Diurnal patterns in methane fluxes vary along a boreal hydrological gradient

Abstract. Methane (CH₄) exchange in peatland landscapes is strongly controlled by hydrological conditions and exhibits pronounced spatial and seasonal variability. However, much less is known about how CH₄ exchange varies over the course of a day and whether the drivers of diurnal variability differ across hydrological gradients. We investigated seasonal and diurnal CH₄ flux dynamics across a hydrological gradient in the Siikaneva peatland complex, Finland, comprising a well-drained upland forest, an intermediate transition site, and an ombrotrophic bog. Automated chamber measurements were used to quantify CH₄ fluxes at high temporal resolution over an annual cycle and to identify environmental controls on diurnal variability.

Methane exchange ranged from persistent daily mean uptake of up to −6.6 mg CH₄ m^-2 d^-1 at the upland forest to persistent emissions of up to 108.5 mg CH₄ m^-2 d^-1 at the bog. All sites exhibited pronounced diurnal variability, but the direction and seasonal expression of these patterns varied along the hydrological gradient. The bog and intermediate sites generally showed greater CH₄ emissions or weaker uptake during nighttime than daytime, whereas the upland forest exhibited stronger CH₄ uptake at night. While the bog remained a net CH₄ source throughout the year, the intermediate site shifted seasonally between continuous uptake during autumn and winter and daytime uptake combined with nighttime emissions during spring and summer. Maximum day–night differences reached 146.0 mg CH₄ m^-2 d^-1 at the bog but only 4.2 and 6.5 mg CH₄ m^-2 d^-1 at the upland and intermediate sites, respectively.

The environmental controls on diurnal CH₄ variability differed systematically among sites. At the upland forest, CH₄ flux variability was most strongly associated with ecosystem respiration (ER), ambient atmospheric CH₄ concentrations, and air temperatures, potentially reflecting links between ecosystem carbon cycling and methanotrophic activity and enhanced near-surface CH₄ concentrations under stable nocturnal boundary-layer conditions. At the intermediate site, CH₄ variability was associated with ambient CH₄, PAR, and an interaction between shallow soil temperature and soil moisture, indicating that temperature effects on CH₄ exchange became stronger under wetter conditions. This may reflect greater sensitivity of CH₄ oxidation to diurnal temperature fluctuations when high water tables restrict the aerobic layer. At the bog, CH₄ variability was most strongly associated with ER, soil temperature, and water table position, consistent with interactions between ecosystem carbon supply, temperature-sensitive CH₄ production and oxidation, and hydrological control of their balance. Together, the results suggest that processes governing short-term CH₄ variability differ from those controlling seasonal mean fluxes and that CH₄ oxidation can contribute substantially to diurnal variability even at sites where CH₄ production dominates net CH₄ exchange.

Our findings indicate that neglecting diurnal variability can substantially influence CH₄ budget estimates and apparent differences among ecosystems, highlighting the importance of high-frequency measurements for both process understanding and accurate quantification of ecosystem-scale CH₄ exchange across heterogeneous boreal landscapes.