Effects of gamma irradiation on soil biological communities and C and N pools in a clay loam soil

Gamma irradiation is becoming a promising technique in soil ecological studies because it has a particular advantage in selectively eliminating the target organism. But this selective sterilization technique is still in its initial exploratory stage and the subsequent impacts on soil carbon (C) and nitrogen (N) pools are relatively unknown for the majority of soils. Therefore, the responses of soil respiration, soil collembola, nematodes and microbial communities, and soil C and N pools (extracted with KCl, K2SO4 and H2SO4) to a range of gamma irradiation doses (0, 5, 10, 20 and 40 kGy) were determined under a clay loam soil in a 4-week incubation study. A flush of CO2 was observed at the beginning of the incubation period (1-2 days) post-irradiation, and then strongly decreased relative to the unirradiated soils. At the middle of incubation period (11-14 days), there was a recovery of CO2 efflux in the 5 kGy treatment. The effects of radiation on biological communities were dependent on taxa groups. The majority of collembola (>80%) and nematodes (>90%) were killed immediately in the higher doses (>5 kGy), but at lower doses of 5 kGy they were killed within 2 weeks after irradiation. The relative abundance of saprophytic fungi and protozoa decreased with increasing irradiation dose throughout the incubation period, while an opposite trend was found for some special bacterial taxa (19:1ω8c and i17:1ω9c). The resistance threshold of the entire microbial community to gamma irradiation was 10 kGy. The C and N contents in the KCl/K2SO4-extracted pools (except the dissolved organic C), in the H2SO4-extracted labile pool II (LP II) and in the recalcitrant pool (RP) decreased with increasing irradiation dose across the incubation period. The decreases in LP II and RP were accompanied by the increase in labile pool I. The variation in the level of all soil C and N pools was significantly affected by the radiation doses higher than 5 kGy. Our results indicate that the gamma dose between 5 and 10 kGy is sufficient to eliminate soil fauna without significant effect on microbial community compared to the unirradiated treatments. Moreover, a radiation dose of 5 kGy for selective defaunation has minor impacts on soil C and N pools of a clay loam soil. Our results also suggest that dose optimization is necessary due to high variability associated with irradiation levels effect on biological taxa in different soils.