Impact of pollen on throughfall biochemistry in European temperate and boreal forests

The aim of this study is to get a better view on the effects of pollen on throughfall (TF) biochemistry for the main tree species groups in Europe (oak, beech, spruce, pine). We used generalized additive mixed modelling to study the relationship between long- term series of measured TF fluxes in spring (April–June) at 100 ICP Forests Level II plots and corresponding airborne pollen concentrations (Seasonal Pollen Integral, SPIn) from nearby aerobiological monitoring stations. We also conducted a 7-day laboratory dissolution experiment with bud scales and flower stalks of beech, pollen of beech, common oak, silver birch, Scots pine, Corsican pine, Norway spruce and sterilized pollen of silver birch in a nitrate (NO3-N) solution (11.3 mg N L). Throughfall fluxes of potassium (K+), ammonium (NH4+-N), dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) showed a positive relationship with SPIn; NO3-N fluxes a negative relationship. The experiment broadly confirmed the findings based on field data. Within two hours, pollen released large quantities of K+, phosphate, DOC and DON, and lesser amounts of sulphate, sodium and calcium. After 24-48 hours, NO3-N started to disappear, predominantly in the treatments with broadleaved pollen, while concentrations of nitrite and NH4 +-N increased. At the end of the experiment, the inorganic nitrogen (DIN) was reduced, presumably because it was lost as gaseous nitric oxide (NO). Our results show that pollen dispersal might be an overlooked factor in forest nutrient cycling and might induce complex canopy N transformations.