The role of calcium uptake from deep soils for spruce (Picea abies) and beech (Fagus sylvatica)

Chemical analyses of the soil from up to 28 neighboring stands of pure spruce and mixed spruce–beech stands on two different bedrock materials for soil formation (Flysch: nutrient rich; Molasse: acidic) in Austria showed a marked enrichment of Ca in the O-horizon of the mixed stands as well as much wider Ca/Mg and Ca/K ratios. Because in mixed stands the Ca/Mg and Ca/K ratios in the O-horizon reflected the Ca content of the subsoil, we hypothesized that this relative Ca enrichment results both from higher transpiration of beech and uptake of water from deeper soil horizons, where the soil solution is expected to contain more Ca (Ca-pump of beech hypothesis). However, dendrochemical analyses justify the conclusion that acidification caused by pure spruce mobilizes base cations, increasing Ca stem wood concentrations. According to these studies, we suggested that top soil mobilization of Ca by acidifying effects of spruce is the main Ca source. Higher Ca soil solution concentrations in deep soil horizons under pure spruce than under mixed stands agreed with this revised hypothesis. Acidifying effects of spruce were much more pronounced on Flysch than on Molasse and preferential leaching of Ca from the top soil might explain the observed differences of Ca/Mg and Ca/K ratios in the O-horizon of mixed stands on these two bedrocks, which were reflected in the foliage as well. Comparisons of 87Sr/86Sr ratios of tree leaves and roots at different depths indicated that beech is indeed a real Ca-pump. This method confirmed that Ca of the current foliage is mainly of atmospheric source, due to similar 87Sr/86Sr ratios as measured in precipitation, and is hardly supplied by bedrock weathering. We conclude that even in mixed spruce–beech stands Ca is leached in high amounts together with NO3, SO4 and organic anions through the shallow top soil, which is rooted by spruce. Uptake of Ca in deep soil layers by beech minimizes substantial loss of Ca and other base cations, which would be inevitable under pure spruce stands.