In Vitro Nitrogen Transformations by Pollen From Temperate Tree Species

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 (PO3-) 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.