Relationship between soil CO2 concentrations and forest-floor CO2 effluxes

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Abstract

To better understand the biotic and abiotic factors that control soil CO2 efflux, we compared seasonal and diurnal variations in simultaneously measured forest-floor CO2 effluxes and soil CO2 concentration profiles in a 54-year-old Douglas fir forest on the east coast of Vancouver Island. We used small solid-state infrared CO2 sensors for long-term continuous real-time measurement of CO2 concentrations at different depths, and measured half-hourly soil CO2 effluxes with an automated non-steady-state chamber. We describe a simple steady-state method to measure CO2 diffusivity in undisturbed soil cores. The method accounts for the CO2 production in the soil and uses an analytical solution to the diffusion equation. The diffusivity was related to air-filled porosity by a power law function, which was independent of soil depth. CO2 concentration at all depths increased with increase in soil temperature, likely due to a rise in CO2 production, and with increase in soil water content due to decreased diffusivity or increased CO2 production or both. It also increased with soil depth reaching almost 10 mmol mol−1 at the 50-cm depth. Annually, soil CO2 efflux was best described by an exponential function of soil temperature at the 5-cm depth, with the reference efflux at 10 °C (F10) of 2.6 μmol m−2 s−1 and the Q10 of 3.7. No evidence of displacement of CO2-rich soil air with rain was observed.

Effluxes calculated from soil CO2 concentration gradients near the surface closely agreed with the measured effluxes. Calculations indicated that more than 75% of the soil CO2 efflux originated in the top 20 cm soil. Calculated CO2 production varied with soil temperature, soil water content and season, and when scaled to 10 °C also showed some diurnal variation. Soil CO2 efflux and concentrations as well as soil temperature at the 5-cm depth varied in phase. Changes in CO2 storage in the 0–50 cm soil layer were an order of magnitude smaller than measured effluxes. Soil CO2 efflux was proportional to CO2 concentration at the 50-cm depth with the slope determined by soil water content, which was consistent with a simple steady-state analytical model of diffusive transport of CO2 in the soil. The latter proved successful in calculating effluxes during 2004.

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:F=DCzwhere 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.

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