The impact of short-rotation hybrid aspen (Populus tremula L. × P. tremuloides Michx.) plantations on nutritional status of former arable soils

• Changes in soil chemistry after 10 years of tree growth in hybrid aspen plantations were studied.
• Significant changes had occurred both in A-horizon and B-horizon.
• Significant decreases were detected in pHKCl and concentrations of Ca, Mg and B.
• Significant increases were detected in concentrations of total N, Corg, Cu and Mn.
• The initial level of the nutrient and the growth of the trees were the main drivers of changes.

The area of short-rotation forest (SRF) plantations with hybrid aspen (Populus tremula L. × P. tremuloides Michx.) for pulp and energy wood production in approximately 25-year rotation period has increased considerably during the last few decades in Northern Europe on previous agricultural lands. However, little is known about how this new land use will alter the chemical properties of soils in the long-term and how long nutrients from the land’s previous use will sustain high productivity of trees. Little is also known about the dynamics of soil micronutrients and the plantations’ influence on subsoil layers. Repeated soil monitoring was carried out in hybrid aspen plantations at the ages of 5 (young) and 15 years (midterm) to detect changes in soil chemical characteristics, including soil reaction (pHKCl), and concentrations of nutrients and organic carbon (Corg) in A- and B-horizon on 51 permanent experimental plots. The main soil- and stand-related factors responsible for changes in soil chemical properties, were analyzed. Significant changes were observed both in soil A-horizon and B-horizon. In A-horizon, pHKCl (−0.2 units), Ca (−19.4%) and B (−25.8%) had decreased while total N (+5.8%), Cu (+20.3%) and Mn (+21.3%) had increased. In B-horizon, Ca (−23.4%), Mg (−12.8%) and B (−45.0%) had decreased, while Corg (+17.2%) and Cu (+53.8%) had increased. The concentrations of available P, K and Corg in A-horizon had remained unchanged. Reduction of soil chemical characteristics mainly occurred in plantations where their initial level had been high (this relationship was observed in A-horizon for total N and B and in B-horizon for Corg, Mg and B). Several soil chemical characteristics decreased more (or increased less) in plantations where tree growth was more vigorous (in A-horizon: pHKCl, total N and B; in B-horizon: Mg and B). Soil cation exchange capacity, base saturation and moisture conditions also explained some of the observed variation in changes. Relatively high loss of B in faster growing plantations indicates possible future limitations of this essential micronutrient. To summarize, first generation hybrid aspen plantations planted on former agricultural fields had significantly altered soil reaction (pHKCl) but did not show significant depletion of primary macronutrients (N, P, K) and soil organic carbon.