ESA Symposium, Savannah 2003

Respiratory control of the global C cycle in a changing environment:
a search for new integrative tools

The symposium will take place on Wednesday, August 6 from 1:30 to 5 pm.

Sponsor

Physiological Ecology Section

Principal Organizer

Miquel A Gonzalez-Meler, Department of Biological Sciences, University of Illinois at Chicago, 845 West Taylor St, Chicago IL-60607. mmeler@uic.edu

Abstract

Human activities are altering the Earth's atmosphere, concentrating greenhouse (i.e.. CO2) and other gases through fossil fuel burning and land use change with the potential to cause global warming, climate change and alterations in ecosystem function and integrity. The impact of these environmental changes can be mitigated if atmospheric CO2 accumulation is slowed or stopped. Strategies to reduce emissions at source points can be coupled with storage of carbon in the biosphere. Terrestrial ecosystems have the potential to remove and sequester part of the carbon that is emitted annually into the atmosphere by increasing photosynthetic carbon gain over respiratory carbon losses. Estimating the potential for increasing carbon storage in ecosystems under different global change scenarios is difficult because of a lack of understanding on controls of the carbon flow from plants to soils and, eventually, back to the atmosphere as CO2.

In ecosystems, the net balance between gross photosynthesis and respiration is a sensitive predictor of functional ecosystem properties. Recent research indicates that enhanced photosynthetic carbon uptake in ecosystems does not necessarily translate into greater ecosystem carbon storage. Ecosystem processes that prevent increased carbon storage are not known except that respiratory processes are involved. As such a limited understanding of respiration is a major barrier to predicting current and future carbon fluxes and one of the current challenges in ecosystem science.

The proposed symposium will meet this challenge by identifying a new framework within which novel approaches and techniques can provide mechanistic understanding of carbon fluxes across temporal and spatial scales. Specific emphasis will be made on dissecting sources of ecosystem fluxes at different spatial and time scales and searching new integrative tools with special attention paid to the role of respiration in setting the mean residence time of ecosystem carbon pools. Ultimately, this cross-disciplinary discussion could strongly influence policy decisions, as provisions for anthropogenic C-storage incorporate terrestrial carbon management strategies. The broad ecological community holds the best position for addressing basic research questions with global implications.

Speakers

Presentation abstracts

The Balance Between Respiration and Photosynthesis in relation to ecosystem C sinks Joseph A Berry, Department of Plant Biology, Carnegie Institution of Washington
We now observe that terrestrial ecosystems are sequestering some of the CO2 entering the atmosphere from fossil fuel combustion and land use conversion. Over thousands of years prior to the industrial revolution, the CO2 concentration of the atmosphere remained quite stable at about 280 ppm. This implies that the rates uptake and release of CO2 must have been in steady-state balance. This inference is also consistent with the structure and kinetics of the carbon cycle. The rate of respiration is roughly proportional to the size of the pool of decomposable substrates, while photosynthesis is largely independent of size of this pool. Therefore, carbon pools should tend toward a state where respiration equals photosynthesis integrated over times longer than the turnover time of carbon in the ecosystem. Perturbations to either process should generate feedbacks that tend to maintain this balance. Is the present sink simply a feedback response of the carbon cycle? Measurements of the responses of photosynthesis to CO2 concentration conducted with the rainforest mesocosm of Biosphere 2 will be used to examine the extent to which the present sink can be accounted for as a response of this coupled system to CO2-fertilization of photosynthesis. Contradictory evidence from other experiments will be discussed.
 
Rate-based respiratory measurements without a system context are impeding understanding of the controls on respiratory fluxes Michael G. Ryan, USDA Forest Service, Rocky Mountain Research Station
Ecosystems scientists and ecophysiologists measure autotrophic and soil surface CO2 efflux ('soil respiration') to understand how these fluxes vary among plant parts and with environmental variation. Such knowledge is essential to develop scaling models and to predict how fluxes and ecosystem carbon storage will change with changes in species, biomass, nutrition, climate, atmospheric decomposition and composition. Early measurements of respiration rates assumed that these rates would remain constant (once the sources of variability had been identified) and that inferences at the system level could be drawn or inferred from measurements of rates. The recent intense focus on the carbon cycle and its regulation has promoted a renewed interest in respiration. However, the fundamental question motivating these measurements remains the same: will environmental changes alter a plant's carbon balance (its ratio of respiration to photosynthesis, R:P) or the carbon balance of the ecosystem? My objective with this review is to convince ecophysiologists to place respiration measurements into the context of a larger plant or ecosystem carbon balance, because short-term measurements of respiration rates cannot be used to infer ecosystem behavior. Autotrophic respiration rates are highly variable and plastic - they vary with temperature, metabolic activity, substrate availability, phenology, and tissue type. Soil surface CO2 efflux is strongly linked to plant processes through root and mycorrhizal respiration, and because much of the heterotrophic respiration consumes recently produced detritus. Both autotrophic and soil respiration rates 'acclimate' if conditions change. Finally, plants can shift carbon allocation in response to shifts in resource availability. This plasticity and structural changes make it difficult or impossible to infer how R:P or ecosystem C storage might change from short-term measurements of autotrophic or soil respiration rates. I provide examples of rates placed in the context of larger carbon balances, and some strategies for assembling such budgets.
 
Interpreting ecosystem respiration fluxes using stable isotopes Dave Bowling and Dan Yakir, Dept of Biology, University of Utah and Dept. of Environmental Sciences and Energy Research and Weizmann Institute of Science, Israel
Respiration by terrestrial ecosystems is one of the most important fluxes in the global carbon cycle. Stable isotopes have been an integral component of carbon cycle studies for several decades, and both, 13C and 18O in CO2 have been used to assess the magnitude of respiration fluxes at the global scale. During the last decade, application of stable isotopes to ecosystem carbon cycle studies gained popularity and progress was made in several areas of biosphere-atmosphere exchange research. In this talk, we will review the application of stable isotope studies to analyses of respiration, focusing primarily on measurements conducted at the ecosystem scale. We will discuss linkages that have been observed between mean annual precipitation, atmospheric vapor pressure deficit, and soil moisture availability with the carbon isotope ratio of total ecosystem respiration d13CR. We will also describe results from continental-scale networks of stable isotopes in ecosystems (BASIN, Carboeuroflux-Stable Isotopes Network) showing variation in d13CR that follows general trends with seasonal, meteorological and geographical parameters. We will explore the potential for using dynamics of d13CR to improve estimates of ecosystem discrimination D13CE for regional and global-scale models of carbon source/sinks in the land biosphere.
 
Anabolic and catabolic contributions to total soil respiration James W Raich, Department of Botany, Iowa State University.
The results of numerous studies indicate that soil respiration rates are likely to increase in response to increases in atmospheric CO2 and to increases in temperature. However, the effects of higher soil respiration rates on soil organic matter (SOM) pools are difficult to ascertain because soil respiration is a net flux that reflects changes in SOM decomposition and root respiration rates, both of which may respond to environmental changes. I review published data and present new data that were collected to distinguish belowground autotrophic (root) and heterotrophic (microbial) contributions to the total soil CO2 efflux. Isotopic data, in particular, have shown promise of allowing these two main CO2-producing processes to be individually quantified, in sites that have recently been subject to a change in vegetation cover. What evidence exists, though, for the remaining 99% of the world's land surface? I suggest that the classic distinction between autotrophic and heterotrophic sources of CO2 in soils needs to be modified to better understand belowground C-cycling processes, and propose a conceptual model that is consistent with existing data, and which allows for the estimation of anabolic and catabolic contributions to the total soil-CO2 efflux. Such an approach would greatly enhance our capacity to interpret the results of experimental studies, and to evaluate the potential responses of terrestrial ecosystems to changing environmental conditions.
 
Sources of soil respiration determined from radiocarbon Susan Trumbore, University of California Irvine
Soil respiration combines CO2 derived from autotrophic (root metabolism) and heterotrophic (organic matter decomposition) sources. A major challenge in ecological research is to determine how much of soil respiration comes from each of these sources, and how each may respond to environmental changes. We have developed methods using the difference in radiocarbon among these substrates to quantify these sources, and in addition to determine the relative contribution of different decomposing substrates to overall heterotrophic C losses. This talk will present radiocarbon measurements of soil respiration and its components from tropical, temperate and boreal ecosystems, as well as discuss how temperature and moisture can influence the different components of soil respiration. Radiocarbon in microbially-respired CO2 provides a measure of the mean residence time of fast-cycling C pools in ecosystems, which vary from several years in tropical systems to decades in boreal forests. In contrast, soil organic matter is often comprised of more stable, slowly cycling components that contribute little to soil respiration. The pools contributing the most to heterotrophic respiration include leaf and root litter; in particular soil respiration rates can be sensitive to the moisture content of the leaf litter layer. More stable, humified, organic matter pools contribute little to respired C on an annual basis, but are important for C balance on longer (decades-centuries) time scales.
 
Mean Residence time of soil carbon: controlling factors and implications for carbon cycling Julie D Jastrow, Argonne National Laboratory
Soil organic matter (SOM) is a heterogeneous mixture of plant, animal, and microbial materials in all stages of decay combined with a variety of decomposition products of differing ages and complexity. Because the turnover of these components varies continuously, any estimate of the mean residence time (MRT) for SOM as a whole simply represents an overall average. The use of whole soil MRT for predicting responses to management practices or environmental perturbations can be misleading because soils with similar average MRTs can have very different distributions of C among pools with varying turnover rates. Recent advances in approaches to fractionating SOM have improved our ability to isolate functionally meaningful pools tied to the mechanisms that control soil C cycling, and isotopic techniques enable the MRTs of soil fractions to be estimated and compared. Even so, predictions of responses to perturbations are problematic because (1) estimates of MRT are made under the assumption of steady state conditions and (2) the MRT of C in physically or chemically protected pools is not controlled by its inherent decomposability. Thus, under aggrading conditions relatively new C can accumulate in pools with long MRTs, and disturbance can accelerate the loss of old C pools by exposing protected but otherwise labile C to decomposers. Similarly, responses to environmental changes that influence decomposer activity, such as temperature or moisture, cannot be directly predicted from estimates of pool MRTs without accounting for effects on the transfer between pools and knowledge of the saturation levels for protective mechanisms. Hence, simulation models that account for variations in turnover rates for different soil C pools and the transfers between pools are necessary to generate realistic predictions of soil C dynamics.

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Revised: 7/9/03