Peak season soil GHG exchange after 23 years of ungulate exclusion in an old-growth temperate forest

Large herbivores have been proposed to promote soil carbon (C) sequestration and their (re)introduction to ecosystems is therefore suggested as a nature-based climate solution. While forest ungulates can alter soil biogeochemical processes such as soil C and nutrient cycling, their effects on net greenhouse gas (GHG) exchange between soil and atmosphere remain underexplored. Old-growth forests are of particular interest in this regard due to their large soil C stocks and methane (CH 4 ) sink capacity. In this study, we conducted a snapshot campaign during peak growing season in the old-growth forest of Białowieża National Park, eastern Poland, to assess potential effects of 23 years of ungulate exclusion on soil GHG exchange. We used chamber measurements across 13 experimental blocks with paired ungulate absence and presence plots. We found that ungulate absence increased soil surface respiration by 19% ± 8% and net CH 4 uptake by 54% ± 11%, while N 2 O fluxes showed no consistent differences. These effects were partly mediated by changes in soil water filled pore space and potentially by shifts in CH 4 cycling microorganisms. Our results suggest that wild ungulates can modulate soil GHG fluxes at measurable levels in an old-growth forest under natural animal densities. More knowledge of ungulate impacts on forest biogeochemistry and climate feedback is timely given increasing climate driven threats to forests and proposals to use ungulate introductions as a mitigation strategy.