What was done
As part of a comprehensive experiment, the authors measured nitrogen contents of green and senesced leaves of red maple (Acer rubrum) and sugar maple (Acer saccharum) seedlings growing in open-top chambers in Tennessee, USA, at atmospheric CO2 concentrations of 360 and 660 ppm and ambient and ambient plus 4°C air temperatures to determine if leaf litter nitrogen concentrations are affected by these different environmental conditions.

What was learned
Elevated air temperature had little to no effect on parameters measured in this study.  However, physiologically-active leaves of the red and sugar maple seedlings growing at 660 ppm CO2 contained 19 and 25% less nitrogen, respectively, than corresponding leaves of seedlings growing in ambient air.  Similarly, after senescence had occurred, leaves that had fallen from the CO2-enriched red and sugar maple seedlings had 18 and 22% less nitrogen, respectively, than fallen leaves collected from beneath the seedlings growing in ambient air.  Thus, the lower nitrogen concentrations observed in CO2-enriched leaf litter reflected the lower nitrogen concentrations of the physiologically-active leaves, suggesting that nitrogen remobilization out of leaves during senescence was not affected by elevated CO2.

What it means
As the air's CO2 content continues to rise, it is likely that CO2-induced reductions in the nitrogen concentrations of physiologically-active leaves of red and sugar maple trees will be passed on to their leaf litter.  If these nitrogen deficits are accompanied by increases in litter carbon concentrations - which is very likely, considering that nearly all of earth's vegetation responds to atmospheric CO2 enrichment by synthesizing more primary and secondary carbon compounds - it is likely that the decomposition rates of such litter will be reduced.  Thus, it is to be expected that deciduous forests containing these maple species will exhibit greater amounts of carbon sequestration in their soils as the air's CO2 content continues to increase.