Effect of measurement CO(2) concentration on sugar maple root respiration

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 atmosphere. Root respiration was measured as both CO(2) production and O(2) consumption on excised fine roots ( 20,000 micro l 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 micro l l(-1)). Respiration rates changed little at CO(2)]s above 3000 micro l l(-1) and were essentially constant above 6000 micro l 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 micro l 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.