The Soil Microfungi of a Chronically Irradiated Oak‐Pine Forest

Abstract

The effects of chronic ionizing radiation on the soil microfungal community of an oak—pine forest was investigated by the dilution plate method and the contact slide technique. Twenty samples were collected and 10 slides inserted in the soil at each of 10 points along a radiation gradient varying from less than 1 to over 1800 R/day 2 cm below ground. A minimum of 55 fungi were selected at random off the isolation plates prepared from each of the 200 soil samples, and 5 slides per dose were examined. The 15,770 isolates obtained were distributed among 267 taxa. Subsurface radiation levels in the vegetated area ranged from less than 1 R/day in the undisturbed forest to 20 R/day under an almost pure stand of Carex pensylvanica. These exposures only indirectly affected the microfungi, producing minor distributional changes. Species diversity remained virtually constant throughout the area. The mean propagule level was approximately 100,000/g dry soil. Moniliaceous and mucoraceous fungi producing upright, morphologically complex reproductive structures characterized the population. The eight most abundant fungi, typically isolated throughout the world from podzolic soils supporting conifer—hardwood forests, were Penicillium terlikowskii, Mortierella isabellina, Aspergillus parvulus, Oidiodendron tenuissimum, P. nigricans, P. spinulosum, Monocillium humicola v. brunneum, and Mortierella nana. These eight taxa represented 71% of the isolates but only 5% of the species. A transitional zone devoid of higher plants except for a few scattered clumps of carex had a subsurface radiation level of 70 R/day. Species diversity was greatest in this region, where mesic and pioneer populations overlapped. The mean number of fungi was approximately 16,000/g dry soil. Only a few dominant forms of low density occurred in this area, notably, P. funiculosum, P. purpurogenum, Trichoderma viride, P. terlikowskii and Monocillium humicola v. brunneum. Distributional patterns of the microfungi correlated with the radiation gradient in the Devastated Zone, a region with no living higher plants. At subsurface radiation exposures of 175—1,250 R/day, no species was abundant and the mean number of propagules was reduced to approximately 3,00/g dry soil. Coefficient of community (CC) values and morphological complexity decreased as exposures to radiation increased. CC50 occurred at 1,600 R/day. Species diversity was reduced but remained constant along the gradient as shifting groups of more and more resistant forms occurred, culminating in a population dominated by the morphologically simple yeasts. A subsurface radiation level of 1,830 R/day, the maximum dose in this study, reduced the fungi to a mean level of only 500 propagules/g dry soil. The population contained yeasts but was characterized by intensely melanized, extremely slowgrowing, sterile isolates capable of producing only a rudimentary pseudomycelium. It is postulated that these are the most radioresistant of the fungi isolated and that resistance is related to their pigmentation. The contact slide results were comparable to those obtained by the dilution plate method except that basidiomycetes, not isolated by the latter, were the most radiosensitive fungi. Their dicaryotic hyphae were not observed on any slides receiving over 500 R/day.