Constraining temporal and spatial variations of methane flux in a temperate wet woodland

Atmospheric methane (CH₄) concentrations are rising globally, and evidence suggests natural sources may be responsible. Forests represent the largest terrestrial sink in the global CH₄ budget, however CH₄ emissions from certain forest ecosystems – wet woodlands (i.e. forested wetlands) – remain poorly constrained. Due to their hydrology, the anoxic soils in wet woodlands provide suitable conditions for methanogenesis. Little is known about the spatial or temporal patterns of CH₄ flux in these ecosystems, and the environmental variables that drive these, due to insufficient understanding of biogeochemical mechanisms and limited observations.

To address this, we used hourly-resolved automatic chambers, complimented by a greater expanse of monthly manual chambers to compare CH₄ and carbon dioxide (CO₂) flux to soil parameters in a temperate wet woodland (Wakehurst, Sussex, UK). From observations over two years, we show that soil temperature is the dominant control of CH₄ flux from the wet woodland soil once within high soil moisture (>40%) or water table depth (WTD) (< 0.2m); at lower moisture, changes in WTD and moisture determine CH₄ flux. Large seasonal variations were present, where CH₄ emissions peaked in summer months (44.05 ±1.15 nmolm^-2s^-1 (mean)), and reduced in winter (7.54 ± 0.078 nmolm^-2s^-1 (mean)), with measurements in drier soil moving from source to sink. A diurnal cycle in CH₄ flux positively correlated with soil temperature was revealed, with diurnal and seasonal variation comparable in magnitude, highlighting the importance of high temporal resolution flux measurements. Diurnal cycles changed significantly on the hottest days (>90^th percentile soil temperature), with diurnal amplitudes of CH₄ higher (~100 ppb) than the general trend (~20 ppb).

The large spatial, seasonal and diurnal variability in methane flux we report are significant for quantifying and understanding CH₄ emissions from these small fragmented forest ecosystems, which are currently highly uncertain or missing in model estimates. The relationship to soil temperature suggests rising summer temperatures may lead to an increase in summer CH₄ emissions in future climate scenarios, and highlights the importance of constraining and understanding this ecosystem within the global CH₄ budget.