Contribution of atmospheric nitrogen deposition to net primary productivity in a northern hardwood forest

Net primary productivity (NPP) in northern temperate forests is an important part of the global carbon cycle. Because NPP often is limited by nitrogen (N), atmospheric N deposition (Ndep) may increase forest NPP. At a northern hardwood forest site in northern Lower Michigan, USA, we quantified rates of N supply by Ndep, canopy retention of Ndep (Ncr), and soil net N mineralization (Nmin); calculated the N requirement of NPP; and estimated the fraction of NPP that could be attributed to atmospheric N inputs. Net N mineralization supplied 42.6 kg N·ha–1·year–1 (84% of the NPP N requirement), and Ndep averaged 7.5 kg N·ha–1·year–1 (15%). The forest canopy retained 38% of Ndep (Ncr = 2.8 kg N·ha–1·year–1), primarily in the forms of organic N and NH4-N. Fine root (62%) and leaf (31%) N requirements dominated the NPP N requirement of 50.7 kg N·ha–1·year–1. Annual N supply by the processes we measured agreed very closely with the NPP N requirement, suggesting that internally cycled N and N of atmospheric origin contribute to the N nutrition of this forest. Our results indicate that up to 15% of the NPP at this site may be driven by atmospheric N inputs.