Title:

Examining the Impact of Shrub Encroachment on Methane Production and Consumption in Northern Peatlands

Peatlands are a globally important carbon store, and climate and environmental change threatens the stability of the approximately 500Gt of carbon stored in peatlands. Increasing air and peat temperature may increase emissions of methane (CH4), a powerful greenhouse gas, from temperate and northern peatlands. Encroachment of shrubs and other woody vegetation can increase carbon losses by drying peatlands, therefore exposing these carbon stores to aerobic decomposition and emission as carbon dioxide (CO2). However, little is known about the impact of shrub encroachment on CH4 cycling in peatland ecosystems. We examined how shrub encroachment affects the microbial production (methanogenesis) and consumption (methanotrophy) of CH4 in Sallie’s Fen, a well-studied a 1.7 hectare temperate fen in Barrington, NH, USA (43°12.50N, 71°3.50W), using (a) incubation experiments and (b) 16s rRNA iTag data from the Global Peatland Microbiome Project. Methane consumption was higher in sites with small shrubs and lower in sites without shrubs but with abundant sedges. Conversely, CH4 production was higher in sites with abundant sedges but without shrubs and lower in sites with shrubs. Methylocystis was the most abundant methanotrophic genus, with a mean relative abundance of 2.2 ± 2.0% across all samples. Methanogens were less abundant, and the most abundant methanogenic orders Methanomicrobiales and Methanobacteriales had mean relative abundances of 0.36 ± 0.32% and 0.32 ± 0.35%, respectively. While rates of CH4 production and consumption varied by vegetation type and depth, the abundance and community composition of methanogens and methanotrophs only varied significantly between depths. Forthcoming stable isotope analysis may resolve the discrepancy between the effect of vegetation on measured rates of CH4 transformations and microbial abundance, as stable isotopes provide a better proxy for microbial activity in the field.

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