Soil respiration fluxes and carbon sequestration of two mountain forests in Switzerland
Rühr, Nadine K.
The major source of CO2 from forest ecosystems, often referred to as soil respiration, is a component flux of root-rhizosphere respiration and microbial decomposition of litter and soil organic matter. Thus, to estimate the CO2 balance of soils both respiration fluxes have to be accounted for separately, and in addition, information on soil C stocks as well as on above- and belowground litter input are needed. Therefore, we combined measurements of soil CO2 efflux with a soil carbon model (Yasso07) using site-specific litter input and climate data to estimate the CO2 balance of two Swiss mountain forests over several years. We found soil respiration rates at both study sites to be strongly related to soil temperature if not limited by soil water availability. Inter-annual differences in soil respiration were found to be small, since the studied years were similar in mean annual temperature and precipitation. However, the three studied years were different in winter temperatures, affecting soil respiration at Lägeren.During themild winter (Jan, Feb,March) in 2007, the cumulative soil respiration flux was nearly doubled compared to the harsh winter in 2006. Moreover, SR was found to be dominated by microbial respiration (MR) during winter, and in addition, MR was found to be highly temperature sensitive. During winter MR is not limited by substrate supply after autumnal leaf litter fall but rather by temperature. During the entire study period microbial and root-rhizosphere respiration were found to contribute each about half to soil respiration at both study sites, well within the range of other partitioning studies. Simulated soil C stocks were found to be slightly lower than measured/interpolated soil C stocks, while simulated MR was comparable to measurement results. During the study period (1989–2008), the soils at Davos were found to be a significant C sink with 21 g C m−2y−1 (95% confidence interval of [5.8, 36.2]), while the soils at Lägeren were neither a significant C sink nor a significant C source with 3.4 g C m−2y−1 (95% confidence interval of [-16.0, 22.7]) on average. These numbers agreed very well to estimates of tree biomass C storage and net ecosystem productivity (NEP). The difference between NEP and tree biomass C storage does most likely represent the C sink of soils, estimated at Davos to be about 20–30 g C m−2y−1 and at Lägeren to be about 0 g C m−2y−1 (see BAFU report by Zweifel et al. 2009). Higher temperatures, as measured during the past 20 years (mean temperature during 1989–2008 was about 1°C higher than during 1958–1998), were found to affect the soil C sink strength. Excluding this temperature increase from the simulation enlarged soil C storage rates. Thus, the higher temperatures during the past 20 years have probably dampened the soil C sink strength at both study sites. In addition, we found simulated microbial respiration and soil C storage rates to be highly variable during the study period caused by variations in temperature and precipitation. Thus, by using the simple soil carbon model Yasso 07 and comparing it to measurement results, we could show that the soils of our study sites are most likely a sink or at least not a clear source of C to the atmosphere under the current climate conditions. However, we are not able to make predictions on soil C storage rates in the future, since the effect of climate change on productivity (C-input) and on decomposition rates (C-output) is still far from clear Show more
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