دانلود رایگان مقاله اثر تابش اشعه گاما بر اجتماع بیولوژیکی خاک و مخزن C و N در خاک لوم رسی

Abstract

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 (19:1v8c and i17:1v9c). 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.

5. Conclusions

The abundance of soil organisms and the concentration of soil C and N pools were significantly changed with gamma irradiation in a clay loam soil. The dose-dependent effect of irradiation was observed in soil C and N pools, and in soil collembola, nematodes and protozoa communities, but not in soil microbial community. This might be because some bacterial taxa have high tolerance to ionizing radiation, such as 19:1v8c and i17:1v9c. When the gamma irradiation dose lies in the range between 5 and 10 kGy, the majority of collembola and nematodes can be effectively eliminated without significant effect on the entire soil microbial community. To maintain a comparable soil C and N pools between sterilized and unsterilized soils, 5 kGy is an appropriate dose for selective defaunation. But this elimination would take 2 weeks to complete. Thus, we propose that a 14 day pre-incubation after radiation in a clay loam soil is needed for evaluating the contribution of re-inoculated organism to soil C and N pools, especially to DOC and DON. This research also suggests that evaluation of the response of soil biological and chemical properties to irradiation is necessary prior to employing selective sterilization technique by gamma irradiation, because the optimal dose will vary with faunal groups and soil types.