The role of tree pollen in forest nitrogen cycling: A laboratory perspective

Pollen is a critical component of the nitrogen (N) cycle in forests, but its role in N uptake, release and transformation during precipitation events remains poorly understood, contributing to uncertainties in N deposition estimates. In the frame of the COST Action CLEANFOREST a laboratory experiment was conducted to assess the biochemical activity of tree pollen and its effects on N compounds in precipitation. Pollen from green alder (Alnus viridis), pedunculate oak (Quercus robur), European beech (Fagus sylvatica), silver birch (Betula pendula), Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) was suspended in a synthetic nitrate (NO₃^-) solution isotopically labelled with ¹⁵N under non-sterilized conditions and two sterilization treatments: addition of (i) thymol and (ii) a broad-spectrum antibiotic mixture (PSA) containing penicillin, streptomycin, and amphotericin B. Over one week, water samples were analysed daily for NO₃^-, nitrite (NO₂^-), ammonium (NH₄⁺) and total dissolved nitrogen (TDN) from which dissolved organic nitrogen (DON) was calculated. The results showed significant NO₃^- removal from the solution in broadleaved species, particularly oak, beech, and alder, in all treatments, but most clearly in the non-sterilized treatment. Most species showed a significant decrease in DON during the first two-three days, in all treatments, but especially in the sterilized (PSA) treatment, which was subsequently converted into NH₄⁺ (mineralization). The use of ¹⁵N as a tracer clearly shows that the labelled N was actively taken up by the pollen in both the non-sterilized and PSA-treated samples. Notably, pollen from all tree species, predominantly the broadleaves, enzymatically transformed extracellular NO₃^- into NO₂^-, highlighting its active role in the N cycle. These findings offer valuable insights into N release, uptake, and transformation during precipitation events and reveal important interactions between pollen and microorganisms. The differences observed between sterilized and non-sterilized treatments underline the significant influence of microbial activity on N conversion. By expanding our understanding of canopy-level N processes, this research contributes to improving N deposition models and introduces innovative approaches to studying the forest N cycle. Further studies are essential to clarify the mechanisms by which pollen and microbial communities influence N transformations at ecosystem scales.

Keywords: Broadleaves; Conifers; Pollen; ¹⁵N; Ammonium; Nitrate; Nitrite