Jose Adolfo Amador, University of Rhode Island, Kingston, RI, Faith L. Anderson, Dept. of Natural Resources Science, University of Rhode Island, Kingston, RI and Jennifer Cooper, Soil and Water Science, University of Florida, Belle Glade, FL
Water-filled pore space (WFPS) affects substrate and O2 availability, exerting an important control on N removal in soil-based wastewater treatment. Wastewater has high levels of organic C, nutrients and microorganisms, and a low O2 level, is treated in subsurface horizons, and has a residence time in soil on the order of hours. Nevertheless, our understanding of how WFPS affects N removal from wastewater is based on long-term incubation of agricultural surface soils with clean, oxygenated water. We used mesocosms to examine the short-term (1-h) response of N2O and N2 production to changes in WFPS in B (silt loam) and C (very gravelly coarse sand) horizon soil. We used deionized water (DW), sand-filtered effluent (SFE) or septic tank effluent (STE) amended with 15NH4 to adjust WFPS, which provided a range of concentrations of organic C, nutrients, dissolved O2 and microorganisms. Production of 15N2 was 50 to 100x that of 15N2O at nearly all values of WFPS in B and C horizon soil for all water types. Conversion of 15NH4 to N gases was highest at the lowest WFPS value for all water and soil types, and was 4–6 % for DW and 1–2% for STE and SFE. Production varied linearly 15N2 with 15N2O for most water and soil types, suggesting strong coupling of processes. When normalized by the amount of substrate added, the effect of WFPS on 15N2 and 15N2O was greater at the lowest WFPS for all water types and soils tested. The response of N gas production in surface agricultural soil to changes in WFPS does not appear to represent what takes place in soil-based wastewater treatment. Microbial production of N2 and N2O in subsurface horizons happens quickly over a broad range of WFPS values, and both wastewater type and soil texture can affect N gas production.