High soil carbon dioxide concentrations inhibit root respiration of Douglas fir

summary Total and basal respiration (RtandRb, respectively) of intact and undisturbed roots of one‐year‐old Douglas fir seedlings,Pseudotsuga menziesiivar.glauca[Beissn] Franco, were measured at experimentally varied soil carbon dioxide concentrations ([CO2]). Use of specially designed root boxes and a CO2gas‐flow compensating system designed around an infrared gas analyzer (IRGA) allowed controlled delivery of CO2to roots and simultaneous measurements of CO2released by roots. Root respiration rate responded to each inlet [CO2], independent of whether the previous concentration had been higher or lower, within two to three hours (pairedttest = 0.041,P= 0.622, andn= 13). Total and basal respiration rates decreased exponentially as soil [CO2] rose from 130 ppm, well below atmospheric [CO2], to 7015 ppm, a concentration not uncommon in field soils. Analyses of variance (ANOVA) showed that the effects of soil [CO2] on rates of total and basal root respiration were statistically significant. Root respiration rates decreased by 4 to 5 nmol CO2g−1dry weight of roots s−1for every doubling of [CO2] according to the following equations: ln(R1) (nmol CO2g−1s−1) = 5.24–0.30*ln[CO2] withr= 0.78,P< 0.0001, andn= 70; and ln(Rb) (nmol CO2g−1s−1) = 6.29–0.52 * ln[CO2] withr= 0.82,P< 0.0001, andn= 35. The sensitivity of root respiration to [CO2] suggests that some previous laboratory measurements of root respiration at atmospheric [CO2], which is 3 to 10‐fold lower than [CO2] in field soils, overestimated root respiration in the field. Further, the potential importance of soil [CO2] indicates that it should be accounted for in models of below‐ground carbon budgets.