Agricultural Practices Modulate the Beneficial Activity of Bacterial-Feeding Nematodes for Plant Growth and Nutrition: Evidence from an Original Intact Soil Core Technique

Free-living nematodes have beneficial effects on plant growth and nutrition. Exploring how agricultural practices modulate these beneficial effects is still challenging. A study was conducted in Ferralsols from Madagascar from one unmanaged grassland and 16 upland rainfed rice fields, representative of different agricultural practices: rotation, agroforestry and monoculture. Intact soil cores in plastic cylinders were sampled in the field to assess the effects of agricultural practices on changes in plant growth and nutrition induced by the presence of bacterial-feeding nematodes. The soil cores were fumigated to kill the nematodes and moistened with a filtered fresh soil suspension containing only microbial cells. A rice seed was introduced in the core, which was then incubated under natural climatic conditions for 40 days with or without inoculation of the bacterial-feeding nematode Acrobeloides sp. The inoculation of the nematodes induced lower, similar or higher plant biomass and nutrient content in comparison to the control according to the agricultural practices. Positive effects of Acrobeloides sp. on plant functions were frequent in soil cores sampled from fields with high plant diversity, especially from agroforestry systems. The intact soil core technique appears to be a robust means of mimicking field conditions and constitutes a promising tool to assess effects on soil processes of the ecological intensification of agricultural practices.

Determining the environmental fate of a filamentous fungus,Trichoderma reesei, in laboratory-contained intact soil-core microcosms using competitive PCR and viability plating

Trichoderma spp. are used extensively in industry and are routinely disposed of in landfill sites as spent biomass from fermentation plants. However, little is known regarding the environmental fate of this biomass. We tracked the survival of T. reesei strain QM6A#4 (a derivative of strain QM6A marked with a recombinant construct) over a 6-month period in laboratory-contained, intact soil-core microcosms incubated in a growth chamber. Survival was tested in 3 different soils and the effect of a plant rhizosphere (bush lima beans, Phaseolus limensis) was investigated. Levels and viability of the fungus were determined, respectively, by quantitative competitive polymerase chain reaction analysis of total soil DNA extracts and dilution-plating of soil on a semiselective growth medium. Whereas chemically killed QM6A#4 became undetectable within 3 d, QM6A#4 added as a live inoculum decreased ~4- to ~160-fold over the first 1–3 months and then reached a steady state. After 4 months, soil cores were subjected to a 1.5-month simulated winter period, which did not significantly affect QM6A#4 levels. Throughout the experiment, QM6A#4 remained viable. These results indicate that, following release into the environment, live T. reesei will persist in soil for at least 2 seasons.Key words: competitive PCR, genetically engineered microorganisms (GEMs), genetically modified organism (GMO), survival of microorganisms, microcosm, Trichoderma.