The distribution of anaerobic bacteria along a soil drainage catena

Strict anaerobic culture techniques were used to enumerate the anaerobic bacteria present in three soil sites located along a drainage catena near Blacksburg, Virginia. An anaerobic cooked meat plus 0.5% glucose medium cultured the largest number of anaerobes from the poorly drained soil. The population of obligate anaerobes ranged from 10⁶/ g dry weight soil on the poorly drained soil (% moisture = 112.06) to 10⁵/g dry weight soil on the intermediate soil (% moisture = 34.51) to 10⁴/g dry weight soil on the well drained soil (% moisture 20.81). The population of organisms able to grow anaerobically (facultative plus obligate) ranged from 10⁶/g dry weight soil on the poorly drained site to 10⁵/g dry weight soil on the well drained site. This same population on the poorly drained site was relatively constant over a nine month period with the exception of a sharp rise in early spring. The clostridia constituted at least one third of the obligately anaerobic bacteria present on the poorly drained soil. A sizeable percentage of the obligate anaerobic isolates on this site were either clostridia which formed spores unable to germinate in the medium employed, clostridia which were very pleomorphic in cell shape and gram reaction, or nonsporeforming obligate anaerobes. These results indicate that strict anaerobes and possibly nonsporeforming strict anaerobes exist in soils of different drainage character even though facultative organisms appear to be more successful competitors on the more well drained sites.