Simulated warming decreases acidity of forest topsoil under Norway spruce (Picea abies (L.) H.Karst.) stands in the Western Carpathians

Abstract

Global warming is affecting the montane forests of the temperate zone, causing a number of changes in all elements of the ecosystem, including the soil. Norway spruce forests, weakened by ongoing decay over several decades, are likely particularly sensitive to climate-induced changes. The response of soil cation exchange capacity components of spruce stands to warming is not well understood. We have assumed that warming will cause significant changes in pH, disrupting ion-exchange relationships in the soil. To the best of our knowledge, the present study is the first devoted to examining the mechanisms of soil pH change in temperate forest under the influence of warming. The forest in-situ experiment, simulating the soil warming for 578 days, was performed. Laboratory analyses included measurements of pH, and concentrations of carbon and exchangeable forms of calcium, potassium, magnesium, sodium, and aluminum. Shortly after the beginning of simulated warming (84 days), we observed a significant increase in the pH of the topsoil horizons. On average, in the warmed organic (O) horizon, the pH increased by 0.21, and in the mineral (A) horizon by 0.29 pH units. Based on previous studies, the pH increase was most likely triggered by an increase in ammonium-nitrogen content in the warmed soil. Using the Henderson–Hasselbalch equation to model the pH revealed that, in the O horizon, the pH increase is a result of the presence of weaker organic acids, less able to release hydrogen ions during dissociation. In the A horizon, the increase in pH was explained by an increase in abundance of aluminum ions, probably migrating upward from deeper soil layers. We consider that the ion-exchange mechanisms described here will contribute to an improved understanding of the complex processes of spruce forest dieback in the context of climate warming.