Accurate estimates of root respiration are crucial to predicting belowground C cycling in forest ecosystems. Inhibition of respiration has been reported as a short-term response of plant tissue to elevated measurement [CO 2 ]. We sought to determine if measurement [CO 2 ] affected root respiration in samples from mature sugar maple (Acer saccharum Marsh.) forests and to assess possible errors associated with root respiration measurements made at [CO 2 ]s lower than that typical of the soil

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... phere. Root respiration was measured as both CO 2 production and O 2 consumption on excised fine roots (≤ 1.0 mm) at [CO 2 ]s ranging from 350 to > 20,000 µl l −1 . Root respiration was significantly affected by the [CO 2 ] at which measurements were made for both CO 2 production and O 2 consumption. Root respiration was most sensitive to [CO 2 ] near and below normal soil concentrations (< 1500 µl l −1 ). Respiration rates changed little at [CO 2 ]s above 3000 µl l −1 and were essentially constant above 6000 µl l −1 CO 2 . These findings call into question estimates of root respiration made at or near atmospheric [CO 2 ], suggesting that they overestimate actual rates in the soil. Our results indicate that sugar maple root respiration at atmospheric [CO 2 ] (350 µl l −1 ) is about 139% of that at soil [CO 2 ]. Although the causal mechanism remains unknown, the increase in root respiration at low measurement [CO 2 ] is significant and should be accounted for when estimating or modeling root respiration. Until the direct effect of [CO 2 ] on root respiration is fully understood, we recommend making measurements at a [CO 2 ] representative of, or higher than, soil [CO 2 ]. In all cases, the [CO 2 ] at which measurements are made and the [CO 2 ] typical of the soil atmosphere should be reported.