Assembly of root-associated N(2)O-reducing communities of annual crops is governed by selection for nosZ clade I over clade II

The rhizosphere is a hotspot for denitrification. The nitrous oxide (N(2)O) reductase among denitrifiers and nondenitrifying N(2)O reducers is the only known N(2)O sink in the biosphere. We hypothesized that the composition of root-associated N(2)O-reducing communities when establishing on annual crops depend on soil type and plant species, but that assembly processes are independent of these factors and differ between nosZ clades I and II. Using a pot experiment with barley and sunflower and two soils, we analyzed the abundance, composition, and diversity of soil and root-associated N(2)O reducing communities by qPCR and amplicon sequencing of nosZ. Clade I was more abundant on roots compared to soil, while clade II showed the opposite. In barley, this pattern coincided with N(2)O availability, determined as potential N(2)O production rates, but for sunflower no N(2)O production was detected in the root compartment. Root and soil nosZ communities differed in composition and phylogeny-based community analyses indicated that assembly of root-associated N(2)O reducers was driven by the interaction between plant and soil type, with inferred competition being more influential than habitat selection. Selection between clades I and II in the root/soil interface is suggested, which may have functional consequences since most clade I microorganisms can produce N(2)O.