Contrasting effects of water deficits and rewetting on greenhouse gas emissions in two grassland and forest ecosystems

Climate change is expected to increase the frequency and intensity of water deficits and extreme rainfall events in temperate regions, with significant effects on greenhouse gas (GHG) emissions. In this study, we investigated the impact of water deficits and drying and rewetting events on GHG fluxes in two Irish sites with adjacent forest and grassland ecosystems. We deployed rain-out shelters to simulate drought and applied water to mimic the extreme precipitation events. The effects of warming on these events were also examined using soil cores collected from the field. Water deficits increased carbon dioxide (CO2) emissions at the evergreen coniferous forest site but decreased it at the broadleaf deciduous forest site, likely due to differences in the prevailing soil moisture contents and the availability of oxygen for microbial activity. Rewetting triggered pulses of CO2 (1.1 - 7.2 fold), methane (CH4) (> 20 fold), and nitrous oxide (N2O) (3.3 - 71.7 fold) emissions in both ecosystems. Warming amplified the effects of water additions, leading to a 1.9 - 3.4-fold increase in CO2 and N2O fluxes, compared to the pre-wetting levels and a 1.2 - 1.5-fold increase compared to the controls. Cumulative CO2 emissions over 24 hours showed a negative response to increasing soil moisture and a positive response to the changes in soil moisture (difference between the initial value before water addition and the final soil moisture after water addition). CH4 fluxes exhibited an opposite trend. Multiple linear regression revealed that at higher soil carbon concentrations CO2 emissions were reduced but CH4 emissions increased, for the same change in soil moisture. Given that future climate scenarios predict an increase in extreme rainfall events a better understanding of the influence of soil drying-rewetting events on GHG emissions is required that accounts for multiple influencing factors, including differences in regional and site characteristics.