Recovery of soil microbial community structure and activity following partial sterilization with gamma irradiation

Sterilization (partial or full) is one of the most fundamental tools for studying the ecological roles of soil biota and notably soil fauna, but there is a lack of knowledge on how these techniques affect other soil-inhabiting micro-organisms (non-target), and on their recovery after sterilization. We investigated the effect of a partial soil sterilization with gamma (gamma) irradiation (target dose of 6 kGy, proven to selectively remove the entire nematode community and higher soil fauna) on the soil microbial community and its recovery in a microcosm experiment during 12 weeks. We specifically studied the effects, following partial sterilization, of recovery time (4 sampling events after 1, 4, 8 and 12 weeks) on the microbial activity (activities of dehydrogenase and 8-glucosidase, and nitrification), microbial biomass and microbial community composition (using PLFA analysis), in two soils of contrasting texture (silt loam and loamy sand). We also studied whether inoculation of finely sieved and ho-mogenized soil powder (SP) from these same soils (containing prokaryotes, fungi and protists, but no metazoan organisms) would stimulate the microbial recovery process. Dehydrogenase activity was strongly reduced by gamma-irradiation in the silt loam (and SP did not stimulate its recovery) but not in the loamy sand, whereas there was no effect of gamma-irradiation on 8-glucosidase activity in either soil. Nitrification was strongly affected in the loamy sand and did not yet recover by the end of the incubation (week12), but the SP inoculation brought nitrification to the level of the unirradiated control by week4. Surprisingly, nitrification rate was not affected by gamma-irradiation in the silt loam. The factors "recovery time", "irradiation" and "SP inoculation" affected the microbial com-munity composition (based on PLFA), but the direction of these changes did not seem to follow a clear pattern. These results urge to carefully consider the re-establishment of microbial activity in studies on the roles of micro -and mesofauna following selective sterilization, even after several months. Also, SP inoculation can be a useful way to stimulate the recovery of the nitrification process, which would be crucial in studies of the role of soil fauna in N dynamics.