Tree dieback and related changes in nitrogen dynamics modify the concentrations and proportions of cations on soil sorption complex

Abstract

The soil sorption complex in an unmanaged mountain forest (Plešné Lake catchment, Czech Republic) changed substantially during the decade following forest dieback in 2004–2008, when a bark beetle outbreak killed >90% of mature Norway spruce stands. All downfallen biomass remained in the catchment. Leaching of Ca2+, Mg2+, K+, H+ and ionic Al from the catchment increased after tree dieback and was predominantly coupled with nitrate export. However, the proportions and amounts of individual cations retained on the soil sorption complex and their contribution to leaching differed in time. The average concentration of exchangeable non-acidic cations (base cations and NH4+) doubled from 119 to 247 µeq g−1 in the upper ∼20 cm of soil because their release from dead biomass exceeded losses via leaching and tree uptake. The surplus of non-acidic cations replaced almost 50% of the exchangeable acidity (Al3+ and H+), which was leached out from the soil sorption complex to receiving waters. It was mainly exchangeable Ca2+ that drove the increase of soil base saturation and the decrease of exchangeable acidity. Its concentration continually rose, as well as its contribution to the non-acidic cations (up to 85%). Besides, exchangeable NH4+ played an important role in the first 4 years following dieback, when its proportion increased to ∼10–20%. K+ only significantly contributed in the first two years. Consequently, tree dieback substantially increased soil base saturation with cations released from dead biomass and thus accelerated soil recovery from long-term acidification. Described changes in soil chemistry represent natural conditions and the uppermost limit of an ecosystem response to all possible management practices based on biomass removal in similar mountain forest areas.