HIGH SOIL CARBON-DIOXIDE CONCENTRATIONS INHIBIT ROOT RESPIRATION OF DOUGLAS-FIR

Total and basal respiration (R(t) and R(b), respectively) of intact and undisturbed roots of one-year-old Douglas fir seedlings, Pseudotsuga menziesii var. glauca [Beissn] France, were measured at experimentally varied soil carbon dioxide concentrations ([CO2]). Use of specially designed root boxes and a CO2 gas-flow compensating system designed around an infrared gas analyzer (IRGA) allowed controlled delivery of CO2 to roots and simultaneous measurements of CO2 released 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 (paired t test = 0.041, P = 0.622, and n = 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 CO2 g(-1) dry weight of roots s(-1) for every doubling of [CO2] according to the following equations: In(R(t)) (nmol CO2 g(-1) s(-1)) = 5.24-0.30*ln[CO2] with r = 0.78, P < 0.0001, and n = 70; and ln(R(b)) (nmol CO2 g(-1) s(-1)) = 6.29-0.52*ln[CO2] with r = 0.82, P < 0.0001, and n = 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.