Intact soil-core microcosms compared with multi-site field releases for pre-release testing of microbes in diverse soils and climates

Intact soil-core microcosms were used to compare persistence of Pseudomonas chlororaphis 3732RN-L11 in fallow soil and on wheat roots with field relea ses at diverse sites. Parallel field and microcosm releases at four sites i n 1996 were repeated with addition of one site in 1997. Microcosms were obt ained fresh and maintained at 60% soil water holding capacity in a growth c hamber at 70% relative humidity, a 12-hour photoperiod, and constant temper ature. Persistence of 3732RN-L11 was measured at each site in field plots a nd microcosms at 7-21 day intervals, and in duplicate microcosms sampled at an independent laboratory. Linear regression slopes of field plot and micr ocosm persistence were compared for each site, and between identical microc osms sampled at different sites, using log(10) transformed plate counts. Mi crocosm persistence closely matched field plots for wheat roots, but persis tence in fallow soil differed significantly in several instances where pers istence in field plots was lower than in microcosms. Analysis of weather va riations at each site indicated that rainfall events of 30-40 mm caused dec reased persistence in fallow soil. Cooler temperatures enhanced persistence in field plots at later time points. Inter-laboratory comparison of regres sion slopes showed good agreement for data generated at different sites, th ough in mio instances, longer sampling periods at one site caused significa nt differences between the sites. Soil characteristics were compared and it was found that fertility, namely the carbon to nitrogen ratio, and the pre sence of expanding clays, were related to persistence. These microcosm prot ocols produced reliable data at low cost, and were useable for pre-release risk analyses for microorganisms.