Separating Soil Respiration into Plant and Soil Components Using Analyses of the Natural Abundance of Carbon‐13

In presence of vegetation, the CO 2 –C produced by respiration activity in soils originates from plant C (rhizosphere respiration, R rh ) and from soil C (soil respiration, R s ). Quantitative estimates of the CO 2 produced by each source are required in many studies of C dynamics in the soil–plant system. In this study, we (i) used measurements of the 13 C value of soil CO 2 to separate total soil respiration ( R t ) into subcomponents R rh and R s in a maize ( Zea mays L.) field under undisturbed conditions and (ii) compared these R rh estimates with values obtained using the root‐exclusion approach. The maximum contribution of R rh to total respiration was 45%, observed in August. Estimates of R rh increased from zero 30 d after planting to 2 g CO 2 –C m −2 d −1 70 d after planting, remained relatively constant at that level in August, and then decreased until the end of the growing season. The total C losses as R rh were 17% of the crop net assimilation. Estimates of R s gradually declined from 3.3 g CO 2 –C m −2 d −1 in late June to 1.4 g CO 2 –C m −2 d −1 at the end of the season. Losses of soil C represented ≈6% of total soil C. Variable values of δ 13 C of the soil CO 2 in the control plot after Day 250 made the technique less reliable late in the season. However, several observations indicated that the approach has potential to provide quantitative estimates of R rh and R s First, the seasonal pattern of the R rh estimates coincided with that of the plant growth and physiological activity. Second, the cumulated R rh across the growing season agreed well with published data obtained using 14 C labeling techniques. Third, in the maize plot, variation in the estimated R s was closely correlated with changes in soil temperature with a Q 10 of 1.99 Finally, the estimates of R rh obtained using the isotopic approach agreed well with those obtained using the root exclusion technique.