Abstract
Globally peatlands store 500 Gt carbon (C), with northern blanket bogs accumulating 23 g C m−2 y−1 due to cool wet conditions. As a sink of carbon dioxide (CO2) peat bogs slow anthropogenic climate change, but warming climate increases the likelihood of drought which may reduce net ecosystem exchange (NEE) and increase soil respiration, tipping C sinks to sources. High water tables make bogs a globally important source of methane (CH4), another greenhouse gas (GHG) with a global warming potential (GWP) 34 times that of CO2. Warming may increase CH4 emissions, but drying may cause a reduction. Predicted species composition changes may also influence GHG balance, due to different traits such as erenchyma, e.g., Eriophorum vaginatum (eriophorum) and non-aerenchymatous species, e.g., Calluna vulgaris (heather). To understand how these ecosystems will respond to climate change, it is vital to measure GHG responses to drought at the species level. An automated chamber system, SkyLine2D, measured NEE and CH4 fluxes near-continuously from an ombrotrophic fen from August 2017 to September 2019. Four ecotypes were identified: sphagnum (Sphagnum spp), eriophorum, heather and water, hypothesizing that fluxes would significantly differ between ecotypes. The 2018 drought allowed comparison of fluxes between drought and non-drought years (May to September), and their recovery the following year. Methane emissions differed between ecotypes (p < 0.02), ordered high to low: eriophorum > sphagnum > water > heather, ranging from 23 to 8 mg CH4-C m−2 d−1. Daily NEE was similar between ecotypes (p > 0.7), but under 2018 drought conditions all ecotypes were greater sources of CO2 compared to 2019, losing 1.14 g and 0.24 g CO2-C m−2 d−1 respectively (p < 0.001). CH4 emissions were ca. 40% higher during 2018 than 2019, 17 mg compared to 12 mg CH4-C m−2 d−1 (p < 0.0001), and fluxes exhibited hysteresis with water table depth. A lag of 84–88 days was observed between rising water table and increased CH4 emissions. A significant interaction between ecotype and year showed fluxes from open water did not return to pre-drought levels. Our findings suggest that short-term drought may lead to a net increase in C emissions from northern wetlands.
Highlights
Peatlands are found across the northern hemisphere and are a globally important store of approximately 500 Gt carbon (C) (Yu, 2012)
Due to the anaerobic conditions caused by high water tables, these bogs are a major source of methane (CH4) (IPCC, 2014), an important greenhouse gas (GHG) with a global warming potential (GWP) 34 times that of CO2 (IPCC, 2014)
Hotspots of CH4 production are known to occur at suitable microsites within peat soil profiles (Laing et al, 2010) and we suggest that our data indicate that, despite water table dropping as low as 40 cm below the vegetation-air interface, there remained sufficient anaerobic microsites within the soil profile to sustain methanogenesis at all times
Summary
Peatlands are found across the northern hemisphere and are a globally important store of approximately 500 Gt carbon (C) (Yu, 2012). Simulated drought has been shown to decrease the productivity of the common blanket bog species, sphagnum mosses (Sphagnum spp.) (Lees et al, 2019), eriophorum (Eriophorum vaginatum) (Buttler et al, 2015) and heather (Calluna vulgaris) (Ritson et al, 2017). Such a reduction in primary productivity, alongside increased soil respiration (Rs) due to warming (Bond-Lamberty et al, 2004) will, by definition, lead to reduced net ecosystem exchange (NEE) of CO2 (reduced C uptake) and might cause these important C sinks to become sources
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