The deposition of reactive nitrogen (N) onto terrestrial ecosystems can change ecosystem processes and the size of the global land carbon sink. When N is added to ecosystems, the carbon response to the added N is often larger in studies that apply N to leaves or compare sites on N deposition gradients, as compared to studies where N is applied to soil. This suggests that foliar interception and direct uptake of gaseous N deposition could play an important role in future responses of the land carbon sink. However, limits to field techniques make it challenging to quantify the impact of direct foliar uptake of N deposition on carbon uptake across ecosystems and to scale this response to the globe. In this study, we quantified the sensitivity of Earth system model estimates of terrestrial carbon uptake to direct foliar uptake of atmospheric N deposition. We accomplished this by a) adding this route of plant N acquisition into an Earth system model (Community Land Model version 4.5) and b) running global simulations where plants could take up to 10% or 30% of N deposition through leaves, with a corresponding reduction of plant N demand from soil.
Preliminary results, in which foliar N uptake simply substituted for soil uptake, induced only minor changes in global annual averages of gross primary productivity (GPP) and net primary productivity (NPP), on the order of 1%. Global annual average GPP and NPP were larger under present-day N deposition than under pre-industrial conditions, likely due to reductions in plant N limitation from higher rates of N deposition. Only small responses in GPP and NPP occurred with the substitution of foliar N uptake for soil N uptake under both sets of conditions. Although these results suggest that foliar uptake of N deposition has a minor effect on the global land carbon sink, additional simulations will examine a role for foliar N uptake to increase plant N, rather than simply substituting for soil N. Moreover, this version of the model did not account for cost differences between actively acquiring N through roots and passively through leaves. Accounting for carbon cost differences between methods of plant N acquisition may reveal a larger effect of leaf uptake of N deposition on the global carbon sink.