Aluminum and iron chemistry in the O horizon changed by a shift in tree species composition

In the present study we characterized the buffering system including aluminum in the organic surface horizon (O) of field experiments with replicated plots of pure Norway spruce (Picea abies (L.) Karst) and replicated plots of spruce with a birch (Betula pendula Roth and B. pubescens Ehrh.) admixture. Our results show that a change in tree species composition may have a rather large, short-term (12 years) effect on the concentration of organically bound aluminum in the humus layer (H) of the O horizon. The pure spruce plots had a significantly higher concentration of exchangeable aluminum (Al-e) and organically bound (pyrophosphate extractable) aluminum (Al-P) in the H layer and a lower concentration of each base cation. Furthermore, our results confirm earlier findings that aluminum has base cation properties in acidic organic horizons. Thus, the change in tree species composition did not affect the pH in spite of a change in base saturation, because base cations were mainly compensated for by aluminum. The change in organically bound aluminum was accompanied by a similar change in organically bound pyrophosphate extractable iron (Fe-P). The observed differences between the mixed and pure spruce plots in the amounts of Al-P and Fe-P in the H layer could not be explained by estimated differences in biocycling of Al and Fe either by above-ground litterfall or by root turnover.