Canopy uptake of atmospheric N deposition at a conifer forest: part I -canopy N budget, photosynthetic efficiency and net ecosystem exchange

Abstract

Global carbon cycle assessments of anthropogenic nitrogen (N) deposition influences on carbon sequestration often assume enhanced sequestration results. This assumption was evaluated at a Rocky Mountains spruce-fir forest. Forest canopy N uptake (CNU) of atmospheric N deposition was estimated by combining event wet and throughfall N fluxes with gradient measured HNO₃ and NH₃ as well as inferred (NO_x and particulate N) dry fluxes. Approximately 80% of the growing-season 3 kgN ha.1 total deposition is retained in canopy foliage and branches. This CNU constitutes ~ 1/3 of canopy growing season new N supply at this conifer forest site.

Daytime net ecosystem exchange (NEE) significantly (P = 0.006) and negatively (CO₂ uptake) correlated with CNU. Multiple regression indicates ~20% of daytime NEE may be attributed to CNU (P >0.02); more than soil water content. A wet deposition N-amendment study (Tomaszewski and Sievering-part II), at canopy spruce branches, increased their growing-season CNU by 40-50% above ambient. Fluorometry and gas exchange results show N-amended spruce branches had greater photosynthetic efficiency and higher carboxylation rates than control and untreated branches. Namended branches had 25% less photoinhibition, with a 5-9% greater proportion of foliar-N-in-Rubisco. The combined results provide, partly, a mechanistic explanation for the NEE dependence on CNU.