No effect on soil C stock, but increased tree C stock 17 years after site preparation: Results from two hemiboreal field experiments

Forest management practices can influence carbon and nitrogen dynamics, which are critical for climate change mitigation. We investigated the effects of disc trenching during stand establishment, in combination with repeated nitrogen fertilisation of the previous stand, on carbon and nitrogen stocks in soil, tree biomass, ground vegetation, and fungal community composition in two 17-year-old Pinus sylvestris stands in Sweden (Hagfors and Nissafors). The study applied a split-plot design with factorial combinations of fertilisation (0–1800 kg nitrogen ha⁻¹ in the previous rotation) and disc trenching as sub-plots. Seventeen years after disc trenching, the total soil carbon stocks were not significantly different between disc-trenched and non-disc-trenched treatments, but organic matter was redistributed. At both sites, disc trenching increased tree biomass carbon and nitrogen by ∼50% compared to the non-disc-trenched treatment, resulting in significantly higher ecosystem carbon stock at Nissafors; and an increasing trend at Hagfors (albeit non-significant). In contrast, fertilisation of the previous stand had no persistent effects on soil, biomass, or ecosystem carbon and nitrogen stocks in year 17 of the subsequent rotation. Disc trenching and previous fertilisation had limited effects on fungal communities in the soil organic layer. At Hagfors, however, ectomycorrhizal richness was higher in unfertilised non-disc-trenched sub-plots. Overall, disc trenching enhanced ecosystem carbon stock through increased tree growth without causing long-term soil carbon or nitrogen losses, while previous nitrogen fertilisation showed no legacy effects. These results suggest disc trenching can support forest regeneration without causing long-term soil carbon loss.