Characterizing the aerobic and anaerobic microbial activities in surface and subsurface soils

Abstract

AbstractStudies were conducted to characterize the aerobic and anaerobic microbial activities in surface and subsurface soils of septic tank tile fields. Under aerobic conditions, the indigenous microflora in the soils degraded the natural substrates and many of the synthetic chemicals tested. Half‐lives (t1/2) for the biodegradation (14CO2 production) of glucose and glutamic acid ranged from 2 to 5 h in surface soils and from 8 to 25 h in subsurface soils. Degradation of stearic acid was slower in both surface (t1/2 = 118 h) and subsurface (t1/2 = 82 to 312 h) soils. 14CO2 evolution from the synthetic chemicals methyl parathion and 2,4‐D was more variable than that from the natural substrates and occurred in the surface soil but in only one of the two subsurface soils tested. The aerobic biodegradation of nitrilotriacetate (NTA) was studied only in the two subsurface soils from the tile fields. NTA was mineralized in both soils and the half‐life for 14CO2 production ranged from 86 to 161 h. When the surface and subsurface soils were incubated under anaerobic (nitrate‐reducing) conditions, active denitrification by the indigenous microflora was observed. In surface soils, denitrification occurred immediately after an anaerobic preincubation (12 to 24 h) and the N2O production curve was linear. In subsurface soils, N2O production occurred only after 32 to 44 h of anaerobic incubation and was described by an S‐shaped curve. Biodegradation of the natural substrates (glucose, glutamic and acetic acid) under these anaerobic conditions ranged from 1 to 2 h for the surface soil to 2 to 20 h for subsurface soils. The half‐life for the anaerobic mineralization of NTA ranged from 49 to 125 h in the subsurface soils. The results indicate that the indigenous microflora in the subsurface have lower activities than do those in surface soils; however, the subsurface microflora can effectively mineralize natural and certain synthetic substrates under aerobic and denitrifying conditions.