Carbon Release from Roots to Microbes Prevents Nitrogen Limitation Under CO2 Enrichment


A forest’s ability to store carbon depends on resource limitations, such as nitrogen. The Progressive Nitrogen Limitation (PNL) theory suggests that under elevated CO2, a forest will immobilize nitrogen in biomass, limiting nitrogen needed for enhanced growth. DOE scientists show, for the first time, that mature trees exposed to CO2 enrichment increase the release of soluble carbon from roots to soil, and that such increases are coupled to the accelerated turnover of nitrogen pools in the rhizosphere. Over the course of three years, the team measured in situ rates of root exudation from intact loblolly pine (Pinus taeda L.) roots at the Duke Forest, near Chapel Hill, North Carolina. Trees fumigated with elevated CO2 increased exudation rates by 55% during the primary growing season, leading to a 50% annual increase in dissolved organic inputs to fumigated forest soils. These increases in root-derived carbon were positively correlated with microbial release of extracellular enzymes involved in breakdown of organic nitrogen in the rhizosphere, indicating that exudation stimulated microbial activity and accelerated the rate of soil organic matter turnover. Trees exposed to both elevated CO2 and nitrogen fertilization did not increase exudation rates and had reduced enzyme activities in the rhizosphere. These results provide field-based empirical support suggesting that sustained growth responses of forests to elevated CO2 in low fertility soils are maintained by enhanced rates of microbial activity and nitrogen cycling fuelled by inputs of root-derived carbon. However, the decomposition of soil organic matter by the stimulated microbes may prevent a large soil carbon pool from accumulating in forest soils.


Phillips, R. P, A. C. Finzi, and E. S. Bernhardt. 2011. “Enhanced Root Exudation Induces Microbial Feedbacks to N Cycling in a Pine Forest Under Long-Term CO2 Fumigation,” Ecology Letters 14, 187–94.

Drake, J. E, A. Gallet-Budynek, K. S. Hofmockel, E. S. Bernhardt, S. A. Billings, R. B. Jackson, K. S. Johnsen, J. Lichter, H. R. McCarthy, M. L. McCormack, D. J. P. Moore, R. Oren, S. Palmroth, R. P. Phillips, J. S. Pippen, S. G. Pritchard, K. K. Treseder, W. H. Schlesinger, E. H. DeLucia, and A. C. Finzi. 2011. “Increases in the Flux of Carbon Belowground Stimulate Nitrogen Uptake and Sustain the Long-Term Enhancement of Forest Productivity Under Elevated CO2,” Ecology Letters 14, 349–57.