B21C-01 INVITED
Potential for Abrupt Changes in Soil Carbon
Predictions of terrestrial feedbacks to climate warming differ dramatically in the magnitude and even the sign of soil C responses over the next century. These differences reflect uncertainties inherent in how models parameterize two key responses: the temperature dependence of decomposition rates and the rate of C accumulation given hypothesized increases in vegetation productivity due to CO2 fertilization. Abrupt changes in soil C (gains or losses of C at rates of 1MgC ha-1 yr-1 or more) are observed in early stages of soil formation, following vegetation change, or with changes in soil drainage, pH or nutrient status. Radiocarbon measurements provide a tool for determining the source and age of the C being stored or lost, and also provide a means of detecting processes destabilizing C that has previously been stable over decades to thousands of years. To be important for the global C cycle, abrupt changes need to occur over large land areas, which focuses attention on high latitude soils and vegetation change at low to mid-latitudes. This talk will provide examples and explore the observational constraints of the potential for soils to act as globally important C sources or sinks over the next century. It will underscore the need to understand the complex and interacting factors – physical and chemical as well as biological – that can allow organic matter to persist in and be lost from the soil environment on a range of timescales
B21C-02 INVITED
Ecosystem-Scale Sensitivity of Carbon Cycling Processes to Climate Warming
The coupled carbon-climate models reported in the literature all demonstrate a positive feedback between terrestrial carbon cycles and climate warming. The positive feedback results from decreased net primary production (NPP) in most models and increased respiratory carbon release by all the models under climate warming. Past experimental research has been focused on sensitivity of soil carbon decomposition to climate warming. Research results so far are controversial on directions and magnitudes of decomposition sensitivity to warming largely because the latter is not only dependent on quality and kinetics of carbon substrates but also tightly coupled with ecosystem carbon uptake. In this talk, we show that ecosystem warming in a North American prairie stimulated plant biomass production, enhanced dominance of C4 plants, extended growing seasons, enhanced root/shoot ratio, and increased nitrogen uptake and use efficiency. The stimulated biomass growth led to increases in litter mass and soil respiration, but no significant change in soil carbon storage. Our results demonstrate that plant ecophysiological processes play a critical role in regulation of ecosystem-scale responses to global change and future climate warming. http://bomi.ou.edu/luo
B21C-03
Sensitivity of soil C loss to temperature sensitivity algorithms that vary with soil C lability
Soil C stocks are sensitive to changes in temperature and it is commonly believed that even minor increases in temperature may lead to large releases of C from soils to the atmosphere. Most of this thought is based on short- term incubation data and model output that implicitly assumes soil C pools are comprised of organic matter fractions with uniform temperature sensitivities. Several lines of evidence suggest that resistant soil C fractions are more sensitive to temperature than resistant soil C fractions, but carbon cycle models typically apply equal sensitivity for both labile and resistant fractions. Based on long-term soil C and 13C laboratory incubation data, we have derived three new temperature response functions that account for differential temperature sensitivities of labile and resistant soil C – one with a direct relationship between soil C resistance to decomposition and temperature sensitivity, and two with curvilinear relationships in which increasing sensitivity responses are transient (i.e., limited to a fraction of the resistant soil C). We test these algorithms against independent laboratory measurements and field soil respiration measurements. We then evaluate predictions of these algorithms against those with uniform temperature sensitivities across different soil C pools. Our preliminary results indicate that the three models are all capable of accurately modeling soil CO2 fluxes in the field, but that the algorithms with a soil C lability-dependant temperature sensitivity most accurately model laboratory responses and predict substantially larger reductions in soil C stocks with a changing climate.
B21C-04
The Response of Soil Carbon Decomposition to Temperature
Global warming may accelerate the decomposition rate for soil carbon (C), potentially converting this massive terrestrial C reservoir from a net sink of atmospheric CO2 to a net source. We used three independent methods to examine the sensitivity of non-labile soil C decomposition (more than 90 percent of total soil C) to temperature: laboratory incubations paired with chemical fractionation and radiocarbon analyses; incubations where substrate supply was artificially manipulated; and isotope-based estimates of in situ soil C decomposition across gradients in mean annual temperature. Across methodologies, we found that non-labile soil C decomposition responded to changes in temperature with a Q10 values between 1.0 and 1.4. We conclude that for non-labile carbon (that remaining after 100 days of incubation or isolated slow and recalcitrant fractions through chemical fractionation) decomposition responses to temperature were significantly lower than the response of labile carbon (less than 5 percent of total soil C). We show that our findings cannot be explained by masking of recalcitrant carbon loss by labile carbon, and that, in contrast to modeling evidence, such masking is not tenable when soil C mass loss is accurately represented.
B21C-05
Is the pool of soil carbon undergoing decomposition changing with temperature?
Forest harvesting alters the structure of the mineral soil organic carbon pool but the response of an altered pool structure to changes in temperature is not well known. A site with more carbon and/or a higher quality carbon pool would, intuitively, produce more CO2 and, based on kinetic theory, exhibit a lower temperature sensitivity. We identified a set of sites that differ only in their time since harvest and that are separated by no more than 5 km. To examine the temperature dependence of decomposition as a function of carbon quality and quantity, we incubated soils from 6 depth strata in the top 50 cm of mineral soil from each site across the region's annual temperatures range (1 C to 31 C) at constant moisture. The headspace of the incubation chambers was measured weekly for 5 weeks for CO2 production and δ13C after flushing the headspace with CO2 free air. Rates of respiration indicate multiple temperature optima, and respiration rates and δ13C ratios of respired C suggest that the pool of available substrate shifts with temperature. A substrate addition experiment corroborated our findings suggesting that the diversity and structure of the microbial community may be as important as temperature in determining the pool of carbon that is actually available for decomposition.
B21C-06
The Degree of Permafrost Thawing Determines Arctic Tundra Carbon Balance
In interior Alaska, we measured gross photosynthesis (GP), ecosystem respiration (ER), and net ecosystem exchange (NEE) near a permafrost monitoring borehole that has recorded an increase in permafrost temperatures during the last 22 years. From May 2004 to May 2006, ecosystem C exchange measurements were made with static and automatic chambers in a moist acidic tundra ecosystem. A gradient in degree of permafrost thaw was used to select three sites (Minimal, Moderate and Severe Thaw) that corresponded to an increase in thermokarst occurrence. Between June 1 and August 30, the Severe and Moderate Thaw sites had significantly greater C uptake (GPP) (p<0.05) than the Minimal Thaw site. This growing season GP correlated with aboveground productivity. However, greater winter ER from the Severe Thaw site caused it to be a source (negative NEE, -128 g C m-2) of carbon over three years. The Moderate Thaw site was a carbon sink (52 g C m- 2), while the Minimal Thaw site was near carbon neutral (-7 g C m-2). In the spring and fall, carbon uptake correlated with the occurrence of plant functional groups (sedges, evergreen shrubs) that maintain green foliage and can photosynthesize under cold conditions. Both of these functional groups decreased inside thermokarst. In the winter, the Severe Thaw site lost 34% more C than the other two sites, likely due to warmer deep soil temperatures. As permafrost thaw proceeds, increasing ecosystem C loss may occur during the winter, spring, and fall due to the unique biotic and abiotic characteristics of thermokarst.
B21C-07
The Contribution of Old Carbon to Respiration from Alaskan Tundra Following Permafrost Thaw
More than 450 Pg of soil carbon (C) has accumulated in high latitude ecosystems after the retreat of the last major ice sheets. Recent studies suggest that, due to climate warming, these ecosystems may no longer be accumulating C, and in some cases may be losing stored C to the atmosphere. We used radiocarbon measurements of carbon dioxide to detect the age of C respired from tussock tundra near Denali National Park, Alaska. At this tundra site, permafrost has been observed to warm and thaw over the past several decades, causing the ground surface to subside as ice volume in the soil decreased. We established three sites within this area that differed in vegetation and surface topography; both characteristics varied in relation to the degree of permafrost thaw. We made radiocarbon measurements of ecosystem respiration, incubations of soil organic matter, and incubations of above and belowground plant biomass to determine the age and isotopic value of C respired from these sites. Over the study period from 2004 to 2006, ecosystem respiration radiocarbon values averaged from +35‰ to +95‰ in different months across sites. For soil incubations, surface soil radiocarbon was elevated relative both to ecosystem respiration and the current atmospheric radiocarbon value, demonstrating the significant contribution from C fixed over the past years to several decades. The deeper soil, in contrast, had respiration isotope values that averaged below zero, reflecting the significant effect of radioactive decay on the isotope content of deeper soil layers. The plant and soil incubations were combined in a multi- source mixing model to determine probable contributions from these different sources to ecosystem respiration. Deep soil respiration generally averaged between 5-15% of total ecosystem respiration, but reached as high as 40% in some months. When aggregated across the growing season, the two sites undergoing more disturbance from permafrost thaw had on average 2-3 times the loss of old deep C as compared to the least disturbed site.
B21C-08
Modeling impact of permafrost dynamics on carbon dioxide and methane cycling in northern high latitudes
Over the last several centuries, permafrost conditions in northern high latitudes have changed, and are likely to continue changing. A growing body of evidence indicates that the terrestrial ecosystems in the region are undergoing substantial changes associated with the warming that has been occurring during recent decades. The fate of the large amount of carbon stored in the region is uncertain under these changes. In this study, a process-based biogeochemistry model, coupled with a permafrost model, is used to estimate the budget of carbon dioxide and methane of the region for current climate conditions and for future scenarios by considering effects of permafrost dynamics and fire disturbances. The simulations indicate that currently the region is a net source of carbon to the atmosphere. Throughout this century, the region will most likely continue as a net source of carbon due to permafrost thawing and fire disturbances. In particular, the methane emissions will more than double the amount of current emissions. However, the coupled carbon and climate model simulations show that these emissions will exert relatively small radiative forcing on global climate system compared to large amounts of anthropogenic emissions. The further analysis suggests that the complex interactions of permafrost, fire, and topographical and hydrological changes should be considered in the future quantification of these gases budget for the region.