In Appalachian ecosystems, forest disturbance has long-term effects on biogeochemical processes such as nitrogen (N) cycling. For example, at the Coweeta Hydrologic Laboratory in western North Carolina, N export from several historically clear-cut watersheds remains elevated decades after forest recovery. However, long-term effects of forest disturbance on soil microbial community structure and biogeochemical functions remain poorly understood. To elucidate potential relationships between soil communities and watershed-level biogeochemical cycling, we selected four historically disturbed watersheds and four undisturbed reference watersheds at Coweeta. Historical disturbances included forest clear-cutting, cable-logging, conversion to pasture, and conversion to pine monoculture. In each watershed, we established six plots, collected five 10cm depth soil cores from each plot, and composited soil samples by plot. We determined microbial community structure by Illumina sequencing of the 16s rRNA gene and used sequence data to make metagenomic predictions for genes associated with N-cycling pathways such as nitrification. We also used a 15N stable isotope pool dilution approach to measure gross nitrification rates in collected soils. We predicted that communities from disturbed forest soils would have higher abundance of taxa associated with N-cycling (i.e. Nitrospirae), higher predicted nitrification gene counts, and higher measured rates of nitrification.
Results/Conclusions
Our results show that disturbed soil communities were distinct from reference communities at the phylum level, with disturbed communities having lower abundance of Acidobacteria and higher abundance of Proteobacteria and Nitrospirae. Additionally, disturbed communities showed higher OTU richness and Shannon diversity, likely driven by higher soil pH in historically disturbed forests. Also, multivariate analyses showed that disturbed sites were distinct from reference sites in terms of OTU-level community structure. Metagenomic profiles predicted from sequence data showed higher predicted nitrification gene counts, which were significantly correlated with actual gross nitrification rates measured using 15N pool dilution. These results suggest that disturbance and subsequent vegetation changes associated with forest succession restructure the composition and functional capacity of soil microbial communities, which feeds back into watershed-scale processes such as N export. In future work, we will determine effects of historical disturbance on diversity and ecosystem functions of soil fungal communities.