Gone with the Wind: Rethinking How Water-Filled Pore Space Affects Gaseous Nitrogen Production in Soil-Based Wastewater Treatment.

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.