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Estimating Root Plus Rhizosphere Contributions to Soil Respiration in Annual Croplands

^a Dep. of Ecology, Evolution and Organismal Biology, 353 Bessey Hall, Iowa State University, Ames, IA 50011-1020
^b Dep. of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011

View larger version (19K): [in a new window]   Fig. 1. Carbon fluxes through the crop–soil system. 1 = net crop production, generating organic matter from atmospheric CO2. 2 = aboveground crop residues returned to the soil surface. 3 = belowground C allocation, that is, fluxes of C from shoots to root systems. 4 = root mortality and other inputs of organic matter from root systems to the soil. 5 = root, mycorrhizal, and rhizosphere respiration. 6 = CO2 produced by the heterotrophic oxidation of detritus in the soil. 7 = surface soil CO2 efflux, or soil respiration.

View larger version (24K): [in a new window]   Fig. 2. Hypothetical representation of seasonally variable CO2 sources from different root processes contributing to total soil respiration. Root processes that generate CO2 directly (root growth and root respiration) or indirectly (microbial respiration of root exudates) are shown to be seasonal, turning on and off depending on crop growth and phenology. A strongly seasonal pattern of total CO2 production by roots (Rrrh) is generated, even though none of the individual process rates correlate with temperature.

View larger version (24K): [in a new window]   Fig. 3. Observed (solid symbols) and model-estimated (open symbols) rates of soil-CO2 production resulting from (a) root + rhizosphere respiration (Rrrh) and (b) the decomposition of soil organic matter (Rsom) in a maize (Zea mays L.) field at Ottawa, Canada. Observed data are from Rochette et al. (1999).

View larger version (15K): [in a new window]   Fig. 4. Model-predicted root + rhizosphere respiration (Rrrh) in a maize field in Ontario, shown in relation to soil temperature. Each point represents a single days data. The two variables are not correlated (linear regression, n = 18, r2 = 0.05, P = 0.36). The relationship between predicted and isotopically determined Rrrh is shown in Fig. 3.