Excessive fertilizer use leads to nutrient imbalances and losses of these to the environment through leaching, runoff and gaseous emissions. Nutrient use efficiency (NUE) in agriculture is often low and improving it could increase the sustainability of agricultural systems. The main aims of this thesis were to gain a better understanding of plant-soil-microbe interactions in order to improve agricultural NUEs. The studies included experimentally tested how crops respond to addition of high carbon amendments, fertilizer application rates and timing, and crop rotations. Furthermore, methods for measurement of roots were compared and a protocol for measurement of roots was developed.
The first experiment simulated an agricultural field using mesocosms. In this setting, we tested the effect of 4 previous crops (precrops), which either had or did not have a symbiosis with arbuscular mycorrhizal fungi (AMF)/rhizobia, on the focal crop (winter barley). We also tested the addition of high carbon amendments (wheat straw/sawdust) for immobilization of residual soil nitrogen (N) at harvest of the previous crop. Overall, the findings were that non-AMF precrops had a positive effect on winter barley yield compared to AMF precrops. Wheat straw reduced N leaching, whereas sawdust addition had a negative effect on the yield of winter barley.
Root traits are often measured in static environments, whereas agricultural fertilizer is applied once or multiple times at different crops growth stages. The second experiment tested the effect of different fertilizer (N/phosphorus (P)) application timings on plant traits grown in rhizoboxes. Overall, delaying N application had a more detrimental effect on plant biomass than delaying P application. The root system increased its root length initially due to N-deficiency, but was quickly thus N-limited that root length was relatively lower than the control group. This study emphasizes the need to dynamically measure roots for a mechanistic understanding of root responses to nutrients.
Because of the many root related measurements in the second experiment, a step-by-step method for measuring root traits under controlled and field conditions was developed and included in this thesis. This method paper describes precisely how root traits of interest can be measured, and helps with deciding which approach should be taken depending on the experimental design. Additionally, we compared the bias and accuracy of several popular root measurement methods. Although methods well correlated with a reference method, most methods tended to underestimate the total root length.
Overall, these results highlight the importance of crop choice in crop rotations and the plasticity of root systems in relation to nutrient application. Our results show high carbon amendments could reduce nitrate leaching after the harvest of crops, especially those with high risk of nitrate leaching, although they had only small impacts on yield. Future research should investigate the applicability in a farm setting, also taking into account financial and practical aspects. Non-AMF precrops could possibly increase yield of the next crop due to a shift to parasitism in agricultural fields, but whether this plays a large role in crop yields should be further investigated for specific soil, crop, and climate conditions. Our results also show the plasticity of root systems in response to nutrients. Understanding and using this plasticity can be useful for improving NUEs by optimizing fertilizer application and selecting root traits that are beneficial for specific environmental conditions.
Social insects like honeybees (Apis mellifera) and stingless bees (Apidae: Meliponini) face a relatively high risk to be attacked by pests and pathogens. To decrease the risk of infection, in addition to an innate immune system, these species have evolved various cooperative defense mechanisms such as hygienic behavior or allo-grooming, which contribute to the overall health of the colonies and are therefore also referred to as social immunity. The collection and use of plant resin is another important strategy of social immunity. Resin is a sticky, often aromatic substance with antimicrobial and deterrent properties secreted by plants for protection of the vegetative tissue. Honeybees and stingless bees take advantage of these properties by using resins for nest construction (often mixtures of resin and wax called “propolis” or “cerumen”) and as defense against pests and pathogens. Plant resins, thus, play a crucial role for the ecology of these species and are an important resource for them. Nevertheless, how bees exploit available resin sources and if resin collection can protect colonies from diseases received comparatively little attention in the past. Therefore the aim of this thesis is to provide new insights into the plant origin and significance as well as the influence of resin resource diversity on bee colony health. Resource use and availability form fundamental prerequisites, having decisive influence on the viability of individuals and maintenance of populations. Information on the resources required by a species is thus important to effectively promote and preserve it. For honeybees (A. mellifera) in temperate regions, precise information about which resin sources they use is largely lacking. By chemical comparing bee-collected resins and tree resins, I traced back the resin sources used by individual bees. Results show that honeybees collect distinct resin types that are related to different tree species (several poplar species: Populus balsamifera, P. xcanadensis; Betula alba; Aesculus hippocastanum; several poplar species). With this study I provided the first evidence, that A. mellifera in temperate regions use a variety of different tree species as resin sources and, moreover, show preferences for specific resin sources. Maintenance of colony health is probably one of the major purposes of resin collection. Nevertheless, studies investigating the benefits of resins at the colony level are rare and there are only few evidences on the effects of raw propolis (unlike commonly used ethanol extracts) on colony health. For this reason, I conducted an experimental field study in which I investigated whether propolis, as it is naturally deposited in the nests, can protect honeybee colonies against some of the most important pathogens (Varroa destructor mite, Deformed Wing Virus). The results of this study showed that propolis in (semi-) natural conditions can increase the disease resistance of honeybee colonies, underscoring the importance of resins for honeybee health. Resin collection by stingless bees is comparatively well studied and it is known that these species commonly forage on a variety of different plant species. To increase knowledge on whether and how bees may profit from a diversity of resin resources, I tested how the protective function of a resin varied among different sources (and their mixtures) and various potential aggressors (predators, parasites and pathogens). The results of this study revealed that resins from different trees vary in their effectivity against different target organisms. Moreover, resin blends were more effective than some of the individual resins, suggesting that bees can benefit from a variety of resin resources. In summary, honeybees in temperate regions, similar to tropical stingless bees, use a variety of different tree species as resin sources. Because resins from different tree species varied in their protective function, this indicates that bees can profit from a variety of different resins/resin sources by improving the defense against diverse pests and pathogens. Conversely, the lack of resin had a negative impact on the disease resistance of colonies. Consequently, availability as well as the variety of suitable resin sources is of great importance for the health of bees. In addition to nectar and pollen, resin, as a further important resource, should therefore find more attention in beekeeping. Resin collection as the natural disease defense of bees should find more respect in beekeeping praxis and should be more strongly included in future consideration on how to promote bee colony health.
The worldwide decline of plant and insect species during the last decades has far-reaching consequences for the functionality of ecosystems and their inherent processes. Pollination as one of them is an indispensable ecosystem service for human wellbeing. More than 85% of the worldwide flowering-plant species depend to some degree on pollination by insects (pollinators). Similarly, many pollinators depend on the flowers of the plants, as they need nectar and pollen as food resources for themselves and their offspring. However, an increasing number of pollinator and plant species are threatened by multiple, interacting, and sometimes synergistic causes (habitat loss, fragmentation, diseases, parasites, pesticides, monocultures) that are becoming a growing threat to ecosystem functioning. Given the loss of plant species diversity, it is increasingly difficult for pollinators to find food throughout the year. Therefore, this study analyses the influence of plant diversity on pollinators. The study was conducted in the course of the Jena Experiment, which is a long-term biodiversity experiment (since 2002) with 60 plant species, common to Central European Arrhenatherum grasslands. With a plant diversity gradient of 1, 2, 4, 8, 16, and 60 plant species per plot, time-series data resulted from a wide range of ecosystem processes, ranging from productivity, decomposition, C-storage, and N-storage to herbivory, and pollination. These were studied to investigate the mechanisms underlying the relationships between biodiversity and ecosystem processes.
Chapter 2 studies the spatio-temporal distribution of pollinators on flowers along an experimental plant diversity gradient. For this purpose, the pollinators were divided into four different functional groups, i.e. honeybees, bumblebees, solitary bees and hoverflies. In particular, the spatial pollinator behaviour was examined, that is, in which flowering height the flowers were visited within the plant community. In order to study the temporal component, pollinator visits were observed over the course of the day and the season. As a result, an unprecedented high resolution of plant-pollinator interactions was found. For the first time it was possible to demonstrate that the different pollinator functional groups can complementarily use different spatio-temporal niches which was most pronounced in species-rich plant mixtures,. This leads to the conclusion that species-rich plant mixtures provide sufficient resources that can be used by generalists, such as honeybees and bumblebees, as well as other pollinator functional groups, such as hoverflies and solitary bees.
Chapters 3 and 4 continues on the chemical composition of flower nectar (nectar) of various plant species. Nectar is used as food resource for adult pollinators, but is also largely used as a supply for their offspring, making it the most important pollinator reward. The chemical composition of the nectar was analysed for the two most important macronutrients, carbohydrates (C) and amino acids (AA), using high performance liquid chromatography (HPLC). Subsequently, their contents were analysed in terms of concentration, proportional content and the ratio of carbohydrates to amino acids (C:AA).
In Chapter 3, the nectar of 34 plant species from the grasslands of the Jena Experiment was compared. In doing so, similarities and/or differences of the nectar compositions were investigated with respect to the most important macronutrients carbohydrates and amino acids between the individual species but also between the most representative plant families. This should lead to a better understanding about how plant diversity influences consuming pollinators and which factors, e.g. phylogenetics, morphology or ecology, can lead to different nectar compositions. We could show that each plant species differs in terms of carbohydrate content, amino acid content and C:AA-ratio. In addition, there were clear differences between the four representative plant families Apiaceae, Asteraceae, Fabaceae and Lamiaceae regarding the proportions of essential amino acids. The proportions of the individual sugars and the C:AA-ratios also differed greatly between the four plant families. Therefore, it can be assumed that these nectar contents are family-specific. The need for differences in carbohydrate content are probably due to the different morphology of the flowers, as plants with open flowers and exposed nectar, as in Apiaceae and Asteraceae, can protect their nectar from evaporation if the nectar has a higher osmolality, which can be achieved by a higher hexose (fructose and glucose) content. Thus, the nectar can remain dilute for a longer time and consequently remain consumable for pollinators, which in turn can contribute to the pollination of plants. Fabaceae and Lamiaceae showed different results. Here the nectar was probably protected from evaporation by closed flowers, which explains the high proportion of sucrose, leading to a lower osmolality that would enhance evaporation for exposed nectar. The metabolic pathways controlling the family-specific C:AA-ratios are yet to be explored. In conclusion, it can be suggested that this study contributes to elucidating the morphological and phylogenetic characteristics that control each plant species’ nectar composition.
In Chapter 4, nectar was investigated in the context of diversity effects on the example of the plant species Field Scabious, Knautia arvensis. It was analysed to what extent the nectar quality (nutrient content) differs between plant individuals of one species. The underlying factors causing these differences in nectar composition have never been studied before. In order to investigate these coherences, plant communities in the Jena Experiment of different plant species richness levels containing the target plant species K. arvensis were used. In particular, we examined whether the nectar of K. arvensis is influenced by other neighbouring plant species, e.g. through competition for pollinators. The carbohydrate and amino acid content in nectar varied both between individuals of K. arvensis and between the different plant species richness levels. However, there were significant non-linear differences in the proportions of certain essential and phagostimulatory amino acids, which were produced proportionally more in the nectar of K. arvensis plants in species-rich plant communities, while histidine, one of the generally inhibiting amino acids tended to be less present. Our findings therefore suggest that the nectar of K. arvensis is more palatable when the plants grow in species-rich plant communities.
Overall, these studies indicate how fragile plant-pollinator interactions are but also how important plant species-rich grasslands are to support plant-pollinator interactions. Increased plant species diversity is essential to ensure the availability of flowering resources throughout the year. Pollinators, such as honeybees, bumblebees, solitary bees, and hoverflies can use the niches in time and in vertical space complementarily. However, in plant species-poor grasslands there may be more niche overlaps, which is probably due to a reduced availability of resources. This points to the need to include different plant species belonging to different plant families, whose nectar may have evolved in response to morphological flower traits and metabolic pathways. Therefore plant species diversity can supply pollinators with nectar differing in carbohydrate and amino acid content and thus differing in quality. Also C-AA ratios have proven to be a useful measurement to reveal differences between plant species. In addition, C:AA ratios were not differing in nectar of K. arvensis individuals growing in different plant species richness levels, although their nectar seemed to be more attractive in mixtures with 16 plant species, likely due to higher content of essential and phagostimulatory amino acids than in plant species-poor mixtures. Thus further research investigating diversified farming systems, including pollinator-friendly practices to reveal the attractiveness of different plant species. More diversified field margins and grasslands, for the maintenance of pollinator services for sustainable provision of crop pollination.
