Implication of elevated atmospheric carbon dioxide for greenhouse gas exchanges from forest floor and tree stems

Globally, natural ecosystems including forests are responsible for ~60% of the total nitrous oxide
(N2O) gas emitted from soils, and consume ~25% of the total atmospheric methane (CH4). N2O and
CH4 are potent greenhouse gases, thus playing a critical role in global warming. However, the
magnitude of greenhouse gas fluxes from mature temperate deciduous forests under elevated
atmospheric CO2 (eCO2) has never been studied before to accurately predict global warming
potential of forests under future climates.

As soil water and carbon availability, and demand for soil nitrogen by trees increases in forests
exposed to eCO2, thus significant shifts in N2O and CH4 fluxes under future climates is expected.
Unraveling the mechanistic controls of greenhouse gas production is urgently needed to accurately
model the balance between carbon uptake by forests against greenhouse gas emission. The aim is to
disentangle the impact of eCO2 on greenhouse gas production and consumption processes and its
transport across soil surface and tree stems into air to reduce uncertainty when predicting the role
of forests in global warming.

It is hypothesized that an increase in soil water, carbon and demand for soil nitrogen by trees will
down-regulate gross N2O production and enhance N2O reduction , thus reducing net N2O
emissions. As CH4 flux constitutes a balance between microbial uptake under oxic and production
under anoxic conditions; we hypothesize that under increased soil water conditions, net CH4 uptake
in forest soils will decrease thus providing a positive feedback to global warming. Since the water
budget of trees under eCO2 is altered, thus we hypothesize that trees mediated transport of
greenhouse gases from soil to air relative to fluxes from forest-floor may change warranting
partitioning to avoid under/over estimation.

This research sites at the interface of and complementing on-going and planned research in soil and
tree water and CO2 fluxes, root dynamics, invertebrates ecology, and photosynthesis at BIFoR-FACE,
to unravel previously unexplored pathways of greenhouse gas production in mature forests under
eCO2. The cross disciplinary outcomes links to global scale modelling of carbon and nitrogen budgets
of forests under future climates, which is aligned to the grand ethos of Forest Edge.

This research will be undertaken at the only global Free Air CO2 Enrichment experiment in a mature
temperate forest under the Birmingham Institute for Forest Research (BIFoR). Soil greenhouse
production and consumption will be investigated through isotope tracing techniques (15N and 13C),
while soil profiles gas concentration and fluxes from forest floor and trees will be characterized using
automated and manual cover-chambers. Soil properties including whole tree water budgets will be
characterized using TDR probes, tree-sap flows and weather sensors to couple biogeochemical and
hydrological processes at similar spatio-temporal scales, and to enable extrapolation of the
outcomes to global forests.