Estimating root plus rhizosphere contributions to soil respiration in annual croplands

Although soil respiration represents an important C transfer from terrestrial ecosystems to the atmosphere, the effects of environmental and biological factors on soil respiration rates are not adequately understood. This is due primarily to the variety of processes that produce CO, within the soil. Thus, separating the main CO2-producing processes is needed to improve our understanding of soil C cycling and dynamics. Here, we describe and test a model that estimates Soil CO2 emissions derived from anabolic and catabolic processes, representing organic matter decomposition and root + rhizosphere respiration, respectively. Our model is based on the exponential response of organic matter decomposition with respect to temperature, and it requires only measurements of total soil CO2 emissions and soil temperature as inputs. To test the model, we relied on published measurements of soil respiration rates and soil temperatures in a maize (Zea mays L.) field in Ottawa, Canada, and on independent estimations of soil and root contributions for this field made on the basis of stable-C isotope measurements of soil-derived CO2- Modelbased and isotope estimations correlated significantly (r(2) = 0.91, P < 10-9) on a daily basis. Model-based estimations for root + rhizosphere respiration rates for the entire growing season totaled 145 g C m(-2) or 27% of CO2 emissions, and those based on C isotopes totaled 158 g C m(-2) or 30% of the total emissions. The excellent correspondence between model-based and isotope-based estimations suggests that this relatively simple model can be used to distinguish root from soil contributions to soil CO2 emissions in temperate-zone, annual croplands free of significant water stress.