Global warming effects on soil carbon dynamics of forests

Soil organic matter (SOM) dynamics are one of the least understood parts of the response of terrestrial ecosystems to global warming. Increasing soil temperature accelerates the decomposition rate of SOM by stimulating soil microbial processes. This induces an increase in the CO 2 flux from the soil to the atmosphere (soil respiration). The magnitude of the temperature-driven increase in soil respiration depends on SOM (stock, input, quality and accessibility) and the response of decomposing microorganisms. The temperature effect is not necessarily linear over time. Limitation of the availability of easily decomposable SOM can weaken the temperature effect in the long run. A shift of functional groups within the microbial community or physiological adaptations of soil microbes may offset temperature effects by lowering soil respiration rates independent of substrate availability. Because both, substrate depletion and microbial adaptations are slow processes, short-term effects of increased temperature likely do not reflect long-term trends. For a meaningful forecast of future carbon (C) storage in forest ecosystems, however, the long-term response of SOM pools to climate change is relevant. In the soil warming experiment Achenkirch we have increased the soil temperature by 4C during growing seasons since 2004. We have observed a strong increase (~ 40%) of the soil respiration rate between 2004 and 2007 followed by a slightly decreasing response until 2009. The intention is to prolong the warming experiment for further 3 years. Our experiment will be one of few studies world-wide where a long-term trend is investigated. The continuation yields a data set were soil C pool changes can be quantified by two approaches, (i) a 9 years budget of fluxes (soil respiration versus C inputs), and (ii) two C pool comparisons (C stock in archived soil material from 2004 vs. 2013; C stocks in warmed plot vs. control plot soil). By combining the data, we will come up with a long- term scenario of soil C dynamics under elevated temperature for this site. Such results are the basis for the accounting of terrestrial sinks of greenhouse gases within the Kyoto Protocol. For a functional understanding of SOM dynamics, the heterogeneity of SOM requires a separation into different fast and slow cycling pools. We will physically fractionate soil and determine the turnover time of the fractions by radiocarbon ( 14C) analysis. The stocks and turnover times of individual fractions will show how warming affected labile and recalcitrant SOM pools. A detailed analysis of the chemical composition of SOM out of different C pools will provide new insights in the functioning of soil C cycling. In addition to aboveground litter input, belowground litter input will be determined by quantification of fine root turnover. The response of soil microbes to warming will be addressed by re-assessing the microbial community structure and testing for a potential physiological adaptation to elevated temperature. A potential change of the temperature sensitivity in SOM decomposition on warmed plots will be assessed by lab incubation experiments. Beside that we will address unsolved issues regarding the influence of carbonate-rock weathering on the CO 2 efflux from soil.

Worlds forests presently serve as atmospheric carbon sinks and thus mitigate a significant fraction of the global anthropogenic CO2 release. Forests store carbon in their biomass and in the soil. There is concern that global warming accelerates the decomposition of soil organic carbon (above and below ground plant litter and humus) with a corresponding increase in the CO2 efflux from the soil to the atmosphere. This could shift the equilibrium between CO2 uptake by the biomass and CO2 release from the soil towards the latter and turn forest ecosystems into carbon sources. Artificial soil warming is a tool to assess the effects of increasing temperatures on soil organic carbon dynamics in the field. Soil microorganisms increase the rates of soil organic carbon decomposition and respiration (CO2 efflux) with increasing temperature. The response rates to warming may however change with time. Therefore it is especially important to assess the long-term warming effects on soil carbon dynamics as was accomplished in our project. We continued soil warming in a temperate mountain forest from 2011 until 2013 which corresponded to the 6th until the 9th year of artificial soil warming. We assessed warming effects on soil CO2 efflux, soil carbon pools, tree fine roots and microbial physiology. Soil warming by + 4 C persistently increased the soil CO2 efflux by approx. 40 % throughout all 9 years. The expected decrease in the warming response, as a matter of labile substrate depletion or microbial adaptations to the higher temperature, which had been observed in other field warming studies, did not occur at our site/soil. Decomposer microbes did not show any signs of thermal adaptation. Different soil organic matter fractions, which were considered to range from labile to recalcitrant substrates, all responded to soil warming. Increased tree fine root turnover could explain a smaller part of the additional soil CO2 efflux from the warmed soil. Tree fine roots changed their morphology towards longer and thinner root tips and the mycorrhizal colonization was slightly enhanced in the warmed soil. All our results point towards a high susceptibility of the studied carbonate rich forest soil to warming. The organic substrate in the upper layer of the forest soil seems less protected when compared to other forest soils. Global warming may cause substantial carbon loss from this forest soil to the atmosphere.