The role of plant diversity, plant functional groups, and mineral nitrogen for soil microbial functioning and soil mesofauna in temperate grassland
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EnglishSoil-derived ecosystem functions such as decomposition and element cycling are crucial for ecosystem services such as production of food, fodder and biofuels; in short, they are essential prerequisites for human and animal life. They dependent on biodiversity, soil microbial functioning and the soil-inhabiting fauna. Since about 30 years, ecological research focuses on biodiversity-ecosystem function (BEF) relationships, and by now it is general consensus among ecologists that biodiversity is essential for maintaining ecosystem functioning. Although many studies on BEF relationships have been done, further knowledge on the mechanisms underlying the positive BEF relationships is still needed. Especially the role of (plant) biodiversity for belowground processes and soil organisms has been neglected in earlier studies on BEF relationships. Also, studies investigating the stability of soil microbial functioning and its relation to plant diversity are scarce, although stability of ecosystem functions is essential for the sustainable provisioning of ecosystem services. Additionally, the influence of mineral N fertilization on soil organisms and its interaction with plant community properties is of major importance for ecosystem functioning and needs further investigation, as the application of N fertilizer increases worldwide, but is known to has controversial effects on ecosystem functions, such as increasing crop productivity, but decreasing plant diversity. Within the framework of the present thesis I conducted three studies investigating the impact of plant diversity and plant community composition (identity of plant functional groups) on soil microbial properties and their stability in bulk-soil, combined with the investigation of mineral N incorporation into soil microorganisms and its channelling from microorganisms into soil fauna (mesofauna). The studies were conducted on the field site of the Jena Experiment, comprising plant communities with up to 60 plant species and 1-4 plant functional groups (legumes, grasses, small herbs, tall herbs; all species belong to Molinio-Arrhenateretea meadows typical for hay meadows in Central Europe).
In study 1 (Chapter 2, Stability Experiment), we tracked soil microbial properties (basal respiration and biomass C) over a time period of 12 years. We found that plant species richness consistently increased both soil microbial basal respiration and biomass after a time-lag of four years after the establishment of the experiment, and that the positive relationship between plant species richness and soil microbial properties lasted until the end of the study. The delayed response of the soil microbial community to changes in land-use (from former arable field monocultures to semi-natural experimental grassland) points to the long time, plant effects need to materialize in the belowground system. After the time-lag, increasing amounts and variety of plant-derived inputs into the soil with increasing plant diversity presumably fostered soil microbial respiration and biomass. We expected plant diversity to show specific dynamic effects over the three time phases of our long-term study (each phase spanning four years) on the temporal stability of soil microbial properties. Due to the disturbance (=land-use change) at the beginning of the experiment and the following maturation of the plant communities, we expected plant diversity to exert destabilizing effects during phase 1, neutral effects in phase 2, and positive effects in phase 3 on the temporal stability of soil microbial properties. Indeed, we found the effect of plant diversity on the temporal stability of soil microbial properties to turn from being negative to neutral, but this neutral relationship lasted until the end of the study, suggesting that the recovery of soil microbial communities from former arable land-use takes more than a decade. For the spatial stability of soil microbial properties, the presence of plant functional groups was of major importance, with legumes and tall herbs reducing the spatial stability of microbial respiration, and grasses increasing the spatial stability of the latter. Presumably, plant-trait-based mechanisms such as rhizodeposition of N-rich compounds by legumes, patchy C provisioning to the soil by tap-roots of tall herbs, and evenly distributed C provisioning by grass roots provoked the observed effects of plant functional groups on soil microorganisms.
The results of study 2 (Chapter 3, Fertilization Experiment) revealed that mineral N fertilizer and plant diversity acted independent of each other on soil microorganisms. Unexpectedly, fertilization did not increase soil microbial biomass. As the soil system has been shown to react with a time-lag of several years to environmental changes such as management practice or plant diversity (see above), we assume that the duration of our Fertilization Experiment (two years) may have been too short to unravel the full response of soil microorganisms to fertilization. Instead, fertilization superimposed the negative legume effect on soil microbial respiration, although the underlying mechanisms are likely to be different. Legumes are known to fuel the soil system with organic N, thereby increasing soil microbial C use efficiency. In contrast, mineral N fertilizer probably decreases rhizosphere priming effects by delivering inorganic N, and probably also increased microbial C use efficiency in the present study, as suggested by decreased microbial C-to-N-ratios in fertilized experimental plots. Although mineral N fertilizer neither affected soil microbial biomass nor interacted with plant diversity on soil microbial properties within the investigated time frame of two years, the interactive effect between fertilization and legumes on the soil microbial C-to-N-ratio indicates that mineral N was incorporated into the soil microbial biomass.
To investigate the role of plant community properties for the microbial uptake of mineral N, and whether mineral-derived N is channelled from microorganisms to higher trophic levels, we labelled soil with mineral 15N and analysed its incorporation into soil microbial biomass and most abundant mesofauna taxa over three months (Study 3, Chapter 4, Tracer Experiment). Mineral-derived 15N incorporation decreased over time in all investigated organisms (except in the primary decomposer Tectocepheus velatus sarekensis), reflecting the fast incorporation of mineral 15N into microorganisms and its dominant channeling into mesofauna species. Plant species richness reduced the uptake of mineral 15N in microorganisms, presumably because competition for N in soil is aggravated in more diverse plant communities. The effects of plant diversity on the incorporation of mineral-derived 15N into mesofauna species were species-specific, and reflected different nutritional strategies among animal species. For example, plant species richness decreased 15N incorporation into the secondary decomposer Ceratophysella sp., likely because Ceratophysella sp. fed on microorganisms that were also reduced in 15N due to limited N supply in plant communities of high diversity. Interestingly, plant species richness exerted time-dependent effects in other mesofauna species, e.g. in the primary decomposer Tectocepheus velatus sarekensis. Potentially, the increase in 15N in T. velatus sarekensis with plant species richness later in the experiment was due to increased availability of dead plant roots containing 15N from the mineral 15N added. Also plant community composition (plant functional group identity) played a major role for the 15N incorporation into soil organisms. For example, presence of legumes decreased 15N in soil microorganisms, presumably due the release of unlabelled organic N via rhizodeposition. Grasses increased the incorporation of 15N in Ceratophysella sp., suggesting that the diet of Ceratophysella sp. is not restricted to microorganisms but also includes plant roots highly labelled with 15N. The results of study 3 highlight that mineral N is quickly channeled into soil animal food webs via microorganisms. Our results highlight that plant diversity and community composition alter the competition for N in soil and change the nutrient transfer across trophic levels in soil food webs, potentially leading to changes in soil animal population dynamics and community composition. In short, the present thesis indicates that
1| plant diversity and community composition drive soil microbial properties (respiration, biomass), as well as the temporal and spatial stability of these properties.
2| the soil system reacts with a time-lag of several years to land-use change, and soil microbial communities need more than a decade to recover from former agricultural land-use.
3| plant functional groups exert trait-specific effects on soil microbial properties, and these effects complement each other.
4| mineral N fertilization acts independent of plant diversity on soil microbial properties, but exerts interacting effects with certain plant functional groups (legumes),
5| soil microorganisms largely and quickly incorporate mineral N, and channel this N to higher trophic levels of the soil food web.
6| plant diversity and community composition shape the incorporation of mineral N into soil microorganisms and soil fauna. Taken together, the present thesis reinforces and complements the findings of earlier BEF studies, and emphasizes the importance of maintaining grasslands with high plant diversity including all investigated plant functional groups (legumes, grasses, small and tall herbs) with their trait-specific effects for essential soil ecosystem functions and services. Especially in a changing world with increasing anthropogenic impacts such as increasing mineral N fertilizer application, plant diversity may also buffer detrimental effects of mineral N on ecosystems.