Strong nutrient uplift associated with forest growth in northern forests

Northern forests are currently taking up large quantities of carbon due to forest growth. Yet, it is not known for how long the capacity of the soils to provide enough nutrients to support high forest growth will last. Therefore, it is important to understand nutrient dynamics in northern forests.

We analyzed 33,500 forest soils in Sweden in four repeated inventories covering the period 1983 to 2022.

During the four decades, the standing wood volume and the tree stem diameter increased by 15% and 29%, respectively, across all of Sweden. The plant-available magnesium (Mg), calcium (Ca), and manganese (Mn) concentrations of the organic layer increased continuously and significantly over the four decades across all of Sweden by 38%, 21%, and 100%, respectively. In the south of Sweden, where tree growth and biomass are highest, the Mg and Ca concentrations increased particularly strongly by 62% and 31%, respectively, over the four decades. The concentrations of plant-available Mg and Ca of the organic layer and their increases were related to properties of the mineral soil, such as soil texture and the Mg concentration of the parent material. Further, Mg and Ca concentrations of the organic layer were significantly higher and increased more strongly in broadleaf forests and spruce forests than in pine forests. The nitrogen (N) stock of the organic layer was highest in the second inventory, i.e., in the 1990s, and lowest in the fourth inventory. From the first inventory in the 1980s to the fourth inventory, the N stock of the organic layer decreased by 6% across all of Sweden.

Our results suggest that the increase in tree biomass and tree size caused an uplift of Mg, Ca, and Mn from the mineral soil to the organic layer, likely due to tree luxury uptake of these elements in the mineral soil. Furthermore, the N stock of the organic layer decreased over the last decades likely due to tree growth, after it was comparatively high in the 1990s due to high atmospheric N deposition. Taken together, the results indicate that strong nutrient uplift from the mineral soil occurred in response to forest growth and that N rather than base cations might become more strongly limiting for tree growth in northern forests in the future.