Temporal Lags in Post-Rain Greenhouse Gas Cycling and Fluxes from Septic Leach Field Soils and Associated Greenhouse Gas Cycling Microbial Populations

Abstract

Globally, one in six people depends on septic systems for onsite wastewater treatment; however, little is known about leach field system greenhouse gas (GHG) emissions and microbial GHG cycling. Methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) fluxes and subsurface concentrations in leach field soils were monitored for 3 h after a simulated rain event. Greenhouse gas cycling microbial communities were also monitored using the functional genes mcrA and pmoA for CH4 cycling and cnorB and nosZ for N2O cycling and ratios of producers to consumers (i.e., mcrA:pmoA and cnorB:nosZ). Baseline GHG fluxes and subsurface measurements were similar between control and leach field soils on each sampling date; however, emission trends did change over time in both treatments. Over the 4-week period, both leach field (mean±standard deviation −1.3±1.8 mg CH4 m−2 day−1) and control (−12±25 mg CH4 m−2 day−1) soils were net CH4 consumers. However, both control and leach field soils were net producers of CO2 (17,500±4,400 mg CO2 m−2 day−1 and 25,000±11,000 mg CO2 m−2 day−1, respectively) and N2O (0.38±2.3 and 5.9±10 mg N2 Om−2 day−1, respectively) over the same time period. After the simulated rain event, GHG fluxes were noticeably greater from leach field soils compared with control soils for CO2 and N2O, but not for CH4. Microbial populations did not change substantially over the rain event experiment at the DNA level, but shifts were seen at the transcript level. mcrA:pmoA ratios increased from >2 to >9 in both leach field and control, whereas cnorB:nosZ had the opposite trend, decreasing in both treatments. These results indicate that rain events trigger differential GHG emission trends and suggest that GHG fluxes can be substantially higher than previously reported.