Flux of CO2, CH4 and N2O from temperate woodland soil under elevated CO2

Atmospheric fluxes of greenhouse gases (GHG’s) in the form of CO₂, CH₄ and N₂O from temperate forest soils are an important aspect of the net global warming potential and climate change mitigation function of forests. However, it remains unclear how the magnitude of these atmospheric fluxes of GHG’s will respond to rising atmospheric CO₂ concentrations in mature temperate forests. An increase in carbon capture by temperate forests under elevated atmospheric CO₂ concentration (eCO₂) and its subsequent storage in biomass and soils can have direct impact on the activities of soil microbes. In addition to indirect effects through shifting soil moisture regimes, potentially altering GHG production and consumption processes and hence net emissions from temperate forests. The Birmingham Institute of Forest research established a Free Air Carbon Enrichment Facility (BIFoR-FACE) whereby a mature temperate forest in the UK is exposed to +150 ppm CO₂ above the ambient (aCO2), mimicking future CO₂ conditions. Understanding GHG exchange from soils under elevated atmospheric CO₂ levels is critical for addressing this component of the systems response to eCO₂. Fumigation started in 2017 and continues to date, where the ecological and biogeochemical responses of the forest is being studied. In this abstract, the focus is placed on quantifying ~5-years (2019 – 2024) of GHG flux response to eCO₂ to elucidate shifts in fluxes as influenced by eCO₂ and local microclimatic conditions.

The flux of CO₂ from the soil has been continuously measured within fumigated treatment (eCO₂) and ambient control (aCO₂) arrays since 2017 via LI-COR 8100A long-term measurement systems. With capabilities to additionally measure CH₄ and N₂O being added in 2020 through a coupled Picarro-G2508 analyser. Initial trends from 2017 - 2020 indicated that eCO₂ arrays had a higher efflux of CO₂ relative to paired aCO₂ arrays by +20%. However, from 2020 – 2022 a significant decline of -46.6% in the efflux of CO₂ was detected, in addition to a -76.6% reduction in N₂O effluxes and a -44.3% decline in the CH₄ uptake by the soil component. This period corresponds to a significant decline in soil moisture across the soil profile from the surface (0.05m) to a depth of 0.4m, equivalent to a -36% decline in volumetric water content under eCO₂ relative to aCO₂. Which when coupled with the prevalence of drought periods during the growing seasons of 2021 and 2022 suggest an enhanced drying of soil under eCO_2, which is in turn exacerbated by drought events. During 2023 and the wettest July on record for the UK, the moisture deficit between eCO₂ and aCO₂ shrunk, reducing the variance in the efflux of CO₂ to just ~4.5%. Therefore, it is possible that a functional change in the heterotrophic and autotrophic mediated flux dynamic could be occurring, driven by significant soil drying under eCO₂, an affect which is exacerbated during drought events. Inter and intra-seasonal patterns of GHG fluxes will be examined in further detail, whilst also partitioning between autotrophic and heterotrophic contributions.