Optimizing gamma irradiation as a tool in soil ecology: effects on soil free living nematodes and biochemical properties

Despite the advantages of using gamma irradiation as a soil defaunating tool and in quantifying the contribution of nematodes in nutrient cycling, there are concerns on the method, such as, nutrient flush during and after irradiation and the less reproducibility of the method. The indirect effects depend on moisture content as the more water molecules are in the sample; the more mutagenic radicals are released into the soil which kills more biota. As a result, the same dose may have different effects depending on the moisture content of the soil at the time of irradiation. Therefore, in this experiment, we studied the gamma irradiation dose-moisture relationship and determined a dose that completely kills nematodes, but leaves minimum change on microbial biomass and nutrient concentration at a given moisture content. Bulk soil sample was collected and part of it was air dried while the moisture content of the remaining soil was adjusted to 50 and 80 % water filled pore space (WFPS). After irradiation, the moisture content was adjusted 50% WFPS for all the soil cores. An incubation experiment was conducted for 86 days with two fixed factors: gamma irradiation doses (0kGy (CTR), 1, 3 and 5kGy) and moisture content (air dried, 50% WFPS, 80% WFPS and 50% WFPS and leached with water). Samples in three replicates were destructively removed after 7, 21, 42 and 86 days of incubation at 17°C. Nematodes were completely killed in the air-dried soil in all doses and in soil cores with 80% WFPS at higher doses (3 and 5kGy). Total mineral N was highest at 50% WFPS in the CTR cores and all doses, while NH4+ concentration was consistently the highest in the air dried cores irradiated at 3 and 5kGy doses. Cmic was higher in 80% WFPS cores in the CTR and all doses than all the other treatments at the initial stage of the incubation. At higher doses, no significant differences observed in all treatments in Cmic and dehydrogenase enzyme activity. Betaglucosidase enzyme activity was consistently the highest in the 50%WFPS cores, while air dried cores resulted in the lowest activity in all doses and the CTR. Bacteria fungi ratio (B:F) was highest in 50% WFPS cores in majority of the doses and CTR. The results support the hypothesis that the effect of gamma irradiation on killing free living nematodes and changing the biochemical properties of a given soil varies depending on the moisture content of the soil at the time of gamma irradiation. It appears that 3kGy at 80% WFPS sufficiently kills nematodes and brings lesser effects on selected soil biochemical properties than other treatments.