A NUMERICAL MODEL FOR ESTIMATING LONG-TERM RESPIRATION RATES FROM CARBON OCCLUDED IN SOIL GIBBSITE

Carbon occluded in the soil gibbsite crystal structure at the Panola Mountain Research Watershed, Georgia, U.S. is presumed to be in isotopic equilibrium with the CO₂ respired from soil organics by microbes and plant roots. Water-filled pores however reduce the effective porosity and modify gas diffusion and potentially limit the use of analytical Fickian diffusion models that assume a constant diffusion coefficient throughout a soil profile. We assume that the difference between volumetric water content at 0 cm and -300 cm soil water pressure accurately reflects gas-filled porosity in the soil profile studied. The choice of -300 cm soil water pressure is based on the similarity of soil texture (i.e., % sand, silt, and clay) profiles studied by Bruce et al. (1983), who establishes the strong correlation between soil texture and the capillary soil water retention process in a Cecil soil in the southeastern U.S. We therefore employ the physical characteristics of Cecil soils and assume that the measures of volumetric water content over the depth intervals of 10-16, 35-41, 60-66, 87-93, and 129-135 cm represent the distribution function for gas-filled porosity.  Our numerical model, which includes depth-dependent production and diffusion terms, results in long-term respiration rate estimates of 28 to 12 gC m^-2 y^-1. These values range from 15 to 50 times less than the average of modern values for mixed deciduous forests in wet temperate climates. This disparity has several implications for our understanding of the geologic record of climate change, which include: (1) evidence for a cooler and seasonally drier climate during the mid-Holocene in the southeastern U.S., or (2) fluxes of carbon from the soil pool as recorded by soil mineral proxies (i.e., long-term soil respiration rates) under estimate atmosphere annual carbon flux measurements (i.e., short-term measures), and (3) the need to refine soil respiration models used to relate paleosol stable carbon isotopic measurements to paleo-atmospheric PCO₂ estimates.

Bruce, R.R., J.H. Dane, V.L. Quisenberry, N.L. Powell, and A.W. Thomas. (1983) Physical characteristics of soil in the southern region: Cecil. Southern Cooperative Series Bulletin 267, 185 p.