The effects of the colonization of an abandoned grassland by forest trees and the associated microenvironmental changes on soil properties and the microbial activity of soil were studied along a 170 m transect established on a former pasture colonized by Norway spruce (Picea abies Karst.) in Central Slovakia. The transect with a 5×5 m grid of sampling points crosses all successional stages from a closed forest to an open grassland, representing 50 years of secondary succession. Changes of basal soil respiration, substrate-induced respiration and soil catalase activity in association with tree density, light and temperature regime were examined, and their relationships with chemical soil properties were documented. Generally, light conditions, tree influence and soil temperatures were highly significantly correlated with the distance from the lower transect edge, but the correlation coefficients were not very high (|r| ranged between 0.37 and 0.70), indicating that the microsite conditions within the transect are strongly heterogeneous. With the advance of colonization, the amount of soil organic matter was found to increase, probably in association with a higher amount and a lower quality of litter. Catalase activity exhibits a linear trend along the transect, which is explained by a lower direct insolation of soils (r = 0.28***) and decreasing temperatures (r = 0.36***) in the upper part. Basal respiration is the lowest in the middle of the transect, where colonizing spruces form a dense closed stand with soil covered by a thick layer of raw litter. Both in the lower part covered by species-rich grassland communities and in the upper part, where forest herb species begin to establish, basal respiration is higher, probably due to higher amounts of easily decomposable plant material. In addition to these transect-wide trends, stationary spatial patterns were observed for most soil variables. Generally, microbial activity rates as well as chemical soil properties exhibited spatial continuity up to the distance of approx. 40 to 60 m.
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