Soil respiration in a subtropical montane cloud forest in Taiwan

Tropical/subtropical montane cloud forests (TMCFs) are ecosystems that are less known about their carbon budgets. Especially little information is available about the soil CO2 efflux from these ecosystems, although they have been proved to have special hydrological regime which might be important in controlling soil respiration. In this study, we used an automatic chamber system to measure soil respiration rates at the Chi-Lan Mountain forest site. The half-hourly dataset was used for analyzing the controlling factors and mechanisms of soil respiration. On the other hand a manipulation experiment was conducted in the field by applying 3-fold and 1-fold aboveground litter to soil surface and the respective soil respiration rates were measured using static alkali chamber method. The results showed that soil respiration rates have a positive exponential correlation with soil temperature and a negative exponential correlation with soil water content. An empirical model relating soil respiration (Rs) to soil temperature (T) and soil water content (θ) is Rs = -0.095 + e(0.88+0.10T-6.99θ) with R2=0.83. The annual soil respiration rate calculated using this model was 176 g C m-2 y-1. This extremely low value might be caused by the permanently high soil moisture and the relative lower mean annual temperature compared to other sites that receive similar amount of precipitation. The 3-fold and 1-fold litter treatments result in significantly higher soil CO2 efflux compared to the no litter chambers. The magnitude of difference diminished to minor after six months of treatment. About 10% of the annual soil respiration was contributed by the mineralization of fresh aboveground litter. The carbon mass loss of the decomposing litter during the first six months was mainly due to leaching of dissolved organic carbon (72%) and secondly due to mineralization to CO2. From the results of this study we hypothesized that the TMCFs may be vulnerable to global warming since the drying of the soil may change the soil from carbon sink to carbon source and release the soil organic carbon that has been stored for long time.