Long-term changes in soil composition in unmanaged central European mountain spruce forests after decreased acidic deposition and a bark beetle outbreak

We studied the effect of tree mortality on nutrient concentrations and pools in soils of two unmanaged mountain forest catchments (Plesne, PL; and Certovo, CT; Czech Republic) recovering from acidification due to decreasing acidic atmospheric deposition since the late 1980s. Both catchments were situated at similar elevation, and before a bark beetle outbreak were covered by healthy mature Norway spruce (Picea abies) forests. However, differing bedrock [granite (PL) vs mica-schist and quartzite (CT)] resulted in lower concentrations of base cations (except for magnesium), but higher concentrations of aluminium and iron oxyhydroxides in the CT soils, enabling higher phosphorus (P) accumulation. Despite these differences, soils were similarly acidic, with pH(H2O) from 3.5 to 4.3 and cation exchange capacity (CEC) dominated by exchangeable protons (H-ex(+)) and aluminium in both catchments in 2000. In the PL catchment, >75 % of mature spruce trees died after a bark beetle infestation between 2004 and 2008, and all dead biomass was left on site. Forest damage in the CT catchment was minor. Soil concentrations and pools of exchangeable calcium (Ca-ex(2+)), magnesium (Mg-ex(2+)), potassium (K-ex(+)), and H-ex(+) were tightly related to concentrations of organic carbon (C). Following the tree dieback in the PL catchment, we observed significant (p < 0.05) increases in (1) concentrations and pools of Ca-ex(2+), Mg-ex(2+) , and K-ex(+), (2) base saturation (from 30 to similar to 40 %), (3) CEC:C ratios (i.e., CEC of soil organic carbon), (4) pH(H2O), (5) mineral P forms, and (6) total nitrogen (N) in the upper (O and A) soil horizons. Similar changes were less pronounced or negligible in the less impacted CT soils. The elevated litterfall after tree dieback, resulting in the increased input of base cations to forest floor and increased CEC of soil organic C, played a more important role in the decreasing acidity of the PL soils than the continuing decrease in acidic deposition.