Biological nitrogen fixation in logging residue piles of different tree species after final felling.

Logging residues influence the nitrogen cycling processes that play a key role in risks for nitrogen losses from the ecosystem after the clear cut. Therefore, our aim was to identify the potential ability of logging residues to gain external nitrogen via biological nitrogen fixation. We measured biological nitrogen fixation as nitrogenase activity in logging residues of three different tree species (Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.), and silver birch (Betula pendula Roth.). The study site was located in south-eastern Finland and was clear cut in 2014 and piles of logging residues were established. Sampling was performed in June 2016, September 2018, and August 2019 and nitrogenase activity in branches and needles or leaves was measured using the acetylene (CH) reduction assay. Nitrogenase activity (ethylene production) was shown in all residue types. Nitrogenase activity tended to be higher in branches than in needles or leaves and in coniferous residues than in birch. C-to-N ratios were higher in branches than in needles/leaves and in coniferous residues than in birch. Our results indicate that logging residues can acquire external nitrogen from the atmosphere via biological nitrogen fixation and can thus bring nitrogen to the forest ecosystem and substitute some part of the N losses occurring when residues are retained at the site after clear cutting.