Three-dimensional spatial patterns of trace gas concentrations in baseflow-dominated agricultural streams: Implications for surface-ground water interactions and biogeochemistry

Small streams that drain agricultural landscapes have come under close scrutiny as potentially significant indirect sources of greenhouse gases (GHGs) to the atmosphere. By exploring the stream-ground water connection in three dimensional space (horizontally and vertically beneath the stream channel, and longitudinally along the stream corridor) our results show (1) ground water can be a significant source of greenhouse gases to streams draining agricultural watersheds with concentrations in excess of atmospheric equilibrium by 221 μmol C L ^-1 carbon dioxide, 0. 64 μmol C L ^-1 methane, and 0. 65 μmol N L ^-1 nitrous oxide (N ₂O); (2) changes in the stream-ground water connection can create seemingly erratic patterns in GHG concentrations over short longitudinal distances (order of meters); (3) soil-stream interfaces are hotspots for denitrification and methanogenesis; however, no significant N ₂O production was observed at such an interface under a riparian forest; and (4) nitrate (NO ₃ ^-) and N ₂O can be preserved as electron acceptors in oxic ground waters draining agriculture landscapes; hence, soil nitrification was the major source of N ₂O to stream water, with a legacy in ground water dating back to the 1960s; N ₂O tracked the seepage of NO ₃ ^- into surface waters. In this study, we demonstrate the utility of detailed measurements of multiple trace gases towards revealing spatial and temporal patterns of surface-ground water interactions and biogeochemistry across several small baseflow-dominated stream ecosystems in central Wisconsin, USA.