Vertical partitioning of CO2 production within a temperate forest soil

Eric A Davidson,Kathleen E Savage,Susan E Trumbore,Werner Borken

doi:10.1111/j.1365-2486.2005.01142.x

Eric A Davidson, Kathleen E Savage + Show 2 more

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https://doi.org/10.1111/j.1365-2486.2005.01142.x

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Abstract

AbstractThe major driving factors of soil CO2 production – substrate supply, temperature, and water content – vary vertically within the soil profile, with the greatest temporal variations of these factors usually near the soil surface. Several studies have demonstrated that wetting and drying of the organic horizon contributes to temporal variation in summertime soil CO2 efflux in forests, but this contribution is difficult to quantify. The objectives of this study were to partition CO2 production vertically in a mixed hardwood stand of the Harvard Forest, Massachusetts, USA, and then to use that partitioning to evaluate how the relative contributions of CO2 production by genetic soil horizon vary seasonally and interannually. We measured surface CO2 efflux and vertical soil profiles of CO2 concentration, temperature, water content, and soil physical characteristics. These data were applied to a model of effective diffusivity to estimate CO2 flux at the top of each genetic soil horizon and the production within each horizon. A sensitivity analysis revealed sources of uncertainty when applying a diffusivity model to a rocky soil with large spatial heterogeneity, especially estimates of bulk density and volumetric water content and matching measurements of profiles and surface fluxes. We conservatively estimate that the O horizon contributed 40–48% of the total annual soil CO2 efflux. Although the temperature sensitivity of CO2 production varied across soil horizons, the partitioning of CO2 production by horizon did not improve the overall prediction of surface CO2 effluxes based on temperature functions. However, vertical partitioning revealed that water content covaried with CO2 production only in the O horizon. Large interannual variations in estimates of O horizon CO2 production indicate that this layer could be an important transient interannual source or sink of ecosystem C.

Highlights

Production of CO2 by soil microorganisms and roots varies temporally, spatially across landscapes, and vertically within the soil profile
We observed no change in soil wetness or diffusivity that would account for accumulation of CO2 in the deep soil of pit #2, so we speculate that roots grew near the sampling tubes in the deep soil horizons of pit #2 beginning in 2001, contributing to larger CO2 concentrations there
Based on lower diffusivities in the mineral soil measured by the radon method compared with the model in the present study, our estimate of the importance of the O horizon is likely to be conservative, and its contribution could be as high as 50–60% of total soil CO2 production

Summary

Introduction

Production of CO2 by soil microorganisms and roots varies temporally, spatially across landscapes, and vertically within the soil profile. Soils warm from the top downward in the spring and cool from the top downward in the autumn. It is unclear whether depth-dependent partitioning of CO2 production and its response to temperature would improve empirical models of soil CO2 efflux (Subke et al, 2003). The major driving factors of soil CO2 production – substrate supply, temperature, and water content – all vary vertically within the soil profile, with the greatest temporal variability usually near the soil surface

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