Soil respiration rates and δ~(13)Cco_2 in natural beech forest (Fagus sylvatica L.) in relation to stand structure

摘要

Soil respiration rates were studied as a function of soil type, texture and light intensity at five selected natural beech forest stands with contrasting geology: stands on silicate bedrock at Kladje and Bricka in Pohorje, a stand on quartz sandstone at Vrhovo and two stands on a carbonate bedrock in the Karstic-Dinaric area in Kocevski Rog, Sneina jama and Rajhenav, Slovenia, during the growing season in 2005-2006. Soil respiration exhibited pronounced seasonal and spatial variations in the studied forest ecosystem plots. The CO_2 flux rates ranged from minimum averages of 2.3 μmol CO_2 m~(-2) s~(-1) (winter) to maximum averages of about 7 μmol CO_2 m~(-2) s~(-1) (summer) at all the investigated locations. An empirical model describing the relationship between soil respiration and soil temperature predicted seasonal variations in soil respiration reasonably well during 2006. Nevertheless, there were also some indications that soil moisture in relation to soil texture could influence the soil CO_2 efflux rates in both sampling seasons. It was shown that spatial variability of mean soil respiration at the investigated sites was high and strongly related to root biomass. Based on the δ~(13)Cco_2 data, it was shown that new photoassimilates could account for a major part of the total soil respiration under canopy conditions in forest ecosystems where no carbonate rocks are present, indicating that microbial respiration could not always dominate bulk soil CO_2 fluxes. At Snezna jama and Rajhenav, the abiotic CO_2 derived from carbonate dissolution had a pronounced influence on CO_2 efflux accounting, on average, to ～17%. Further spatial heterogeneity of soil respiration was clearly affected by management practice. Higher respiration rates as well as higher variability in respiration rates were observed in the virgin forest (Rajhenav) than in the management forest (Snezna jama) and could be related to a higher amount of detritus and consequently to less pronounced influence of inorganic pool to CO_2 efflux, lower mixing with atmospheric CO_2 and higher sensitivity to environmental changes. Major differences in soil carbon dynamics among these five forest ecosystems can be explained by the influence of bedrock geology (particularly, the presence or absence of carbonate minerals) and soil texture (affecting gas exchange with overlying air and soil moisture).