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We have developed a model system to assess the influence of earthworm activity on the transfer of plasmid pJP4 from an inoculated donor bacterium, Pseudomonas fluorescens C5t (pJP4), to indigenous soil microorganisms. Three different earthworm species (Lumbricus terrestris, Lumbricus rubellus, and Aporrectodea trapezoides), each with unique burrowing, casting, and feeding behaviors, were evaluated. Soil columns were inoculated on the surface with 10 8 cells per g of soil of the donor bacterium, and after a 2-week incubation period, donor, transconjugant, and total bacteria were enumerated at 5-cm-depth intervals. Transconjugants were confirmed by use of colony hybridization with a mer gene probe. In situ gene transfer of plasmid pJP4 from P. fluorescens C5t to indigenous soil bacteria was detected in all inoculated microcosms. In the absence of earthworms, the depth of recovery was limited to the top 5 cm of the column, with approximately 10 3 transconjugants per g of soil. However, the total number of transconjugants recovered from soil was significantly greater in microcosms containing either L. rubellus or A. trapezoides, with levels reaching about 10 5 CFU/g of soil. In addition, earthworms distributed donor and transconjugant bacteria throughout the microcosm columns, with the depth of recovery dependent on the burrowing behavior of each earthworm species. Donor and transconjugant bacteria were also recovered from earthworm casts and inside developing cocoons. Transconjugant bacteria from the indigenous soil microflora were classified as belonging to Acidovorax spp., Acinetobacter spp., Agrobacterium spp., Pasteurella spp., Pseudomonas spp., and Xanthomonas spp.

Soil physical and biological properties often change when different cropping, tillage, or management systems are imposed. Changes occasionally occur quickly, but usually become evident only after months or years. Infiltration rates are affected by several soil properties and may provide the most sensitive indication of changes in soil properties. To evaluate the use of infiltration measurements for detecting changes in soil properties, we conducted infiltration tests on a cropping systems experiment, a tillage experiment, and two beef cattle grazing experiments. In Pennsylvania, significant changes in infiltration rates did not occur until more than four years after converting from a conventional to a low-input cropping system. Infiltration rates were higher on 14th-year no-till plots compared with moldboard plow and chisel treatments in an Iowa tillage study. Earthworm populations and activity were highest in the no-till treatment. Infiltration rates correlated negatively with increased stocking rates in a long-term beef grazing study in Oklahoma. The number of earthworms did not correlate positively with infiltration in this study, suggesting a complex interaction. A short-term study of overwinter beef corn-stalk grazing in Iowa did not show consistent patterns in infiltration rate or other soil properties with different stocking rates. Infiltration appears to be a good indicator of soil structural changes associated with cropping, tillage, and management systems.

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