Microbial communities controlling methane and nutrient cycling in leach field soils by C.P. Fernandez-Baca, CSI member Ruth Richardson, and colleagues; email: rer26@cornell.edu
Septic systems inherently rely on microbial communities in the septic
tank and leach field to attenuate pollution from household sewage.
Operating conditions of septic leach field systems, especially the
degree of water saturation, are likely to impact microbial
biogeochemical cycling, including carbon (C), nitrogen (N), and
phosphorus (P), as well as greenhouse gas (GHG) emissions to the
atmosphere. To study the impact of flooding on microbial methane (CH4)
and nutrient cycling, two leach field soil columns were constructed.
One system was operated as designed and the other was operated in both
flooded and well-maintained conditions. CH4 emissions were significantly higher in flooded soils (with means between 0.047 and 0.33 g CH4 m-2 d-1) as compared to well-drained soils (means between -0.0025 and 0.004 g CH4 m-2 d-1). Subsurface CH4 profiles were also elevated under flooded conditions and peaked near the wastewater inlet. Gene abundances of mcrA, a biomarker for methanogens, were also greatest near the wastewater inlet. In contrast, gene abundances of pmoA, a biomarker for methanotrophs, were greatest in surface soils at the interface of CH4 produced subsurface and atmospheric oxygen. 16S rRNA, mcrA, and pmoA
amplicon library sequencing revealed microbial community structure in
the soil columns differed from that of the original soils and was driven
largely by CH4 fluxes and soil VWC. Additionally, active
microbial populations differed from those present at the gene level.
Flooding did not appear to affect N or P removals in the soil columns
(between 75 and 99% removal). COD removal was variable throughout the
experiment, and was negatively impacted by flooding. Our study shows
septic system leach field soils are dynamic environments where CH4
and nutrients are actively cycled by microbial populations. Our results
suggest proper siting, installation, and routine maintenance of leach
field systems is key to reducing the overall impact of these systems on
water and air quality.
