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Subsoil microbiology


The objective is to obtain quantitative information on the microbial activity in the subsoil which will aid in evaluating the ecological risks and the economic and social benefits of pesticide use.
Field stations were built in 3 countries and equipped with weather stations in order to investigate the risks of chemicals to groundwater purity and subsoil microbiology. Soil air and temperature sampling probes were developed and installed at each station and subsoil temperatures and oxygen partial pressures were monitored. The highest variations in temperatures, oxygen concentrations and carbon dioxide concentrations were in the upper 1 m of soil. Below 2 m and down to 8 m, temperatures at all 3 sites were high enough to allow microbial activity throughout the year. The oxygen content in all layers of the unsaturated zones was high enough to allow year round microbial activity. Soil cores were collected down the profiles at each station. The distribution, composition and activity levels of the microflora in the top soil and in the unsaturated zones of the 3 profiles were investigated. Aerobic and anaerobic microorganisms were found at all depths. In general, the numbers of both aerobic and anaerobic bacteria decreased with depth. However, at 2 different sampling sites on the silt loam, bacterial numbers decreased to down to a depth of 3.5 m, and then significantly increased. The microbial biomass of this soil followed a similar pattern. In the other soils, the microbial biomass decreased with increasing depth. With one exception, fungi were not found at any of the sites below 2.5 m. The microflora of the 3 unsaturated zones could readily mineralize various forms of nitrogen, and radiolabelled substrates. They could also degrade and mineralize carbonyl-dichlorophenoxy acetic acid and ring labelled 2,4-dichlorophenoxy acetic acid. In subsoil samples from all 3 sites, there was evidence for adaptation and growth of the microflora on 2,4-dichlorophenoxy acetic acid. Using data from adsorption isotherm and degradation studies, attempts were made at modelling carbofuran degradation in the different layers of the profiles.
In Germany, France and the United Kingdom, field stations, one per country, will be built over aquifers, on frequently occurring agricultural soils, to allow fluxes in prevailing conditions at different depths in the subsoil to be monitored (ie temperature, water potential and the oxygen content of the soil atmosphere). Both classical and new methods will be used to characterize the microflora in homogenized soil samples prepared from cores in 25 cm or 50 cm steps down to the water table. Methods to be used include: colony counts of aerobic bacteria, streptomyces and fungi by plate dilution; counting of anaerobic microorganisms after incubation in chambers depleted of oxygen; counting in soil smears; and analysis of the fungal colonization quotient of soil particles down the soil profile with a soil wash technique. The rate of degradation of naturally occurring carbon and nitrogenous compounds will also be measured.

For comparative purposes, degradation studies with one natural substrate and one radiolabelled pesticide (2,4-D) will be conducted under standardized laboratory conditions. In further tests, degradation of these compounds will also be followed using conditions which simulate those in the individual fields from which the subsoils were collected. These tests will, if possible, also be conducted with a second radiolabelled pesticide (carbofuran and/or atrazine). In addition, the side effects of several chemicals after in situ labelling of the subsoil microflora will be tested.


Bayer AG
Bayerwerk Q18
5090 Leverkusen

Participants (2)

Institut National de la Recherche Agronomique
Rue De Sully
21034 Dijon
United Kingdom
Stanhope Gate 15
W1Y 6LN London