Effects of hard frost and freeze-thaw cycles on decomposer communities and N mineralisation in boreal forest soil

Decomposition and mineralisation rates generally increase with increasing moisture and temperature. The expected global climate change may enhance precipitation and raise the temperatures at boreal latitudes, but absence of snow together with occasional low temperatures may cause disturbances in soil processes and faunal communities. To test the effects of disturbances such as hard frosts and freeze-thaw cycles on decomposer populations and N mineralisation, we performed two experiments. In the field experiment, carried out in a pine forest, we induced low soil temperatures by preventing snow covering the ground. In the laboratory test we established three "winter" temperature regimes: constant -2degreesC, freeze-thaw cycles of -2degrees to +2degreesC, and the latter with two hard frost periods (-16degreesC). The microcosm experiment included two decomposer communities; the "simple" community included microbes, protozoa and nematodes, and the "complex" community microbes, protozoa, nematodes, enchytraeids and microarthropods. In the field experiment, the populations of enchytraeids and several microarthropod taxa, as well,as microarthropod species richness, decreased in snowfree plots. In the microcosms, virtually all enchytraeids died and microarthropod populations and taxa decreased in the hard frost treatment, but increased in the freeze-thaw treatment. In the laboratory systems with simple decomposer community structure the amount of mineral nitrogen decreased in the freeze-thaw treatment. The experiments revealed that exceptionally low soil temperatures exert a stronger influence on soil fauna than constant benign temperatures or freeze-thaw cycles, but the rapid recovery of populations may counteract the detrimental effect of occasional frost periods. (C) 2002 Elsevier Science B.V. All rights reserved.