Relationship between soil CO2 concentrations and forest-floor CO2 effluxes
Introduction
Worldwide concern with global climate change and its effects on our future environment requires a better understanding of the global carbon cycle. Soils are of particular importance in the global carbon cycle (Houghton et al., 1995, Schimel, 1995) as they contain more carbon than live biomass (Eswaran et al., 1993), and the emission of CO2 from the soil is a major flux of C into the atmosphere (Schlesinger and Andrews, 2000). Soil CO2 efflux represents 40–80% of forest ecosystem respiration (Janssens et al., 2001, Law et al., 1999) and is, therefore, one of the major processes to consider when determining the carbon balance of forests.
Over the last decade, research has focussed on the measurement of fluxes at the soil surface using a variety of chamber and micrometeorological methods. However, there is considerably less information available on CO2 dynamics below the soil surface, apparently due to the difficulty of sampling and measuring soil CO2 concentrations. Though process-based models (e.g. Fang and Moncrieff, 1999, Jassal et al., 2004, Simunek and Saurez, 1993) are valuable tools in increasing our understanding of various processes governing the CO2 exchange within the soil, they need to be validated using measurements.
In a limited number of studies on the measurement of soil CO2 concentrations, samples are either extracted using syringes from gas sampling tubes (e.g. Davidson and Trumbore, 1995, Drewitt et al., 2005), which have been installed in the soil at different depths, or withdrawn by a pump (e.g. Fang and Moncrieff, 1998, Hirsch et al., 2002). Such sampling, however, causes disturbance to the soil environment, and can, therefore, lead to bias in the measurements. Also, such sampling techniques are not suited to the continuous monitoring of soil CO2 concentrations needed to investigate diurnal changes in CO2 storage in the soil. Recently, fast response, industrial solid-state sensors that can be used to measure soil CO2 concentrations have become available. Liang et al. (2004) and Tang et al. (2003) reported continuous measurements of soil CO2 concentrations with such sensors buried in a Japanese larch forest and a relatively dry silt loam soil in a Mediterranean savanna ecosystem in California, respectively. We adapted, calibrated and tested similar solid-state sensors to continuously measure CO2 concentrations in a relatively wet temperate forest ecosystem soil in British Columbia, Canada (Jassal et al., 2004).
Emission of CO2 from soil is the result of CO2 production in the soil and its transport to the surface. Under most field soil conditions, when changes in barometric pressure are small, transport of gases in the soil is mainly by diffusion in air-filled pores. But our understanding of production and transport of CO2 in soil and how these processes are affected by changes in meteorological and soil variables is poor. Production of CO2 in soil is the result of microbial (heterotrophic) and root (autotrophic) respiration. These are functions of the type and distribution of organic matter and roots in soil, respectively, and are governed by mainly soil temperature and water content. Soil CO2 diffusivity changes with air-filled porosity, which in turn is affected by soil bulk density and soil water content. Soil temperature also affects diffusivity. Thus, both the soil CO2 efflux and soil CO2 concentrations are regulated by the production and transport of CO2 in the soil and are, therefore, interdependent. The first objective of this paper is to study diurnal and seasonal variations in long-term continuously measured belowground soil CO2 concentrations and simultaneously measured forest-floor CO2 effluxes, and examine relationships between the two. Simulations with a process-based model (Jassal et al., 2004) showed that in a rapidly draining soil at the same site as in this study, the CO2 efflux, at time scales as low as 30 min, appeared to be well approximated by the rate of total CO2 production in the soil profile, i.e. near steady-state conditions. This was attributed to relatively rapid CO2 diffusion compared to changes in the rate of CO2 production. The second objective of this study is to confirm these results and examine the applicability of a simple steady-state model to calculate the efflux from measurements of soil CO2 concentration.
Section snippets
Site description and soil characteristics
Measurements were made during 2003 in a 54-year-old Douglas-fir stand located about 10 km southwest of Campbell River (49°51′ N, 125°19′ W, 300 m above mean sea level), on the east coast of Vancouver Island, Canada. The site naturally regenerated after a forest fire in 1949 resulting in an almost homogeneous stand. Tree density was about 1100 stems ha−1, tree height was about 33 m, and mean tree diameter at the 1.3 m height was 29 cm. The aboveground estimate of organic carbon (OC) was about 19 kg m−2
Soil CO2 diffusivities
Soil CO2 flux, F and concentration, C are related through effective diffusivity, D as:where D = Dmɛτ, in which Dm is the molecular diffusivity of CO2 in air, ɛ is the soil air-filled porosity and τ is the tortuosity accounting for the zigzag path length through the soil air pores. The product ɛτ (=D/Dm) has been defined as the tortuosity factor, ξ (Jury et al., 1991) and is normally studied as a function of ɛ (Rolston, 1986). Penman (1940) proposed a linear relationship between ξ and ɛ,
Discussion
We found that soil CO2 concentrations at all depths and soil temperature at the 5-cm depth varied in phase diurnally as well as seasonally. Hirsch et al. (2002) studied deep (>20-cm depth) soil CO2 concentration following thaw in a mature boreal forest and found that the seasonal pattern of daily mean soil CO2 concentration and soil temperature at depths up to 23 cm were in phase, but diurnal cycles of CO2 concentration and soil temperature were not in phase. The latter was explained as an
Conclusions
- 1.
A simple steady-state method of measuring CO2 diffusivity in undisturbed soil cores is described. The method accounts for CO2 production in the soil and uses an analytical solution to the diffusion equation. The diffusivity was related to air-filled porosity with a power law function, which was independent of soil depth.
- 2.
Effluxes calculated from near-surface concentration gradients and diffusivities agreed well with chamber-measured effluxes.
- 3.
Calculations showed that more than 75% of the CO2
Acknowledgements
This research was funded by the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS) as a part of Development of a Canadian Global Coupled Carbon Climate model (GC3M) project, a Natural Sciences and Engineering Research Council (NSERC) operating grant, and the Fluxnet Canada Research Network. We thank Andrew Sauter and Rick Ketler for field and laboratory work support during the course of this research.
References (51)
- et al.
Winter wheat carbon exchange in Thuringia
Germany. Agric. For. Meteorol.
(2004) - et al.
Field N2O, CO2 and CH4 fluxes in relation to tillage, compaction and soil quality in Scotland
Soil Till. Res.
(1999) - et al.
Microbial respiration and substrate weight loss. I. A general model of the influence of abiotic variables
Soil Biol. Biochem.
(1977) - et al.
Measuring forest floor CO2 fluxes in a Douglas-fir forest
Agric. For. Meteorol.
(2002) - et al.
A simple and fast technique for measuring the CO2 profile in the soil
Soil Biol. Biochem.
(1998) - et al.
A model of CO2 production and transport. 1. Model development
Agric. For. Meteorol.
(1999) - et al.
A model of the production and transport of CO2 in soil: predicting soil CO2 concentrations and CO2 efflux from a forest floor
Agric. For. Meteorol.
(2004) - et al.
In situ comparison of four approaches to estimating soil CO2 efflux in a northern larch (Larix kaempferi Sarg.) forest
Agric. For. Meteorol.
(2004) - et al.
Temporal and spatial variation of soil CO2 efflux in a Canadian boreal forest
Soil Biol. Biochem.
(2000) - et al.
Influence of high-frequency ambient pressure pumping on carbon dioxide efflux from soil
Agric. For. Meteorol.
(2004)
Assessing soil CO2 efflux using continuous measurements of CO2 profiles in soils with small solid-state sensors
Agric. For. Meteorol.
Considerations for measuring ground CO2 effluxes with chambers
Chem. Geol.
Drying and wetting effects on carbon dioxide release from organic horizons
Soil Sci. Soc. Am. J.
Soil Physics with BASIC
Diffusion within the soil microstructure—a structural parameter for soils
J. Soil Sci.
Soil water content and temperature as independent or confounded factors controlling soil respiration in a temperate mixed hardwood forest
Global Change Biol.
Gas diffusivity and production of CO2 in deep soils of the eastern Amazon
Tellus
Organic carbon in soils of the world
Soil Sci. Soc. Am. J.
Soil Aeration and its Role for Plants
Automated measurement of soil CO2 exchange at the moss surface of a black spruce forest
Tree Phyiol.
Sensitivity of boreal forest carbon balance to soil thaw
Science
Direct measurement of the deep soil respiration accompanying seasonal thawing of a boreal forest soil
J. Geophys. Res.
Cited by (239)
Burying straw interlayers decreases CO<inf>2</inf> emissions in deep saline soil
2023, Sustainable Production and ConsumptionSoil pore space gas probes for use in agricultural research
2021, Soil Security