Soil greenhouse gas fluxes in floodplain forests of the Danube National Park: effects of flooding and soil microclimate

The relevance of soil greenhouse gas (GHG) fluxes from temperate floodplain forests has yet remained elusive. We studied the soil methane (CH 4 ), nitrous oxide (N 2 O) and carbon dioxide (CO 2 ) dynamics at three forest sites along a flooding gradient in the Danube National Park (Austria) to estimate annual GHG budgets and to assess if and how seasonal flooding affects individual GHG fluxes. Soil surface GHG fluxes were determined along with GHG concentrations in soil air and pore-water at a non-flooded (NF), an infrequently-flooded (IF), and a frequently-flooded (FF) site. Both study years were characterized by dry summers, and only the FF site was flooded during the study period. Soils at all sites were annual CH 4 sinks (NF: − 4.50 ± 0.85, IF: − 2.54 ± 0.57, FF: − 0.67 ± 1.06 kg CH 4 -C ha −1 year −1 ) and the sink strength correlated positively with soil moisture. Pulse-like CH 4 emissions were not observed during or after flooding. Soil N 2 O fluxes showed large temporal and spatial variations, without any significant differences between sites (average NF: 6.5 ± 7.1, IF: 10.4 ± 14.3, FF: 9.4 ± 10.5 µg N 2 O-N m −2 h −1 ). Pulse N 2 O emissions (up to ~ 80 µg N 2 O-N m −2 h −1 ) occurred during freeze/thaw events, but not during or after flooding. Mean annual soil CO 2 effluxes at NF and IF were 9.4 ± 1.1 and 9.4 ± 2.1 t C ha −1 year −1 , respectively. Soil CO 2 efflux was significantly higher at the FF site (18.54 ± 6.21 t C ha −1 year −1 ). High soil air CO 2 concentrations (> 10%) in aerated deeper soil layers indicated a substantial contribution of the usually waterlogged sub-soils to the summertime soil CO 2 efflux at the FF site. Overall, our results suggest that the studied temperate floodplain forest soils do not absorb/emit substantially more CH 4 and N 2 O than soils of comparable upland forests, whereas low groundwater level can lead to periodically enhanced CO 2 emissions from normally waterlogged soil layers.