The nitrogen budget of a pine forest under free air CO₂ enrichment

Elevated concentrations of atmospheric CO₂ increase plant biomass, net primary production (NPP) and plant demand for nitrogen (N). The demand for N set by rapid plant growth under elevated CO₂ could be met by increasing soil N availability or by greater efficiency of N uptake. Alternatively, plants could increase their nitrogen-use efficiency (NUE), thereby maintaining high rates of growth and NPP in the face of nutrient limitation. We quantified dry matter and N budgets for a young pine forest exposed to 4 years of elevated CO₂ using free-air CO₂ enrichment technology. We addressed three questions: Does elevated CO₂ increase forest NPP and the demand for N by vegetation? Is demand for N met by greater uptake from soils, a shift in the distribution of N between plants, microbes, and soils, or increases in NUE under elevated CO₂? Will soil N availability constrain the NPP response of this forest as CO₂ fumigation continues? A step-function increase in atmospheric CO₂ significantly increased NPP during the first 4 years of this study. Significant increases in NUE under elevated CO₂ modulated the average annual requirement for N by vegetation in the first and third growing seasons under elevated CO₂; the average stimulation of NPP in these years was 21% whereas the average annual stimulation of the N requirement was only 6%. In the second and fourth growing seasons, increases in NPP increased the annual requirement for N by 27-33%. Increases in the annual requirement for N were largely met by increases in N uptake from soils. Retranslocation of nutrients prior to senescence played only a minor role in supplying the additional N required by trees growing under elevated CO₂. NPP was highly correlated with between-plot variation in the annual rate of net N mineralization and CO₂ treatment. This demonstrates that NPP is co-limited by C availability, as CO₂ from the atmosphere, and N availability from soils. There is no evidence that soil N mineralization rates have increased under elevated CO₂. The correlation between NPP and N mineralization rates and the increase in the annual requirement for N in certain years imply that soil N availability may control the long-term productivity response of this ecosystem to elevated CO₂. Although we have no evidence suggesting that NPP is declining in response to >4 years of CO₂ fumigation, if the annual requirement of N continues to be stimulated by elevated CO₂, we predict that the productivity response of this forest ecosystem will decline over time.