Methane and nitrous oxide cycling microbial communities in soils above septic leach fields: Abundances with depth and correlations with net surface emissions

摘要

Onsite septic systems use soil microbial communities to treat wastewater, in the process creating potent greenhouse gases (GHGs): methane (CH4) and nitrous oxide (N2O). Subsurface soil dispersal systems of septic tank overflow, known as leach fields, are an important part of wastewater treatment and have the potential to contribute significantly to GHG cycling. This study aimed to characterize soil microbial communities associated with leach field systems and quantify the abundance and distribution of microbial populations involved in CH4and N2O cycling. Functional genes were used to target populations producing and consuming GHGs, specifically methyl coenzyme M reductase (mcrA) and particulate methane monooxygenase (pmoA) for CH4and nitric oxide reductase (cnorB) and nitrous oxide reductase (nosZ) for N2O. All biomarker genes were found in all soil samples regardless of treatment (leach field, sand filter, or control) or depth (surface or subsurface). In general, biomarker genes were more abundant in surface soils than subsurface soils suggesting the majority of GHG cycling is occurring in near-surface soils. Ratios of production to consumption gene abundances showed a positive relationship with CH4emissions (mcrA:pmoA,p < 0.001) but not with N2O emission (cnorB:nosZ,p > 0.05). Of the three measured soil parameters (volumetric water content (VWC), temperature, and conductivity), only VWC was significantly correlated to a biomarker gene,mcrA(p = 0.0398) but notpmoAor either of the N2O cycling genes (p > 0.05 forcnorBandnosZ). 16S rRNA amplicon library sequencing results revealed soil VWC, CH4flux and N2O flux together explained 64% of the microbial community diversity between samples. Sequencing ofmcrAandpmoAamplicon libraries revealed treatment had little effect on diversity of CH4cycling organisms. Overall, these results suggest GHG cycling occurs in all soils regardless of whether or not they are associated with a leach field system.