Tuesday, November 14, 2006 - 2:35 PM
181-5
Vertical Partitioning of CO2 and N2O Production within Soil Profiles Using Diffusivity Models.
The major driving factors of soil CO2 and N2O production – substrate supply, temperature, and water content – vary vertically within the soil profile, with the greatest temporal variation usually near the soil surface. Several studies have used profiles of gas concentrations, soil water content, and porosity to model diffusivity and production of soil gases as a function of soil depth. Here I present results from a deep tropical forest soil and a shallow temperate forest soil to illustrate the strengths and weaknesses and this approach and the ecological insight that it provides. Although concentrations increase with soil depth, production of these gases is greatest near the soil surface. The O horizon produced at least 40% of the annual soil CO2 efflux in the temperate forest on an Inceptisol. Vertical partitioning revealed that water content covaried with CO2 production only in the O horizon, and that this layer could be an important transient interannual source or sink of ecosystem C. The presence of large rocks in glaciated soil presents a special challenge for diffusivity models. In the tropical Oxisol, CO2 production scaled with fine root biomass to a depth of 7m, where a small deep soil CO2 source caused the concentration to increase with depth throughout the measured profile. In contrast, the concentration of N2O peaked within the top 3m during the wet season, indicating that production below 3m depth, if it occurred at all, was less than net downward diffusion from sources above. However, a modest increase in N2O concentrations to 11m depth during the dry season indicates a possible modest deep soil source. Uncertainties in quantitative estimates of CO2 and N2O production by depth remain large. The method is most effective for revealing relative patterns of gaseous production as a function of depth.
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