Estimating Root Plus Rhizosphere Contributions to Soil Respiration in Annual Croplands

but measurements of total soil-CO2 emissions represent only their sum. Thus, distinguishing among different soil Although soil respiration represents an important C transfer from CO2–producing processes is required if soil respiration terrestrial ecosystems to the atmosphere, the effects of environmental and biological factors on soil respiration rates are not adequately measurements are to be used to investigate controls over understood. This is due primarily to the variety of processes that those individual processes. It would be particularly useful produce CO2 within the soil. Thus, separating the main CO2–producing to distinguish the CO2 produced during SOM decompoprocesses is needed to improve our understanding of soil C cycling sition from the CO2 produced by root and rhizosphere and dynamics. Here, we describe and test a model that estimates respiration (Fig. 1) because this distinction would enable soil CO2 emissions derived from anabolic and catabolic processes, soil-CO2 efflux measurements to be used to evaluate in representing organic matter decomposition and root rhizosphere situ SOM decay and turnover rates or, alternatively, in respiration, respectively. Our model is based on the exponential resitu rates of root and rhizosphere respiration. sponse of organic matter decomposition with respect to temperature, Figure 1 illustrates the flux of C through the crop–soil and it requires only measurements of total soil CO2 emissions and system. For simplification, erosional losses of SOM and soil temperature as inputs. To test the model, we relied on published measurements of soil respiration rates and soil temperatures in a organic matter inputs from outside the system, such as maize (Zea mays L.) field in Ottawa, Canada, and on independent manure applications, have been excluded; in locations estimations of soil and root contributions for this field made on the where those processes are important they would need basis of stable-C isotope measurements of soil-derived CO2. Modelto be incorporated into the mass balance. Crop harvests based and isotope estimations correlated significantly (r 2 0.91, P are not included in Fig. 1 because they reflect removals 10 9) on a daily basis. Model-based estimations for root rhizosphere of biomass before its incorporation into the soil. Figure respiration rates for the entire growing season totaled 145 g C m 2 1 indicates that only the quantity of C that actually or 27% of CO2 emissions, and those based on C isotopes totaled 158 g passes through the soil food web (flows 1 → 2 4 → C m 2 or 30% of the total emissions. The excellent correspondence 6 in Fig. 1) is useful for quantification of soil C loss between model-based and isotope-based estimations suggests that this rates, or determination of SOM turnover rates. Flowrelatively simple model can be used to distinguish root from soil contributions to soil CO2 emissions in temperate-zone, annual croppath 1 → 3 → 5 represents photosynthetic products lands free of significant water stress. that are consumed directly by roots, mycorrhizae, and rhizosphere-associated organisms in their respiratory pathways (i.e., catabolic processes), without ever being diverted to secondary metabolic pathways (i.e., anabolic T release of carbon dioxide from soils to the processes) that lead to the formation of proteins, strucatmosphere is the single largest pathway by which tural materials, and secondary compounds that are enzyC is lost from soils in most annual cropping systems matically decomposed within soils. Observed temporal (Buyanovsky et al., 1987; Paustian et al., 1990; Paul et variations in soil respiration rates (Flow 7 in Fig. 1) may al., 1999). Measurements of soil CO2 emissions therefore reflect changes in either SOM decomposition (Flow 6) provide useful insights into soil C cycling, and provide or in root and rhizosphere respiration (Flow 5). Thus, a basis for evaluating soil C dynamics and potential C observed correlations between environmental variables sequestration under different crop management systems and soil respiration rates cannot be assumed to reflect (e.g., Lundegårdh, 1927; Monteith et al., 1964; Alvarez the independent effects of those environmental variet al., 1995; Franzluebbers et al., 1995; Duiker and Lal, ables on the individual processes generating CO2 within 2000). The use of soil respiration measurements to evalthe soil. uate soil C balances is confounded, however, by the For example, a highly significant, positive correlation whole-soil nature of the flux. Carbon dioxide is probetween soil temperature and soil-CO2 emissions is freduced in soils by a variety of processes, including both quently observed in field studies (e.g., Rochette et al., root respiration and heterotrophic oxidation of soil or1991; Alvarez et al., 1995; Luo et al., 2001; Tufekcioglu ganic matter (SOM). The specific effects of environmenet al., 2001). Such data suggest that higher temperatures tal variables on root and microbial processes in soils may stimulate the heterotrophic oxidation of SOM and may differ (e.g., Kirschbaum, 1995; Boone et al., 1998), deplete soil C pools (e.g., Schleser, 1982; Jenkinson et J. Raich, Dep. of Ecology, Evolution and Organismal Biology, 353 al., 1991; Raich and Schlesinger, 1992; Amundson, Bessey Hall, Iowa State University, Ames, IA 50011-1020; G. Mora, 2001). Soil-warming experiments generally support the Dep. of Geological and Atmospheric Sciences, Iowa State University, hypothesis that increased temperatures stimulate soil Ames, IA 50011. Received 29 July 2004. Soil Biology and Biochemisrespiration rates (Rustad et al., 2001), but higher rates try. *Corresponding author ([email protected]). of soil respiration do not necessarily indicate faster rates Published in Soil Sci. Soc. Am. J. 69:634–639 (2005). of SOM decomposition (Flow 6 in Fig. 1); they may doi:10.2136/sssaj2004.0257 © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: DOY, day of year; SOM, soil organic matter.