Fine-root respiration in a loblolly pine (Pinus taeda L.) forest exposed to elevated CO2 and N fertilization

Forest ecosystems release large amounts of carbon to the atmosphere from fine-root respiration (R-r), but the control of this flux and its temperature sensitivity (Q(10)) are poorly understood. We attempted to: (1) identify the factors limiting this flux using additions of glucose and an electron transport uncoupler (carbonyl cyanide m-chlorophenylhydrazone); and (2) improve yearly estimates of R-r by directly measuring its Q(10)in situ using temperature-controlled cuvettes buried around intact, attached roots. The proximal limits of R-r of loblolly pine (Pinus taeda L.) trees exposed to free-air CO2 enrichment (FACE) and N fertilization were seasonally variable; enzyme capacity limited R-r in the winter, and a combination of substrate supply and adenylate availability limited R-r in summer months. The limiting factors of R-r were not affected by elevated CO2 or N fertilization. Elevated CO2 increased annual stand-level R-r by 34% whereas the combination of elevated CO2 and N fertilization reduced R-r by 40%. Measurements of in situ R-r with high temporal resolution detected diel patterns that were correlated with canopy photosynthesis with a lag of 1 d or less as measured by eddy covariance, indicating a dynamic link between canopy photosynthesis and root respiration. These results suggest that R-r is coupled to daily canopy photosynthesis and increases with carbon allocation below ground.