Abstract
The effects of pollen on dissolved inorganic nitrogen (DIN) compounds in
throughfall water are not completely understood. We conducted a 7-day
leaching experiment with pollen from silver birch (including a
sterilized control), European beech, sessile oak, Scots pine, Corsican
black pine and Norway spruce using an immersion medium containing
nitrate (11.295 mg N l-1). Within 2 hours, pollen
released substantial amounts of potassium (K+),
phosphate (PO43-) and organic
compounds. Solute concentrations of ammonium
(NH4+) were built up over time. In
treatments with pollen from birch, oak and beech, nitrate
(NO3-) concentrations started to
decrease after 24–48 hours, while simultaneously nitrite
(NO2-) emerged, but part of the inorganic nitrogen could
no longer be detected in solution. For birch, sterilisation of the
pollen made no difference, indicating that microorganisms on the pollen
played no substantial role in the observed N transformations. Conditions
in the samples were oxic (1.82–6.12 mg O2
l-1), rendering microbial denitrification unlikely.
Our findings revealed that pollen from broadleaved deciduous trees can
transform throughfall NO3- into
NO2- and likely also nitric oxide
(NO), probably through the nitrate reductase pathway. The synthesis of
NH4+ might be part of a natural
defence mechanism protecting reproductive organs against pathogens
during pollination.