Biotic and abiotic controls of nitrogen and phosphorus cycling in Central European forests

The functioning and services of Central European forests are threatened by global change and a loss of biodiversity. Nutrient cycling as a key forest function is affected by biotic drivers (e.g., dominant tree species, understory plants, soil organisms) that interact with abiotic conditions (e.g., climate, soil properties). In contrast to grassland ecosystems, evidence for the relationship of nutrient cycles and biodiversity in forests is scarce because the structural complexity of forests limits experimental control of driving factors. Alternatively, observational studies along gradients in abiotic conditions and biotic properties may elucidate the role of biodiversity for forest nutrient cycles. This thesis aims to improve the understanding of the functional importance of biodiversity for nutrient cycles in forests by analyzing water-bound fluxes of nitrogen (N) and phosphorus (P) along gradients in biodiversity in three regions of Germany. The tested hypotheses included: (1) temperate forest canopies retain atmospheric N and retention increases with increasing plant diversity, (2) N release from organic layers increases with resource availability and population size of decomposers but N leaching decreases along a gradient in plant diversity, (3) P leaching from forest canopies increases with improved P supply from recalcitrant P fractions by a more diverse ectomycorrhizal fungal community. In the canopies of 27 forest stands from three regions, 16 % to 51 % of atmospheric N inputs were retained. Regional differences in N retention likely resulted from different in N availability in the soil. Canopy N retention was greater in coniferous than in beech forests, but this was not the case on loessderived soils. Nitrogen retention increased with increasing tree and shrub diversity which suggested complementary aboveground N uptake. The strength of the diversity effect on canopy N uptake differed among regions and between coniferous and deciduous forests. The N processing in the canopy directly coupled back to N leaching from organic layers in beech forests because throughfall-derived N flushed almost completely through the mull-type organic layers at the 12 studied beech sites. The N release from organic layers increased with stand basal area but was rather low (< 10 % of annual aboveground litterfall) because of a potentially high microbial N immobilization and intensive incorporation of litter into the mineral soil by bioturbation. Soil fauna biomass stimulated N mineralization through trophic interactions with primary producers and soil microorganisms. Both gross and net leaching from organic layers decreased with increasing plant diversity. Especially the diversity but not the cover of herbs increased N uptake. In contrast to N, P was leached from the canopy. Throughfall-derived P was also flushed quickly through the mull-type organic layers and leached P was predominantly immobilized in non directly plant-available P fractions in the mineral soil. Concentrations of plant-available phosphate in mineral soil solution were low and P leaching from the canopy increased with increasing concentrations of the moderately labile P fraction in soil and increasing ectomycorrhiza diversity while leaf C:P ratios decreased. This suggested that tree P supply benefited from complementary mining of diverse mycorrhizal communities for recalcitrant P. Canopy P leaching increased in years with pronounced spring drought which could lead to a deterioration of P supply by an increasing frequency of drought events. This thesis showed that N and P cycling in Central European forests is controlled by a complex interplay of abiotic site conditions with biological processes mediated by various groups of organisms, and that diverse plant communities contribute to tightening the N cycle in Central European forests and that diverse mycorrhizal communities improve the limited P availability. Maintaining forest biodiversity seems essential to ensure forest services in the light of environmental change.