Plant and Soil 195: 221–232, 1997. 221 c 1997 Kluwer Academic Publishers. Printed in the Netherlands. Estimating respiration of roots in soil: Interactions with soil CO 2 , soil temperature and soil water content Tjeerd J. Bouma , Kai L. Nielsen, David M. Eissenstat and Jonathan P. Lynch Department of Horticulture, The Pennsylvania State University, 103 Tyson Building, University Park, Pennsylvania 16802–4200, USA ( Author to whom correspondence should be addressed at the following address: Netherlands Institute of Ecology, Center for Estuarine and Coastal Ecology, P.O. Box 140, 4400 AC Yerseke, the Netherlands (fax: +31-113-573616; E-mail: tbouma@cemo.nioo.knaw.nl) Received 9 January 1997. Accepted in revised form 24 June 1997 Key words: citrus, Citrus volkameriana Tan. & Pasq., CO 2 -diffusion gradient, root respiration, soil CO 2 concen- tration, Volkamer lemon. Abstract Little information is available on the variability of the dynamics of the actual and observed root respiration rate in relation to abiotic factors. In this study, we describe I) interactions between soil CO 2 concentration, temperature, soil water content and root respiration, and II) the effect of short-term fluctuations of these three environmental factors on the relation between actual and observed root respiration rates. We designed an automated, open, gas- exchange system that allows continuous measurements on 12 chambers with intact roots in soil. By using three distinct chamber designs with each a different path for the air flow, we were able to measure root respiration over a 50-fold range of soil CO 2 concentrations (400 to 25000 ppm) and to separate the effect of irrigation on observed vs. actual root respiration rate. All respiration measurements were made on one-year-old citrus seedlings in sterilized sandy soil with minimal organic material. Root respiration was strongly affected by diurnal fluctuations in temperature (Q 10 = 2), which agrees well with the literature. In contrast to earlier findings for Douglas-fir (Qi et al., 1994), root respiration rates of citrus were not affected by soil CO 2 concentrations (400 to 25000 ppm CO 2 ; pH around 6). Soil CO 2 was strongly affected by soil water content but not by respiration measurements, unless the air flow for root respiration measurements was directed through the soil. The latter method of measuring root respiration reduced soil CO 2 concentration to that of incoming air. Irrigation caused a temporary reduction in CO 2 diffusion, decreasing the observed respiration rates obtained by techniques that depended on diffusion. This apparent drop in respiration rate did not occur if the air flow was directed through the soil. Our dynamic data are used to indicate the optimal method of measuring root respiration in soil, in relation to the objectives and limitations of the experimental conditions. Introduction More than 50% of plant photosynthates produced dai- ly may be respired by the roots, depending on relative growth rate and nutritional status of the plant (Lam- bers et al., 1996). Thus, quantitative information on root respiration is as important to understanding plant growth as is photosynthesis. Root respiration can be accurately measured in nutrient solutions (e.g., Bloom et al., 1992; Lambers et al., 1996; Veen, 1980). Mea- suring root respiration in soil is less precise due to vari- able environmental conditions, but valuable as it more closely reflects natural conditions. One well-known source of variation is microbial activity (e.g., Rochette et al., 1991). Here we focus on other factors that may affect root respiration measurements, using a sterilized and sieved sandy soil with minimal organic matter to minimize microbial respiration of carbon not originat- ing from the plant. Root respiration rate is affected by various abiot- ic factors. For example, effects of temperature (e.g., Edwards, 1991) and soil water content (e.g., Palta and