Greenhouse Gases dissolved in soil solution - often ignored, but important?

Flux measurements of climate-relevant trace gases from soils are frequently undertaken in contemporary ecosystemstudies and substantially contribute to our understanding of greenhouse gas balances of the biosphere. While thegreat majority of such investigations builds on closed chamber and eddy covariance measurements, where upwardgas fluxes to the atmosphere are measured, fewest concurrently consider greenhouse gas dissolution in the seepageand leaching of dissolved gases via the vadose zone to the groundwater.Here we present annual leaching losses of dissolved N2O and CO2 from arable, grassland, and forest lysimetersoils from three sites differing in altitude and climate. We aim to assess their importance in comparison to directN2O emission, soil respiration, and further leaching parameters of the C- and N cycle. The lysimeters are part of theGermany-wide lysimeter network initiative TERENO-SoilCan, which investigates feedbacks of climate change tothe pedosphere on a long-term scale. Soil water samples were collected weekly from different depths of the profilesby means of suction cups. A laboratory pre-experiment proved that no degassing occurred under those samplingconditions. We applied the headspace equilibration technique to determine dissolved gas concentrations by gaschromatography.The seepage water of all lysimeters was consistently supersaturated with N2O and CO2 compared to water equilibratedambient air. In terms of N2O, leaching losses increased in the ascending order forest, grassland, and arablesoils, respectively. In case of the latter soils, we observed a strong variability of N2O, with dissolved concentrationsup to 23 g N L1. However, since seepage discharge of the arable lysimeters was comparatively small and mostlylimited to the hydrological winter season, leached N2O appeared to be less important than direct N2O emissions. Interms of dissolved CO2,our measurements revealed considerable leaching losses from the mountainous forest andgrassland soils, based on concentrations up to 24 mg C L1 and high seepage discharge. Such losses turned out tobe similarly important like soil respiration, particularly during winter when temperature-dependent soil respirationdeclined.In conclusion, the results of the first year of our measurements provide evidence that dissolved greenhouse gasesshould be considered in studies which aim to assess full greenhouse gas balances, particularly in ecosystems wherehydrological conditions favour microbial activity and high leaching losses