Nonlinear responses of ecosystem carbon fluxes to nitrogen deposition in an old‐growth boreal forest.

Nitrogen (N) deposition is known to increase carbon (C) sequestration in N‐limited boreal forests. However, the long‐term effects of N deposition on ecosystem carbon fluxes have been rarely investigated in old‐growth boreal forests. Here we show that decade‐long experimental N additions significantly stimulated net primary production (NPP) but the effect decreased with increasing N loads. The effect on soil heterotrophic respiration (Rh) shifted from a stimulation at low‐level N additions to an inhibition at higher levels of N additions. Consequently, low‐level N additions resulted in a neutral effect on net ecosystem productivity (NEP), due to a comparable stimulating effect on NPP and Rh, while NEP was increased by high‐level N additions. Moreover, we found nonlinear temporal responses of NPP, Rh and NEP to low‐level N additions. Our findings imply that actual N deposition in boreal forests likely exerts a minor contribution to their soil C storage. [ABSTRACT FROM AUTHOR]