Are soils in Swedish forests depleted in mineral nutrients by forestry and increased tree growth? No, on the contrary, a new study reports that levels of important mineral nutrients have increased in the organic layer of Swedish forest soils over the past 40 years. Reduced acid rain and uplift of nutrients from the mineral soil by trees are the suggested reasons.

Swedish forests have long taken up large amounts of carbon dioxide thanks to increases in the standing wood volume during the last decades. Consequently, there has been widespread concern that tree growth combined with felling would deplete the soil of nutrients, especially since nutrient-rich felling residues such as branches are often also harvested. However, until now, no thorough study has been conducted to determine whether this is the case.

Now, Professor Marie Spohn and colleagues at the Swedish University of Agricultural Sciences have analysed data from thousands of soil samples collected from Swedish forests as part of the Swedish Forest Soil Inventory over the past forty years. They found a strong increase in the concentrations of plant-available magnesium (+38%), calcium (+21%) and manganese (+100%) in the soil organic layer during forty years (1983 to 2022). These mineral nutrients are so-called base cations that are important nutrients for plant growth. The increases were calculated as averages across all forests. Yet, the increases differed between forests, depending on the soil texture, the nutrient concentration of the soil parent material, and the dominant tree species.

“This was not something we had expected, but there are explanations for these dynamics,” says Marie Spohn.

The researchers point out two explanations for the changes in the soil organic layer, which likely both have contributed to the increase in base cations.

One explanation is that the reduction in acid deposition in recent decades has reduced the leaching of base cations from the organic layer to the underlying mineral soil. The other explanation is that there has been an upward transport of nutrients from the mineral soil to the organic layer through trees. Trees can lift nutrients up in the soil profile through tree nutrient uptake in the mineral soil and litterfall that adds nutrients to the soil organic layer. Nutrient uplift through trees is likely driven by the increased tree growth in Sweden over the last decades.

Another change that the researchers saw was that the nitrogen content of the organic layer has decreased over the past four decades. In summary, the researchers see only a low risk for base cation deficiency in Swedish forest soils while, based on their results, they believe that nitrogen will remain the limiting nutrient for tree growth.