Scaling up of methane flux: a case study in the UK uplands

In the context of current and any future climate change, methane (CH4) is an important greenhouse gas (GHG). However, current global trends of changes in atmospheric CH4 are unpredictable and the relative contributions of individual global sources and sinks are inadequately quantified. If net CH4 emissions are to be reduced, an improved understanding of key components, including natural wetlands, is required. A study was conducted in order to construct an annual landscape estimate of CH4 flux for a typical UK blanket bog site near to Lake Vyrnwy, North Wales. Flux measurements were made following an established chamber method and sampling was maintained throughout a calendar year and was spatially stratified by vegetation to facilitate landscape extrapolation. In order to identify which environmental variables controlled CH4 fluxes from the blanket bog, regression analyses were performed using environmental variables measured at the time of flux measurement. In order to identify the longer term influence of environmental conditions, regression analyses were also conducted with running averages of measurements from periods prior to the day of flux measurement. A series of in situ experiments were undertaken to test hypotheses which examined the different controls on the observed variation in CH4 fluxes, related to temporal and spatial patterns of CH4 flux and to putative biases of sampling methods due to the limited footprint of chambers. CH4 fluxes displayed a distinct seasonal pattern with low mean fluxes from January until June, when a dramatic increase in net methane emission occurred; higher CH4 fluxes continued until November and December. The site was a net source of CH4 and the best landscape estimate of CH4 flux (± standard error of the mean) was 9.8 (±3.8) g CH4 m-2 year 1. Errors associated with the extrapolation of measurements to a landscape-scale estimate resulted in estimates that ranged from 8.6 (±3.7) to 11.1 (±3.8) g CH4 m 2 year-1. Soil temperature and water table were the environmental variables which were most consistently associated with CH4 fluxes. However, the relationship between fluxes and water table did not always control CH4 fluxes in an expected manner. At some sites CH4 emissions were lower when the water table was closer to the surface, a result which contradicted the acrotelm-catotelm model of CH4 flux, but may be explained by the hysteresis of fluxes in response to changing water table. Strong hysteresis of CH4 fluxes was also apparent in response to temperature and radiation. Hourly measurements of CH4 flux showed high variability but no significant difference between measurements during day and night. Similarly, a replicated landscape-scale experiment of water table manipulation was expected to cause changes in CH4 flux but, despite controlling for other aspects of spatial variation, the manipulation had no significant effect on CH4 fluxes. Flux estimates were made using chambers with footprints that varied by three orders of magnitude and there was no significant effect on mean CH4 fluxes. However, the variance of CH4 flux estimates strongly correlated with sample area with markedly smaller variance as chamber size increased. Overall estimates of landscape CH4 flux were in the range of previous estimates made for UK peatland sites, but virtually all estimates displayed high variability. Such variability constrained the comparison of different estimates but it is possible to use methods, such as chambers with very large footprints, to improve the results of in situ studies.