Warming and peatland soilseffects on the community structure and carbon fluxes (incremento de temperatura e solos de turbeira: efectos sobre a estrutura das comunidades e sobre os fluxos de carbono)

Resumo

Climate change is expected to have a drastic impact on northern peatland areas. This is of particular concern because they hold about one third of the global soil organic carbon (SOC) stocks. Historically, they have been acting as C sinks as a result of prevailing low temperatures and frequent waterlogged conditions which limit microbial activities. However, increasing temperatures and lowering the water table could accelerate SOC decomposition and peatlands could become carbon sources. Besides climatic factors, the activities of the heterotrofic communities (namely microorganisms and soil fauna) and substrate quality availability (labile versus recalcitrant) and accessability play also an important role in determining the magnitude and direction of C dynamics. Unlike other European countries, peatland systems have a limited presence in Spain and in Galicia they survive in upland areas where cold temperatures and continuous moisture supply allow their presence. The ¿upland active blanket bogs¿ in the Serra do Xistral are of particular importance as they are considered as ¿Priority Natural Habitats¿ (Code 7130), due to its relict nature and because they are the highest in Europe. The overall aim of this study was to investigate the direct and indirect effects of temperature and moisture changes on soil fauna communities in these vulnerable ecosystems and to quantify the potential effects on C losses (mainly as CO2 and DOC). Firstly, the mesofauna communities of two ombrotrophic bogs within the Serra do Xistral and with different microclimate were intensively surveyed during two contrasting seasons (winter and summer) to identify the main responses of the different groups of invertebrates, in terms of population sizes and vertical distribution, to inter- and intra-annual variations of temperature, rainfall and soil moisture contents. The results showed that these abiotic factors had a significant effect on both population numbers and distribution along the soil profile, with the responses being species/group dependent. In general, warmer conditions promoted reproduction rates whereas reduced moisture levels resulted in drastic density reductions. Previous climatic manipulations in the field and in the laboratory have highlighted the key role of the dominant Enchytraeidae worms on C cycling but also the importance of substrate quality on the observed reponses. Therefore, three laboratory experiments were undertaken in order to underpin the mechanisms controlling C dynamics by investigating the effects of each individual climatic driver (temperature and moisture changes) on this mesofaunal group and by manipulating substrate accessability to the microorganisms. Results showed that the combined action of higher temperature and moisture regimes favoured enchytraeid populations, leading to a significant increase in both soil CO2 emissions and DOC leaching from these soils; however, drying the peat had a negative effect on animal survival and resulted in less C being lost from these systems. In the longer term, however, the observed effects of temperature induced changes in enchytraeid populations on C release will become largely dependent on the availability of easily accessed labile substrates and the acclimation of microbial communities to temperature, resulting in different performance in soil enzymatic activities governing the carbon and nitrogen cycles. Taking these findings together, it is possible to conclude that the combined action of abiotic factors and their direct influence on soil fauna (population sizes and vertical distribution) and microbial populations (enzymatic capabilities and temperature acclimation) play a key role in controlling SOM decomposition rates in these peatland soils. Furthermore, the indirect effects of these climatic factors on the top-down population size regulation processes (e.g. microbial grazing, predatory mites) could alter the magnitude and direction of these biological responses and hence the C sink/source function of these organic soils. Therefore, although the scientific debate has mainly focused on the climatic variables, accounting for the abiotic and biotic interactions is required to enable better quantifications of C dynamics and to make more accurate predictions of future changes in the terrestrial carbon pools.