Dynamik von Phosphor im Boden und von Nährstoffen im Kronenraum von Buchenwäldern unterschiedlicher Baumartendiversität

Abstract

Soil nutrient pools, concentrations and turnover as well as soil nutrient inputs due to deposition and canopy exchange have been studied intensively in all kinds of forest ecosystems worldwide. In studies about temperate forests, the main focus concerning the macronutrients was on carbon and nitrogen, whereas phosphorus was rather of interest in tropical forests. However, due to increasing nitrogen deposition, phosphorus became or will become a growth limiting factor of many temperate forests. In addition, the changing climate will affect these ecosystems and their nutrient cycling. To be prepared for these changes the establishment of mixed species forests is recommended since mixed forests are supposed to be more resistant and resilient against disturbances than monocultures. In large parts of Central Europe the potential natural forest vegetation is dominated by beech (Fagus sylvatica L.). Hence, mixed beech forests are assumed to have a high ecological and economic value in the face of climate change. However, not much is known about the effects of increased tree species diversity on soil nutrients and nutrient cycling in temperate beech forests. Studies about the influence of tree species composition on soil phosphorus pools, concentrations and turnover are almost lacking.The dissertation on hand compared the soil phosphorus pools, concentrations and turnover of pure beech stands with those of mixed species stands, investigated the phosphate sorption capacity and transport in forest soils and compared the deposition and canopy exchange of phosphorus and other nutrients of pure beech stands with those of mixed species stands. The study area was situated in the largest coherent broad-leaved forest in Germany and could be divided into three different stand types concerning tree species diversity: (1) beech as the main tree species; (2) beech, lime and ash as the main tree species and (3) beech, lime, ash, maple and hornbeam as the main tree species.The pools and concentrations of both inorganic and especially organically bound phosphorus were smaller in the soils of pure beech stands than in those of mixed species stands. However, these differences mainly could be ascribed to lower clay contents in the pure beech stands, whereas the tree species played a minor role in the storage of phosphorus in these soils. The input of phosphorus to the soil with leaf litter showed a clear tendency to increase with increasing tree species diversity. The turnover time of phosphorus in the organic surface layer was shorter in mixed species stands than in pure beech stands. Hence, the input and turnover was influenced by the tree species. Yet, the lower pH, base saturation and cation exchange capacity of the pure beech stands influenced the decomposition of leaf litter by changes in the composition of the soil biota.The soil samples could be separated into two groups according to their phosphate sorption behavior: there was a tendency that the more acid samples with low clay contents were best fitted by a linear equation and the samples with higher pH values and larger clay contents by the Freundlich equation. Hence, the small-scale heterogeneity of soil properties in the study area seemed to influence the sorption behavior of the soil samples. Desorption of previously added phosphate was incomplete. The application of a transport model on our data showed that phosphate transport could not be described well with this kind of adsorption/desorption model. This was probably due to other processes involved in phosphate storage and transport, like precipitation and dissolution, which were not considered in the model.The deposition of airborne nutrients and acidifying compounds was highest in pure beech stands since they were higher and had rougher canopies compared with the mixed species stands. The canopy leaching of phosphate, potassium, calcium and magnesium was highest in mixed species stands, whereas protons and manganese were mainly leached from pure beech canopies. The differences in ion status and physiology between the tree species may explain the observed differences in canopy exchange. Still, the soil properties also affected the canopy exchange processes. The lower pH of pure beech stands leads to a high mobility of soil manganese and consequently to enhanced manganese uptake by the trees. In addition, the lower base saturation leads to reduced uptake of potassium, calcium and magnesium in the pure beech stands. The beech trees had higher manganese and lower potassium, calcium and magnesium leaf contents than the other tree species, due to both species-specific differences in ion status and differences in soil properties, leading to the described canopy leaching rates.The results of the dissertation on hand show that it is difficult to clearly separate the effects of tree species diversity from those of soil properties on soil phosphorus and nutrient cycling. The soils of the forest stands originally differed in some of the soil properties, namely the clay content. However, trees are known to alter soil properties which underlie rapid changes, i.e., pH, base saturation and cation exchange capacity. Hence, the soil differences between the pure beech and the mixed species stands became even more pronounced. In conclusion, tree species played a direct as well as an indirect role in nutrient storage and cycling in the way that they had an effect on soil properties, which in turn led to changes in the soil phosphorus storage and transport, in the turnover of organic material and in the canopy leaching of ions.