Phd

National Institute for Global Environmental Change

Annual Progress Report for FY 97/98

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Control on soil carbon storage and soil CO2 fluxes at the Harvard Forest core experiment

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Table of Contents:

To jump to a particular section, click the section title.

1. Investigators

2. Funding

3. Objectives

4. Approach

5. Results to Date

6. Figures

7. References

8. Products

o Publications

o Papers/Presentations

o Proceedings

9. Student Participation

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Investigators:

Eric Davidson

The Woods Hole Research Center

Susan Trumbore

University of California

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Funding:

Fiscal Year Amount

1996 118,000

1995 128,500

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Objectives:

Soil Respiration Studies

The advent of micrometeorological methods for measuring net ecosystem exchange (NEE) of CO2 has permitted the quantitative partitioning of forest metabolism into the individual components of photosynthesis, above ground respiration, and below ground respiration. At the Harvard Forest in central Massachusetts, the above-ground processes have been successfully simulated with sophisticated, mechanistically-based canopy process models that account for the effects of variation in light, water vapor, foliar nitrogen content, and other factors on photosynthesis and canopy respiration (Aber et al. 1996, Amthor et al. 1994, Waring et al. 1995, Williams et al. 1996). Below ground respiration is the other major component of NEE, and it has been estimated to comprise 60% to 80% of total ecosystem respiration of this mixed hardwood forest (Amthor et al. 1994, Goulden et al. 1996a, Wofsy et al. 1993). This large soil component of NEE, however, has been related only to a simple exponential "Q10" function of soil temperature. Although soil temperature often does account for a large fraction of seasonal and diel variation in soil CO2 fluxes, we know from laboratory and field studies that other factors, such as soil water content (Linn and Doran 1984), rates of C inputs to soils (Trumbore et al. 1995), and diffusivity (Davidson and Trumbore 1995) also affect CO2 efflux from soils. Because the soil is a complex medium of an organo-mineral matrix of variable depth and supporting a broad array of plants, animals, and microorganisms, reductionist approaches to modeling individual components of soil processes that are comparable to canopy physiology models are extremely difficult, and simplifications, such as temperature dependent Q10 functions, are appealing. Moreover, large spatial heterogeneity of root and microbial activity within the landscape and covariation of potentially important factors, such as temperature and water content, create additional challenges to developing mechanistically based models that account for spatial and temporal variation in soil respiration.

Interest in the factors that control soil respiration is growing because of the potential for changing climate, including temperature and precipitation, to affect net ecosystem productivity and exchange of C between terrestrial ecosystems and the atmosphere (Davidson 1994; Goulden et al. 1996a, Jenkinson et al. 1991, Raich and Schlesinger 1992, Schimel et al. 1994). Trumbore et al. (1996) found a relatively high Q10 value of 3.8 for the temperature dependence of turnover of fast cycling low density soil organic matter along elevational and latitudinal gradients, and they argue that soil responses to interannual variation in temperature could account for much of the interannual variation in the atmospheric CO2 anomaly. The use of Q10 functions for modeling soil respiration is common, although Townsend et al. (1992) and Holland et al. (1995) have shown that estimates of global soil respiration are very sensitive to the selected Q10 value for various biomes. Lloyd and Taylor (1994), Kirschbaum (1995), and Schleser (1982) argue that the Q10 value, itself, is temperature dependent, with higher values typically found in colder climates, and these authors also note that Q10 values are often affected by soil moisture conditions. The CENTURY model (Parton et al. 1987) and the Rothamsted model (Jenkinson and Rayner, 1977) use temperature functions for decomposition of soil organic matter that account for greater temperature sensitivity at lower temperatures. Given the recognized uncertainties associated with assigning the appropriate Q10 value to the appropriate place and season, better understanding is clearly needed of the temperature dependence of soil processes and the factors that interact with or are confounded with temperature.

Several examples exist of empirical relationships that have been established between field measurements of soil respiration and soil temperature and water content (Bunnell et al. 1977; Kiefer and Amey 1992; Hanson et al. 1993; Howard and Howard 1993; Oberbauer et al. 1992; Raich and Potter 1995). Most of these relationships tend to be site specific, and no widely accepted and commonly used model has emerged. The only common theme to these various approaches to modeling soil respiration is that they all include an empirically derived Q10 function, although the range of reported Q10 values is large.

In laboratory studies where roots are excluded and temperatures are controlled, the effects of varying soil water content on microbial respiration have been mechanistically attributed to limitation of diffusion of substrate in water films, to desiccation stress at low water contents (Linn and Doran 1984; Skopp et al.

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