To graze or not to graze: comparing soil carbon and nitrogen storage and cycling under trophic rewilding, passive rewilding and conservation mowing

Rewilding has emerged as a prominent ecological restoration approach in recent decades. It is aimed at restoring natural processes, improving ecosystem functioning, and enhancing biodiversity with minimal human interference necessitated. Different approaches to rewilding exist, including trophic and passive rewilding. Trophic rewilding involves the active introduction of species, most often large herbivores. The latter approach involves passive management with minimal human interference. Being a nascent strategy available to ecosystem managers, comparative empirical research in the context of rewilding is lacking, especially relating to soil functions like carbon (C) and nitrogen (N) cycling and storage.

In this study, we investigated whether the choice of trophic versus passive rewilding had an impact on the quantity and cycling of C and N stored in soils. Additionally, we compared these two approaches to annual mowing and removal of biomass, a typical conservation management strategy for grasslands.

Permanently fenced passive rewilding and conservation mowing plots were established within a trophic rewilding project at Mols Bjerge, Denmark in spring 2017. Plots delineated adjacent to these represented trophic rewilding. Exmoor ponies and Galloway cattle were introduced the previous year and continue to freely roam the 120 ha site with minimal human intervention. The area has previously been used for sheep and cattle grazing research, primarily on aboveground biodiversity. In August 2024, we collected soil samples from three layers (0-5 cm, 5-10 cm, and 10-20 cm; n = 216) in each treatment replicated at 8 locations within the study site. Additional topsoil (0-5 cm; n=72) samples were retrieved from each plot for analyses of microbial activity.

Bulk density, organic carbon (OC), total nitrogen (TN), and pH were determined in samples from all depths. Microbial biomass C and N, respiration, microbial activity and diversity, and net N mineralization rates were analysed in the topsoil samples. Based on preliminary results, trophic rewilding was characterised by the largest stocks of C and N to 20 cm with mean values of 3.62 kg m^-2 and 0.27 kg m^-2, respectively. Passive rewilding and conservation mowing resulted in mean C stock values 11% and 19% lower compared to trophic rewilding, with similar results for N stocks. In contrast, soil C/N ratios were significantly higher under conservation mowing compared to the rewilding treatments. The lowest levels of microbial biomass C, specific (normalised for OC content) C mineralization, and net N mineralization were associated with trophic rewilding, suggesting that nutrient turnover rates are comparatively suppressed. EcoPlate™ results similarly showed reduced microbial activity, as well as diversity, under trophic rewilding with significantly higher results under mowing. These results demonstrate that the decision to include or exclude animals in land management strategies can have a consequential impact on C and N storage and the driving processes related to their cycling in soil. Therefore, this decision should be considered carefully in land management policy development.