Estop-Aragonés, C; Zajac, K; Blodau, C: Effects of extreme experimental drought and rewetting on CO2 and CH4 exchange in mesocosms of 14 European peatlands with different nitrogen and sulfur deposition, Global Change Biology(22), 2285-2300 (2016), online: 2016-03-04, doi:10.1111/gcb.13228
Key words: CO2 exchange; CH4 exchange; drought; humification; mesocosms; nitrogen deposition; peatland; sulfur deposition;

The quantitative impact of intense drought and rewetting on gas exchange in ombrotrophic bogs is still uncertain. Inparticular, 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 CH4fluxes during an initial wet, a pro-longed dry (~100 days), and a subsequent wet period (~230 days) at 12°Cin14Sphagnumpeat mesocosms collectedin hollows from bogs in the UK, Ireland, Poland, and Slovakia. The relationship of N and S deposition with GPP, res-piration, and CH4exchange was investigated. Nitrogen deposition increased CO2fluxes and GPP more than respira-tion, at least up to about 15 kg N ha1yr1. All mesocosms became CO2sources during drying and most of themwhen the entire annual period was considered. Response of GPP to drying was faster than that of respiration andcontributed more to the change in NE; the effect was persistent and few sites recovered “predry” GPP by the end ofthe wet phase. Respiration was higher during the dry phase, but did not keep increasing as WT kept falling andpeaked within the initial 33 days of drying; the change was larger when differences in humification with depth weresmall. CH4fluxes strongly peaked during early drought and water table decline. After rewetting, methanogenesisrecovered faster in dense peats, but CH4fluxes remained low for several months, especially in peats with higher inor-ganic reduced sulfur content, where sulfate was generated and methanogenesis remained suppressed. Based on arange of European sites, the results support the idea that N and S deposition and intense drought can substantiallyaffect greenhouse gas exchange on the annual scale.

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