distribution and at which level of taxonomic organization such patterns can be observed are central questions in microbial ecology. Here we investigated how the total and relative abundances of eight bacterial taxa at the phylum or class level were spatially distributed in a pasture by using quantitative PCR and geostatistical modelling. The distributions of the relative abundance of most taxa varied by a factor of 2.5-6.5 and displayed strong spatial patterns at the field scale. These spatial patterns were taxon-specific and correlated to soil properties, which indicates that members of a bacterial clade defined at high taxonomical levels shared specific ecological traits in the pasture. Ecologically meaningful assemblages of bacteria at the phylum or class level in the environment provides evidence that deep branching patterns of the 16S rRNA bacterial tree are actually mirrored in nature.