Stomatal limitations to mass and energy exchange over boreal black spruce forests may be caused by low needle N concentrations that limit CO2 fixation rates. These low concentrations may be caused by low N uptake rates from cold boreal soils with high soil C:N ratios and by low N deposition rates from boreal atmospheres. A mathematical model of terrestrial ecosystems ecosys was used to examine the likelihood that slow N cycling could account for the low rates of mass and energy exchange measured over a 115-year old boreal spruce/moss forest as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). In the model, net N mineralization was slowed by the high C:N ratios measured in the forest floor and by high lignin contents in spruce litterfall. Slow mineralization caused low N uptake rates and hence high C:N ratios in spruce and moss leaves that reduced specific activities and areal densities of rubisco and chlorophyll. Consequent low CO2 fixation rates caused low stomatal conductances and transpiration rates which in turn caused high soil water contents. Wet soils, in conjunction with large accumulations of surface detritus generated by slow litter mineralization, caused low soil temperatures that further slowed mineralization rates. Model outputs for ecosystem N status were corroborated by low needle N concentrations (<10 mgg-1), stomatal conductances (<0.05 molm-2s-1) and CO2 fixation rates (<6 &mgr;molm-2s-1), and by high canopy Bowen ratios (1.5--2.0) and low canopy net CO2 exchange rates (<10 &mgr;molm-2s-1) measured over the black spruce/moss forest at the BOREAS site. Modeled C accumulation rates of 60(wood)+10(soil)=70gCm-2yr-1 were consistent with estimates from aggregated CO2 fluxes measured over the spruce canopy and from allometric equations developed for black spruce in Canadian boreal forests. Model projections under IS92a climate change indicate that rates of wood C accumulation would rise and those of soil C accumulation would decline from those under current climate. Because these rates are N-limited, they would be raised by increases in atmospheric N deposition. ¿ 2001 American Geophysical Union