Carbon flux response and recovery to drought years in a hemi-boreal peat bog between different vegetation types

Peatlands are a globally important store of approximately 500 Gt carbon (C), with northern blanket bogs accumulating ca. 23 g C m^-2 y^-1 from undecomposed organic material due to prevailing cool wet conditions. As a sink of carbon dioxide (CO₂) they act as an important brake on anthropogenic climate change, but in the warming climate the likelihood of drought will increase. However, it is unknown how drought will affect the GHG balance of peatlands: dryer, warmer conditions will likely reduce net ecosystem exchange (NEE) of CO₂ and increase soil respiration, potentially tipping these landscapes from sinks to sources of C. High water tables mean blanket bogs are major source of methane (CH₄), an important greenhouse gas (GHG) with a global warming potential (GWP) 34 times that of CO₂ over 100 years, but this may change in the future climate. It is further expected that the changing climate will alter blanket bog species composition, which may also influence the GHG balance, due to differences in plant traits such as those which form aerenchyma, e.g. Eriophorum vaginatum (eriophorum) and non-aerenchymatous species, e.g. Calluna vulgaris (heather). In order to understand how these important C stores will respond to climate change, it is vital to measure GHG responses to drought at the species level.   

We used an automated chamber system, SkyLine2D, to measure NEE and CH₄ fluxes near-continuously from an ombrotrophic blanket peat bog. Five general ecotypes were identified: sphagnum (Sphagnum spp), eriophorum, heather, water and mixtures of species, with five replicates of each sampled. We followed the fluxes of CO₂ throughout 2017- 2019 and CH₄ throughout 2017- 2018, hypothesising that GHG fluxes would significantly differ between ecotypes. In 2018, the bog experienced drought conditions, allowing the comparison of NEE between drought and non-drought years, and the potential to recover the following year. Contemporaneous measurements of environmental variables were collected to infer details regarding the drivers of GHG fluxes.

We found significant differences in CH₄ emissions between ecotypes, F= 2.71, p< 0.02, ordered high to low: eriophorum > sphagnum > water > heather> mix, ranging from ca. 1.5 mg CH₄-C m^-2 d^-1 to 0.5 mg CH₄-C m^-2 d^-1. There were no significant differences in NEE between ecotypes, F= 0.54, p> 0.7, however, under 2018 drought conditions all ecotypes were net sources of CO₂. We will also present NEE from 2019, when precipitation levels returned to typical conditions. Our results indicate that drought and shifts in vegetation composition under future climate may alter the C balance of hemi-boreal and potentially act as a positive feedback to climate change in a long-term scenario.