Decomposition, mineralization and soil nutrient release in pure and mixed stands of beech (Fagus sylvatica) and spruce (Picea abies)

Ever since the 19th century, when it became popular to plant Norway spruce (Picea abies) outside its climatic range to reforest devastated forest land in Central Europe, spruce and beech (Fagus sylvatica) stands have been contrasted in their effects on the forest soil. It is nowadays considered prudent, close-to-nature forest practice, to convert secondary spruce stand into mixed spruce-beech stands, even though such mixtures have often not been the natural vegetation at most of the sites in question. Critical reviews on tree-soil interactions concede acidification by spruce but partly question its negative consequences on stand growth. Hence, the assumption that mixed spruce-beech stands are a suitable replacement for secondary spruce stands on former mixed broadleaf sites needs critical reviewing, especially since patterns of properties observed in mixed stands cannot be predicted from patterns observed in monocultures. We propose to study effects of an admixture of beech to spruce on nutrient cycling with special focuses on i) decomposition of mixed beech-spruce litter, ii) mineralization of the top mineral soil and synthesizing these two processes by estimating iii) nutrient release (retention) of the studied forest ecosystems. We hypothesize that i) decomposition and nutrient release of foliage litter of beech and spruce is a function of litter quality and incubation site, ii) net Ca and N mineralization in the top soil un-der beech and spruce are different and iii) nutrient release and retention is a function of forest vegetation type, indicating non-linear effects in species mixtures that would not be obvious in single-species stands. Field incubations of foliar litter enclosed in mesh bags over 2.5 years, will be used to evaluate these hypotheses. To understand the driving forces of litter decomposition and related nutrient release, litter masses and element concentrations before and after incubation will be analyzed, including lignin/N ratio, C/N ratio, and microbial C and N measurements. We further propose to measure in situ net N mineralization and nitrification of the top mineral soil and solute nutrient fluxes (throughfall, soil solution) within the studied forests. Our goals are to i) predict decomposition of mixed species litter from decay rates of the corre-sponding single species, to ii) elucidate the impact of varying mixtures of beech and spruce on N cycling characteristics and to iii) answer the question, how nutrient release and retention may be managed by silvicultural methods. These issues are of practical relevance for, e.g., the productiv-ity of mixed beech-spruce forests and their impact on groundwater quality.

Ever since the 19th century, when it became popular to plant Norway spruce (Picea abies) outside its climatic range to reforest devastated forest land in Central Europe, spruce and beech (Fagus sylvatica) stands have been contrasted in their effects on the forest soil. It is nowadays considered prudent, close-to-nature forest practice, to convert secondary spruce stand into mixed spruce-beech stands. Critical reviews on tree-soil interactions concede acidification by spruce but partly question its negative consequences on stand growth. Hence, effects of admixture of beech to secondary pure spruce stands (three adjacent stands of pure spruce, mixed spruce-beech and pure beech on three nutrient rich sites and three nutrient poor sites; yielding a total of 18 stands) were investigated within this study on nutrient cycling with special emphasis on decomposition, mineralization and nutrient release / retention. Decomposition rates increased from spruce- over mixed- to beech stands. Effects of litter mixtures were small but significant. Although, surprisingly, beech leaves decomposed slower than spruce needles, mixing both fractions hastens decomposition of spruce needles. However, leaf litter decay was slowed down by admixture of needles. It is concluded that accumulation of litter in spruce forests is not caused by recalcitrance of spruce needles to decay but adverse environmental conditions in the litter of spruce stands retard decomposition. Our data show that admixture of beech may improve forest nutrition of spruce in pure spruce stands depending on site conditions. At low deposition rates, there was hardly any linkage between nutrient inputs and outputs. Element outputs were rather driven by internal N (mineralization, nitrification) and S (net mineralization of organic S compounds, desorption of historically deposited S) sources. Nitrate and sulfate seepage losses of spruce-beech mixtures were higher than expected from the corresponding single-species stands due to an unfavorable combination of high spruce-similar soil solution concentrations coupled with high beech-similar water fluxes. Our data show that nutrient leaching through the soil is not simply a "wash through" but is mediated by a complex set of reactions within the plant-soil system. We measured soil respiration which is the largest terrestrial source of CO 2 to the atmosphere. Our data show that soil respiration is affected by tree species composition and that decomposing litter contributed between 11 and 32%. However, net soil C sequestration was primarily controlled by C inputs (leaf and root litter) to the soil and not by losses via decomposition.