RECOLONIZATION OF METHYL-BROMIDE STERILIZED SOILS UNDER 4 DIFFERENT FIELD CONDITIONS

The course of recovery in biological activity was assessed in the top 5 cm of undisturbed soil cores (29.7 cm diameter, 30 cm deep) that had been fumigated in the laboratory with methyl bromide. The cores were returned to their original pasture and forest sites, two with a moderate and two with a high rainfall, and untreated soils at all sites served as baselines. Sampling took place over 166 days (midsummer to midwinter). Microbial biomass (as measured by fumigation-extraction and substrate-induced respiration procedures) and dehydrogenase activity both recovered rapidly, but remained consistently lower in the fumigated than in untreated samples at both forest sites and at the moister of the two pasture sites. Bacterial numbers also recovered rapidly. Fungal hyphal lengths were, on average over 166 days, 25% lower in the fumigated soils. Levels of mineral N were initially highest in the fumigated soils, but declined with time. Fumigation generally had no detectable effects on the subsequent rates of net N mineralization and little effect on nitrification rates. Fumigation almost totally eliminated protozoa, with one to three species being recovered on day 0; the numbers recovered most rapidly in the moist forest soil and slowly in the dry pasture soil. The recolonization rate of protozoan species was similar in all soils, with species numbers on day 110 being 33 and 34 in the fumigated and untreated soils, respectively. Nematodes were eliminated by fumigation; recolonization was first detected on day 26 but by day 166, nematode numbers were still lower in fumigated than in untreated soils, the abundance being 10 and 62 g-1 soil and diversity 10 and 31 species, respectively. Overall, the results suggest that protozoan and nematode populations and diversities could provide a useful medium-term ecological index of the recovery in comprehensive soil biological activity following major soil pollution or disturbance.