Strong Changes in Soil Nutrient Stocks in Northern Forests Over Four Decades.

Northern forests are currently taking up large quantities of carbon due to tree growth. Yet, it is not known how the stocks of soil nutrients have responded to this increase in biomass. Therefore, we analyzed thousands of forest soils in Sweden over the last four decades. We found strong increases in the concentrations and stocks of plant-available magnesium, calcium, and manganese in the organic layer of the soils. Specifically, the concentrations of plant-available magnesium, calcium, and manganese in the organic layer increased by 38%, 21%, and 100%, respectively, over the four decades. These increases were related to soil texture and the magnesium concentration of the soil parent material as well as to the dominant tree species. The increase in nutrients in the organic layer might be caused by an uplift of nutrients from the mineral subsoil to the organic layer due to plant nutrient uptake in the subsoil and litter fall and they might also be driven by decreased leaching of nutrients from the organic layer due to decreased acid deposition. Concurrently, the nitrogen content of the organic layer decreased over the four decades. In conclusion, our results show that stocks and concentrations of plant-available cations in the organic layer of Swedish forest soils increased despite increases in tree biomass and regular tree harvests. Our study indicates that there is a low risk for base cation deficiency and that nitrogen will remain the limiting nutrient for tree growth in northern forests.