As stressed by Schimel et al. 1995 , the uptake of atmospheric CO due to long-term increases in N deposition could increase as human N pollution increases; however, this increase may occur only up to a critical threshold loading value, beyond which any additional N input will cause significant ecosystem degradation (cf. Aber et al.,1989; Schultze,1989; Waring and Running 1998) More detailed studies are needed in order to accept or reject the tentative conclusions that we have drawn here, and we urge further additional