Importance of dissolved greenhouse gases leached from soil: insights from the lysimeter network TERENO-SoilCan

Measurements of climate-relevant trace gases from soils are frequently undertaken in contemporary ecosystem studies and substantially contribute to our understanding of greenhouse gas balances of the biosphere. While the great majority of such investigations builds on closed chamber and eddy covariance measurements, where upward gas fluxes to the atmosphere are measured, fewest concurrently consider greenhouse gas dissolution in soil water and leaching losses via the vadose zone to the groundwater. Here we present annual leaching losses of dissolved nitrous oxide (N2O), carbon dioxide (CO2) and dissolved organic carbon (DOC) from arable, grassland, and forest lysimeter soils from three sites differing in altitude and climate. We aim to assess their importance in comparison to direct N2O emission, soil respiration, further leaching parameters of the C- and N cycle, and finally to the greenhouse gas balance.MethodsThe lysimeters are part of the Germany-wide lysimeter network initiative TERENO-SoilCan, which investigates feedbacks of climate change to the pedosphere on a long-term scale. Soil water samples were collected weekly from different depths of the profiles by means of suction cups. A laboratory pre-experiment proved that no degassing occurred under those sampling conditions. We applied the headspace equilibration technique to determine dissolved gas concentrations by gas chromatography.ResultsSoil solution and seepage water of all lysimeters were consistently supersaturated with N2O and CO2 compared to water equilibrated ambient air. In terms of N2O, leaching losses increased in the ascending order forest, grassland, and arable soils, respectively. In case of the latter soils, we observed a strong variability of N2O, with dissolved concentrations up to 87 μg N L-1. However, since seepage discharge of the arable lysimeters was comparatively small and mostly limited to the hydrological winter season, leached N2O appeared to be less important than direct N2O emissions. Based on dissolved CO2 concentrations, dissolved organic carbon, and seepage water discharge, our measurements revealed annual leaching losses of 6.8, 8.8, and 2.3 mg C m-2 from the forest, grassland and arable lysimeter soils, respectively.ConclusionsIn conclusion, the results of the first year of our investigations provide evidence that dissolved greenhouse gases should be considered in studies which aim to assess full greenhouse gas balances, particularly in ecosystems with small net ecosystem greenhouse gas budgets where hydrological conditions favor microbial activity and high leaching losses