Fertilizer-induced soil carbon rapidly disappears after clearcutting in boreal production forests

Abstract

Forests have a substantial potential to contribute to climate change mitigation, depending on how they are managed. Forest fertilization with nitrogen is used to increase tree productivity in Fennoscandian forests, but it can also increase soil carbon stocks. However, such forests are often harvested through clearcutting, a practice known to impact soil carbon stocks, nitrogen mineralization and biodiversity. To test whether fertilizer-induced soil carbon stocks are persistent, we studied post-clearcut soil carbon and nitrogen stocks, soil respiration, tree growth, ground vegetation and soil fungal communities in 48 previously fertilized and unfertilized production forests in central Sweden. In the first year after clearcutting, clearcuts of previously fertilized forests stored 7 t (+30%) more carbon and 210 kg (+32%) more nitrogen per hectare in the soil organic layer than clearcuts of unfertilized forests. Four to 13 years after clearcutting, there was no significant difference in carbon and nitrogen stocks of the organic layer, or in soil CO2 efflux, between clearcuts of previously fertilized and unfertilized forests. Saprotrophic ascomycetes were more abundant in clearcuts of previously fertilized forests, independent of time since clearcutting. Previous fertilization did neither result in increased growth of regenerating trees nor alter understory vegetation. Synthesis and applications. Overall, the carry-over effects on biodiversity from forest fertilization into stands regenerating after clearcutting were limited. We conclude that soil organic carbon stores induced by fertilization are short-lived and do not persist after clearcutting. Consequently, the potential of forest fertilization to mitigate climate change is likely limited to increases in aboveground biomass and the products that can be produced with the harvested biomass. Our study raises questions about where the added nitrogen and the fertilizer-induced increase in soil carbon have ended up—knowledge that is essential for making well-informed decisions about future fertilization strategies.