Phylogenetic and functional potential links pH and N 2 O emissions in pasture soils.

Denitrification is mediated by microbial, and physicochemical, processes leading to nitrogen loss via N2 O and N2 emissions. Soil pH regulates the reduction of N2 O to N2 , however, it can also affect microbial community composition and functional potential. Here we simultaneously test the link between pH, community composition, and the N2 O emission ratio (N2 O/(NO + N2 O + N2 )) in 13 temperate pasture soils. Physicochemical analysis, gas kinetics, 16S rRNA amplicon sequencing, metagenomic and quantitative PCR (of denitrifier genes: nirS, nirK, nosZI and nosZII) analysis were carried out to characterize each soil. We found strong evidence linking pH to both N2 O emission ratio and community changes. Soil pH was negatively associated with N2 O emission ratio, while being positively associated with both community diversity and total denitrification gene (nir &nos) abundance. Abundance of nosZII was positively linked to pH, and negatively linked to N2 O emissions. Our results confirm that pH imposes a general selective pressure on the entire community and that this results in changes in emission potential. Our data also support the general model that with increased microbial diversity efficiency increases, demonstrated in this study with lowered N2 O emission ratio through more efficient conversion of N2 O to N2 .