Effects of different nature management strategies on soil GHG fluxes: Trophic rewilding, passive rewilding and mowing

Nature-based solutions to climate change, e.g. restoring ecosystem processes that translocate GHG from the atmosphere to biomass, are recognized as cost-effective methods to simultaneously mitigate climate change and reverse ecosystem degradation. The importance of large ungulates as part of nature-based solutions has been emphasized due to their critical role in maintaining and improving diversity in ecosystems, while the extent of large ungulate-mediated effects on radiative forcing and greenhouse gas balance is unclear due to lack of observations and apparent context-dependencies across biomes. We particularly lack direct measurements of large ungulate-mediated feedbacks on soil GHG fluxes despite their substantial influence on atmospheric concentrations of GHG’s. Large ungulates shape their environments e.g. via biomass consumption, alteration and redistribution, seed dispersal and trampling, affecting plant diversity and productivity as well as soil physicochemical conditions. Together, these impacts may govern the direction and magnitude of soil GHG fluxes.

Here, we present a study conducted in a Danish rewilding area, where cattle and horses were released for year-round grazing in 2016. Within the 120 ha area, we studied eight fenced experimental blocks located in common broom (Cytisus scoparius) dominated shrublands on well-drained sandy soils. We aimed to detect effects of three treatments resembling possible nature management strategies: Trophic rewilding (large ungulate presence) passive rewilding (large ungulate absence) and annual mowing (traditional nature management) on soil GHG fluxes. We were particularly interested in identifying the ungulate-mediated effects on soil physicochemical parameters that drive soil GHG fluxes. Our experimental approach included both chamber measurements in the field and laboratory incubations of intact soil cores. During both types of campaigns, we measured fluxes of CO₂, CH₄ and N₂O. To elucidate mechanistic relationships, we also measured soil parameters related to physical structure, soil C & N concentrations, and N mineralization rates.

Initial results from our incubation experiment suggest that trophic rewilding increased soil respiration, which is in contrast to field measurements that showed higher respiration rates from passive rewilding plots. The former result may be attributed to higher soil C concentrations under trophic rewilding, and the latter to greater autotrophic respiration under passive rewilding. Conversely, CH₄ uptake rates and N₂O emissions were reduced under trophic rewilding, which could partially be explained by changes soil structure and nitrification rates. Annual mowing management exhibited similar responses in CO₂ and CH₄ fluxes to trophic rewilding, while the production of N₂O was substantially reduced compared to the other management types. Our study demonstrates that introducing large ungulates in nature management may influence soil GHG fluxes, highlighting their role in soil biogeochemical processes and nature-based climate solutions.