Landscape position influences microbial composition and function via redistribution of soil water across a watershed.

Subalpine forest ecosystems influence global carbon cycling. However, little is known about the compositions of their soil microbial communities and how these may vary with soil environmental conditions. The goal of this study was to characterize the soil microbial communities in a subalpine forest watershed in central Montana (Stringer Creek Watershed within the Tenderfoot Creek Experimental Forest) and to investigate their relationships with environmental conditions and soil carbonaceous gases. As assessed by tagged Illumina sequencing of the 16S rRNA gene, community composition and structure differed significantly among three landscape positions: high upland zones (HUZ), low upland zones (LUZ), and riparian zones (RZ). Soil depth effects on phylogenetic diversity and β-diversity varied across landscape positions, being more evident in RZ than in HUZ. Mantel tests revealed significant correlations between microbial community assembly patterns and the soil environmental factors tested (water content, temperature, oxygen, and pH) and soil carbonaceous gases (carbon dioxide concentration and efflux and methane concentration). With one exception, methanogens were detected only in RZ soils. In contrast, methanotrophs were detected in all three landscape positions. Type I methanotrophs dominated RZ soils, while type II methanotrophs dominated LUZ and HUZ soils. The relative abundances of methanotroph populations correlated positively with soil water content (R = 0.72, P < 0.001) and negatively with soil oxygen (R = -0.53, P = 0.008). Our results suggest the coherence of soil microbial communities within and differences in communities between landscape positions in a subalpine forested watershed that reflect historical and contemporary environmental conditions.