Canopy uptake of atmospheric N deposition at a conifer forest: Part II- response of chlorophyll fluorescence and gas exchange parameters

Spruce foliage was sprayed with NH4 +NO3 − and control solutions to investigate the effect of canopy nitrogen (N) uptake on chlorophyll fluorescence and gas exchange at a RockyMountain subalpine forest (Niwot Forest) at the Niwot Ridge Long-Term Ecological Research site. N-treated branches received NH4 +NO3 − in an ion-matrix solution that was representative of mean precipitation ion concentrations. Branches were sprayed with NH4 +NO3 − to increase the wet N deposition to experimental branches 60% above ambient. Control branches received only the ion-matrix solution (no N), while background branches received only natural precipitation. N content of N-treated new growth and old growth shoots was 2 and 8% greater, respectively, than the background and control shoots’ N content. N-treatment enhanced photosynthetic efficiency (Fv /Fm ) of old growth spruce shoots (≥1 yr old); N-treated shoots’ Fv /Fm was 11–12% greater than control and background shoots’ Fv /Fm (p < 0.05). Greater maximum carboxylation rates accompanied this increased photosynthetic efficiency with the N-treated old growth shoots’ Vcmax 14–15% greater than background and control shoots’ Vcmax (p < 0.05). New growth gas exchange and fluorescence results were similar (although p > 0.05). Both inorganic N assimilation and the incorporation of applied N into the photosynthetic apparatus likely account for the above results of this conifer study.