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Relationship Between Land-Use Practices and Fecal Bacteria in Soils¹

ABSTRACT

Fecal and other bacteria populations in soils from pasture, cornfield, and forest sites were assessed at the Rhode River watershed. Total coliform (TC), fecal coliform (FC), total streptococci (TS), fecal streptococci (FS), and aerobic heterotrophic bacteria (TVC) numbers were determined seasonally. They were also determined with depth to 25 cm in soils from the pasture. Bacterial numbers per gram of pasture soil from the surface 1 cm were: 252-1,603 TC, 10-3% FC, 317-2,270 TS, 163-1,330 FS, and 1.6-65.1 x 10⁶ TVC. The numbers declined markedly with depth, except that TS and TVC numbers remained near the surface values. Fecal coliform numbers varied seasonally, from 22 to 396 MPN/g soil in fall and spring when cattle were grazing, and from 10 to 50 MPN/g soil in winter and summer when cattle were absent from the pasture. The cornfield and forest surface soils contained equally low FC numbers ranging from 3 to 27 MPN/g soil. Aerobic heterotrophic bacteria were similar in magnitude at all three land-use sites. No readily discernable seasonal pattern was noted. The proportion of coliform bacteria differed with land use. In pasture soils 67% of isolates were confirmed as Escherichia coli, 22% were Enterobacter aerogenes, and 11% belonged to other species. In cornfield soils only 8% were E. Coli, 77% E. aerogenes, and 15% other species. In forest soils only E. coli were the dominant species. Thus, the pro- portion of E. coli was directly related to the degree of fecal pollution of the soils, and the proportion of E. aerogenes increased with remoteness of the soil from potential polluting agents.

Key Words: pasture • forest and cornfield soils • total and fecal coliforms • total and fecal streptococci • aerobic heterotrophic bacteria • soil discharge and retention

¹ Research supported in part by the U.S. Environmental Protection Agency, Grant no. R-804536-01, and the Smithsonian Institution's Environmental Science Program.

² Microbial ecologist, Smithsonian Institution-Chesapeake Bay Center for Environmental Studies, P.O. Box 28, Edgewater, MD 21037.

Received for publication January 10, 1981.