Biodiversity Effects on Plant Stoichiometry

In the course of the biodiversity-ecosystem functioning debate, the issue of multifunctionality of species communities has recently become a major focus. Elemental stoichiometry is related to a variety of processes reflecting multiple plant responses to the biotic and abiotic environment. It can thus be expected that the diversity of a plant assemblage alters community level plant tissue chemistry. We explored elemental stoichiometry in aboveground plant tissue (ratios of carbon, nitrogen, phosphorus, and potassium) and its relationship to plant diversity in a 5-year study in a large grassland biodiversity experiment (Jena Experiment). Species richness and functional group richness affected community stoichiometry, especially by increasing C:P and N:P ratios. The primacy of either species or functional group richness effects depended on the sequence of testing these terms, indicating that both aspects of richness were congruent and complementary to expected strong effects of legume presence and grass presence on plant chemical composition. Legumes and grasses had antagonistic effects on C:N (−27.7% in the presence of legumes, +32.7% in the presence of grasses). In addition to diversity effects on mean ratios, higher species richness consistently decreased the variance of chemical composition for all elemental ratios. The diversity effects on plant stoichiometry has several non-exclusive explanations: The reduction in variance can reflect a statistical averaging effect of species with different chemical composition or a optimization of nutrient uptake at high diversity, leading to converging ratios at high diversity. The shifts in mean ratios potentially reflect higher allocation to stem tissue as plants grew taller at higher richness. By showing a first link between plant diversity and stoichiometry in a multiyear experiment, our results indicate that losing plant species from grassland ecosystems will lead to less reliable chemical composition of forage for herbivorous consumers and belowground litter input.

Phosphorus and Nitrogen Regulate Arbuscular Mycorrhizal Symbiosis in Petunia hybrida

Phosphorus and nitrogen are essential nutrient elements that are needed by plants in large amounts. The arbuscular mycorrhizal symbiosis between plants and soil fungi improves phosphorus and nitrogen acquisition under limiting conditions. On the other hand, these nutrients influence root colonization by mycorrhizal fungi and symbiotic functioning. This represents a feedback mechanism that allows plants to control the fungal symbiont depending on nutrient requirements and supply. Elevated phosphorus supply has previously been shown to exert strong inhibition of arbuscular mycorrhizal development. Here, we address to what extent inhibition by phosphorus is influenced by other nutritional pathways in the interaction between Petunia hybrida and R. irregularis. We show that phosphorus and nitrogen are the major nutritional determinants of the interaction. Interestingly, the symbiosis-promoting effect of nitrogen starvation dominantly overruled the suppressive effect of high phosphorus nutrition onto arbuscular mycorrhiza, suggesting that plants promote the symbiosis as long as they are limited by one of the two major nutrients. Our results also show that in a given pair of symbiotic partners (Petunia hybrida and R. irregularis), the entire range from mutually symbiotic to parasitic can be observed depending on the nutritional conditions. Taken together, these results reveal complex nutritional feedback mechanisms in the control of root colonization by arbuscular mycorrhizal fungi.

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.

Plant and arthropod community sensitivity to rainfall manipulation but not nitrogen enrichment in a successional grassland ecosystem

Grasslands provide many ecosystem services including carbon storage, biodiversity preservation and livestock forage production. These ecosystem services will change in the future in response to multiple global environmental changes, including climate change and increased nitrogen inputs. We conducted an experimental study over 3 years in a mesotrophic grassland ecosystem in southern England. We aimed to expose plots to rainfall manipulation that simulated IPCC 4th Assessment projections for 2100 (+15 % winter rainfall and −30 % summer rainfall) or ambient climate, achieving +15 % winter rainfall and −39 % summer rainfall in rainfall-manipulated plots. Nitrogen (40 kg ha−1 year−1) was also added to half of the experimental plots in factorial combination. Plant species composition and above ground biomass were not affected by rainfall in the first 2 years and the plant community did not respond to nitrogen enrichment throughout the experiment. In the third year, above-ground plant biomass declined in rainfall-manipulated plots, driven by a decline in the abundances of grass species characteristic of moist soils. Declining plant biomass was also associated with changes to arthropod communities, with lower abundances of plant-feeding Auchenorrhyncha and carnivorous Araneae indicating multi-trophic responses to rainfall manipulation. Plant and arthropod community composition and plant biomass responses to rainfall manipulation were not modified by nitrogen enrichment, which was not expected, but may have resulted from prior nitrogen saturation and/or phosphorus limitation. Overall, our study demonstrates that climate change may in future influence plant productivity and induce multi-trophic responses in grasslands.