Effects of extreme experimental drought and rewetting on CO2 and CH4 exchange in mesocosms of 14 European peatlands with different nitrogen and sulfur deposition.

The quantitative impact of intense drought and rewetting on gas exchange in ombrotrophic bogs is still uncertain. In particular, we lack studies investigating multitudes of sites with different soil properties and nitrogen (N) and sulfur (S) deposition under consistent environmental conditions. We explored the timing and magnitude of change in CO2 (Respiration, Gross Primary Production - GPP, and Net Exchange - NE) and CH4 fluxes during an initial wet, a prolonged dry (~100 days), and a subsequent wet period (~230 days) at 12 °C in 14 Sphagnum peat mesocosms collected in hollows from bogs in the UK, Ireland, Poland, and Slovakia. The relationship of N and S deposition with GPP, respiration, and CH4 exchange was investigated. Nitrogen deposition increased CO2 fluxes and GPP more than respiration, at least up to about 15 kg N ha(-1)  yr(-1) . All mesocosms became CO2 sources during drying and most of them when the entire annual period was considered. Response of GPP to drying was faster than that of respiration and contributed more to the change in NE; the effect was persistent and few sites recovered "predry" GPP by the end of the wet phase. Respiration was higher during the dry phase, but did not keep increasing as WT kept falling and peaked within the initial 33 days of drying; the change was larger when differences in humification with depth were small. CH4 fluxes strongly peaked during early drought and water table decline. After rewetting, methanogenesis recovered faster in dense peats, but CH4 fluxes remained low for several months, especially in peats with higher inorganic reduced sulfur content, where sulfate was generated and methanogenesis remained suppressed. Based on a range of European sites, the results support the idea that N and S deposition and intense drought can substantially affect greenhouse gas exchange on the annual scale.