News July 2024

Nitrification in the crown canopy greatly obscures the impact of agriculture on Flemish forest ecosystems

INBO participated in the international Horizon 2020 research project NITRIPHYLL, on nitrification in the crown canopy of European forests. Nitrification is the process by which specialized microorganisms (archaea and bacteria) on the leaves or needles, convert ammonium to nitrate. It has long been believed that the crown canopy is an important link in the nitrogen cycle in forests. Only recently have advanced research methods allowed us to unravel these complex processes.

For this research, ten sample plots from the ICP Forests Level II monitoring network were selected. In Flanders, these are the sampling plots of Scots pine in Brasschaat and beech in the Sonian forest. The sites were chosen along a nitrogen deposition gradient. The Flemish plots belong to the category with the highest deposition in Europe. In late summer 2016, samples of throughfall water - that is precipitation after passing through the crown canopy - were examined.

In Brasschaat, 55% of the nitrate in the throughfall was of biological origin (i.e. from microbial conversion of ammonium to nitrate). In the Sonian Forest, the proportion was as high as 69%. Converted, this means that annually 4-5 kg of nitrogen deposition measured as nitrate in the throughfall was originally deposited on the forest in the form of ammonium. Nitrification in the crown canopy thus greatly obscures the proportion of ammonium in the throughfall deposition of inorganic nitrogen.

Ammonia deposition originates mainly from agricultural activities, and oxidized nitrogen from combustion processes (including industry, traffic and households). This research indicates that our measurements of throughfall deposition until today have underestimated the impact of ammonia and overestimated that of oxidized nitrogen.

Arne Verstraeten

Read more: Guerrieri R. et al. (2024). Substantial contribution of tree canopy nitrifiers to nitrogen fluxes in European forests. Nature Geoscience 17, 130-136.


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