Agricultural soils: A sink or source of methane across the British Isles?

Abstract

This study summarizes a large diverse dataset of methane (CH 4 ) fluxes measured from agricultural sites across the British Isles. A total of 53,976 manual static chamber measurements from 27 different sites were investigated to determine the magnitude of CH 4 fluxes from a variety of agricultural fields across the UK and Ireland. Our study shows that contrary to some studies, agricultural soils (both arable and grassland) are small net emitters of CH 4 rather than sinks.Mean fluxes measured from arable and grassland sites (excluding fertiliser and tillage events) were 0.11 ± 0.06 and 0.19 ± 0.09 nmol m −2 s −1 , respectively, and were not found to be significantly different (Welch t-test, p = 0.17). Using the values reported in this study, we estimate that an annual emission of 0.16 and 0.09 Mt of CO 2 -eq is expected from arable and grassland agricultural soils in the UK and Ireland (comparable to 0.3 and 0.7% of the current annual CH 4 emission inventories, respectively). Where CH 4 uptake occurs in soils, it is negligible compared to expected emissions of the application of animal manures and tillage events, which were both found to significantly increase CH 4 emissions in the immediate few days to months after events. Our study highlights that there are significant differences in CH 4 uptake and emissions between sites, and that these differences are partially the result of the moisture content of the soil (i.e., the aerobic status of the soil). We expect uptake of CH 4 to be more prevalent in drier soils where volumetric water content does not exceed 35% and emissions to be exponentially greater where agricultural fields become waterlogged.

Greenhouse Gas Emissions and Crop Yields From Winter Oilseed Rape Cropping Systems are Unaffected by Management Practices

“…Higher soil water content inhibits the diffusive transport of CH 4 and oxygen to active microbial sites, consequently reducing CH 4 uptake in arable soils (Flessa et al, 1995;Drewer et al, 2012). The results of this study are consistent with the recent review by Cowan et al (2020) who found soil volumetric water content as the strongest predictor of CH 4 emissions across agricultural soils in the United Kingdom and Ireland (R 2 < 0.1). This indicates that soil water content and its impact on diffusion processes may override the effect of tillage management in regulating CH 4 exchange in arable soils.…”

Comparing Soil Nitrous Oxide and Methane Fluxes From Oil Palm Plantations and Adjacent Riparian Forests in Malaysian Borneo

“…Results from a comprehensive analysis of a large number of studies (53,976 manual static chamber measurements from 27 different sites) from agricultural mineral soil in temperate regions concluded that agricultural soils can be small sinks of CH 4 (Cowan et al, 2021). We therefore conclude that in order to make a comparable and more robust assessment of CH 4 fluxes a much larger network of sites is required.…”

Carbon monoxide fluxes measured using the eddy covariance method from an intensively managed grassland in Ireland

“…Similar findings have been reported for soil derived methane (CH 4 ) emissions from agricultural soils, where the presence of a high soil moisture content (>80% VWC) hinders the microbial consumption of CH 4 by creating anaerobic soil conditions. 51 The highest rates of monthly net uptake of CO were measured in 2019, namely in August (−6.1 ± 2.8 mg CO–C per m 2 per month) and September (−10.1 ± 2.4 mg CO–C per m 2 per month). Higher net uptake during these months is likely due to the presence of favourable conditions for CO consumption by microbial communities such as high temperatures (13.8–15.8 °C) and adequate soil moisture (46.5–56%) and rainfall (32.7–122.2 mm), in combination with possible CO deposition from adjacent sources for example, car exhausts from the main road located <150 meters from the field site as well as from farm machinery exhausts.…”