How will forest soils ‘breathe’ in 2050? Soil respiration of CO2, CH4 and N2O under elevated atmospheric CO2.

Soil respiration is a measure of the flux of greenhouse gases and accounts for the release and uptake of CO₂, CH₄ and N₂O. It is a function of heterotrophic microbial activity through the mineralisation and immobilisation of organic matter and the symbiotic relations formed in the rhizosphere, contributing to the autotrophic component. It is a primary process within woodlands that contribute to the efflux of CO₂, the sink capacity of CH₄ and the variable flux of N₂O. Understanding how woodland soils will react to rising atmospheric CO₂ levels is critical for budgeting, modelling, and providing insightful management strategies for global forests. The Birmingham Institute of Forest Research is a Free Air Carbon Enrichment facility (BIFoR-FACE), whereby the seasonal and diurnal fumigation of CO₂ is closely controlled. Providing localised and enriched atmospheric CO₂ levels across the canopy of a mature temperate Oak dominant woodland that is representative of 2050 levels.

The flux of CO₂ from the soil has been continuously measured within fumigated treatment (eCO₂) and ambient control (aCO₂) arrays since 2017, with capabilities to additionally measure CH₄ and N₂O being added in 2020. Across all arrays, CO₂ fluxes showed significant negative correlations with soil moisture but significant positive correlations with soil temperature. Initial trends from 2017 - 2020 indicated that eCO₂ arrays had a higher efflux of CO₂ relative to paired aCO₂ arrays, with this pattern switching in 2021. During the 2021 and 2022 fumigation seasons, eCO₂ arrays have seen a decline in the efflux of CO₂, to levels lower than aCO₂ plots. Mean values during 2022 for the efflux of CO₂ within eCO₂ arrays were 2.63 μmol m^-2 s^-1 (n = 18762) versus 3.62 μmol m^-2 s^-1 (n = 22856) for aCO₂ arrays. Uptake of CH₄ and efflux of N₂O were not significantly different between arrays, although eCO₂ arrays had lower CH₄ uptake and N₂O efflux. Indicating a potential decline in the efflux of CO₂ but a reduced uptake of CH₄ from temperate woodlands under future atmospheric conditions. Further investigation will now look to understand the mechanistic drivers behind these changes, focusing on the microbial heterotrophic contribution as a potential mediator of these noted flux rates changes under eCO₂