Wetland soil oxygen (O2) is rarely measured, which limits our understanding of a key regulator of microbial processes responsible for nitrogen loss and greenhouse gas production. We asked: 1) How does soil O2 vary in riparian wetlands?, 2) How does this O2 variation affect denitrification rates?, and 3) How does O2 variation and previous exposure to O2 affect trace gas fluxes? We collected a continuous record O2 and soil moisture data in “wet” and “dry” riparian zones adjacent to a first-order north temperate forest stream in Millbrook, NY, USA. O2 was measured with Apogee O2 sensors connected to dataloggers. We measured denitrification using the Nitrogen-Free Air Recirculation Method (N-FARM) which operates by removing the background di-nitrogen (N2) from a soil core, replacing it with a helium/oxygen mixture, and measuring nitrous oxide (N2O) and N2 production through direct connection to gas chromatographs (GC) with electron capture or thermal-conductivity detectors (ECD/TCD). We measured field trace gas fluxes monthly using the chamber method (n=3 per plot), and analyzed these for carbon dioxide (CO2), N2O and methane (CH4).
Results/Conclusions
In April, soil O2 ranged from 0-13% and consistently increased with increasing distance from the stream. O2 gradually declined until all sensors went anoxic in early September. In mid-fall, a declining water table increased soil O2 to 15-20% within a 2-3 day period. Denitrification rates were similar between the two sites in spring; however, denitrification was 2-3x higher in the wet site in summer and fall. Increasing O2 in soil cores significantly increased N2O production. N2O yield ranged from 0-4% in the wet site and 0-22% in the dry site. O2 may therefore be an important control on N2:N2O ratios. Field N2O fluxes, however, were generally very low ranging from 0-0.75 ng N m-2 hr-1 with no differences between the wet and dry sites. Field CO2 patterns ranged from 2-38 mg C m-2 hr-1 and were significantly lower in the dry site than the wet. Furthermore, the wet site responded more strongly to season, with maximum flux in July and much lower fluxes in spring and fall. Future research should focus on understanding the biotic and abiotic controls on O2 dynamics, and O2 dynamics should be included in models of soil N cycling and trace gas fluxes.
