Cycling of greenhouse gases as a function of groundwater level in a floodplain - carbon dioxide, nitrous oxide and methane: Implications for Biogeochemical Change in a Warming World

Floodplain sediment-water systems play an important role in carbon dioxide, nitrous oxide and methane greenhouse gas cycling. Changes in temperature and precipitation can alter biogeochemical equilibrium as well as production and consumption of greenhouse gases. We monitored CO2, N2O, CH4 greenhouse gas concentrations and C, O, and N stable isotope variations over a period of 4 years in a cross section of five wells located with increasing distance from the Colorado River. Pore space of partially saturated sediments located above an alluvial aquifer was sampled in vertically resolved profiles from 0.5 m to 3 m depth at a periodicity of one month to 2 weeks. Gas concentrations and stable isotopic signatures show annual-scale fluctuations. From 2013 to 2016 during cold seasons, low δ13C of CO2 ( -24‰) and high δ15N of N2O ( -5‰) and minimum concentrations in CO2 (< 5%v), N2O (< 5ppmv) and CH4 (< 0.5ppmv) coincide with low water table elevation and low temperature. At the beginning of summer, which corresponds to maximum water table elevation, we observed the highest concentrations of N2O ( 50ppmv) and of CO2 ( 5.5%v). Low δ15N ( -16‰) and relatively high δ13C ( -21‰) values were also observed for the summer season. CH4 was observed only in the well closest to the river (7ppmv). The variation of CO2, N2O and CH4 concentrations and δ values suggest changes in reducing/oxidizing microbial activity. Strongest biologically mediated reduction is associated with the highest water table, which typically induces reducing conditions. The maximum water elevation coincides with the annual snowmelt in the Rocky Mountains. Climate change directly impacts on biogeochemical cycling in the floodplain by affecting stream and river water discharge. At local and global scales, a drier and warmer climate will decrease N2O and CH4 production. A wetter climate induces higher stream and river water discharge, which will increase the zone and magnitude of N2O and CH4 production.