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We examined the impact of arbuscular mycorrhizal fungi and rhizobia on the living microbial community and microbial necromass under different long-term fertilization treatments at the long-term Static Fertilization Experiment Bad Lauchstädt (Germany). Phospholipid fatty acids (PLFA) and amino sugars plus muramic acid, were used as biomarkers for soil microbial bio- and necromass, respectively, and analyzed from six treatments imposed on two crop rotations, varying only in the inclusion/non-inclusion of a legume. Treatments included: two levels of only farmyard manure (FYM), only mineral fertilizer (NPK), the combined application of both fertilizer types and a non-fertilized control. PLFA profiles differed clearly between the investigated crop rotations and were significantly related to labile C, mineral N, and soil pH. This emphasizes the role of carbon, and of mycorrhizal and rhizobial symbioses, as driver for changes in the microbial community composition due to effects on the living conditions in soil. We found some evidence that legume associated symbiosis with arbuscular mycorrhizal fungi and rhizobia act as a buffer, reducing the impact of varying inputs of mineral nutrients on the decomposer community. While our results support former findings that living microbial populations vary within short-term periods and are reflective of a given crop grown in a given year, soil necromass composition indicates longer term changes across the two crop rotation types, mainly shaped by fertilizer related effects on the community composition and C turnover. However, there was some evidence that specifically the presence of a legume, affects the soil necromass composition not only over the whole crop rotation but even in the short-term.

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Forest soils constitute a major carbon pool in the context of the global carbon cycle. Although ectomycorrhizal fungi (EMF) are responsible for a large part of the input of carbon into this pool, information on the transformation of EMF biomass into soil organic matter (SOM) is scarce. In order to quantitatively analyze the contribution of EMF biomass to SOM formation, we incubated a typical forest arenosol with 13C-labeled fungal biomass from the ectomycorrhizal basidiomycete Laccaria bicolor in a closed bioreactor system for 231 d, enabling the establishment of a full mass balance for the turnover. rDNA fragments specific for the genus Laccaria disappeared after 28 d, indicating death and decay of the organism. The amount of 13C PLFA as a proxy for living biomass decreased to roughly 10% of the initial value in the course of the experiment, again suggesting efficient disintegration of the fungal organism. After 231 d, however, 63.5% of the initially added 13C had been converted to SOM and 52% belonged to the non-living fraction. The analysis of 13CO2 formation (36.5% of the initially added label) suggested two-pool exponential kinetics with k1 = 0.0615 d−1 for the easily degradable and k2 = 0.0005 d−1 for the stable fraction (pool sizes 74% and 26% of the initially added label, respectively). When compared to the degradation of bacterial biomass in agricultural soils, the pool of readily degradable fungal biomass was considerably smaller and the half-lives of both pools (readily degradable and persistent C) were markedly higher for fungal biomass. In summary, our data allow the conclusion that EMF biomass is a significant source of SOM in forest ecosystems.

In spite of some inherent challenges, metabolite profiling is becoming increasingly popular under field conditions. It has been used successfully to address topics like species interactions, connections between growth and chemical stoichiometry or the plant’s stress response. Stress exerts a particularly clear impact on plant metabolomes and has become a central topic in many metabolite profiling experiments in the fields. In contrast to phytochambers, however, external stress is often at least partially absorbed by the environment when measuring under field conditions. Such stress-buffering capacities of (agro)-ecosystems are of crucial interest given the ever-increasing anthropogenic impact on ecosystems and this review promotes the idea of using plant metabolite profiles for respective measurements. More specifically I propose to use parameters of the response of key plant species to a given stress treatment as proxies for measuring and comparing stress-buffering capacities of ecosystems. Stress response parameters accessible by metabolite profiling comprise for example the intensity or duration of the impact of stress or the ability of the plant organism to recover from this impact after a given time. Analyses of ecosystem stress-buffering capacities may improve our understanding of how ecosystems cope with stress and may improve our abilities to predict ecosystem changes.

In our experiments we intended to document the impact of tillage on arbuscular mycorrhizal (AM) fungal communities under temperate climate conditions and intensive agriculture in a Luvisol of a fertile Loess area in Saxony (Germany). AM fungal community structure and diversity have been examined in a long-term field experiment in plots subjected to conditions of intensive agriculture with continuous 3-year rotation of winter wheat, winter wheat and sugar beet and managed by till and no-till treatments for thirteen years. AM fungal diversity was assessed by sequence analysis and terminal restriction fragment length polymorphism (T-RFLP) using the nested PCR-primers LR1/FLR2 and FLR3/FLR4 as well as by morphological spore identification. While both analyses resulted in essentially similar pictures of fungal community composition and of negative effects of tillage for AM fungal diversity, morphological spore identification allowed a considerable better differentiation of fungal taxa and a more sensitive detection of changes in community composition and diversity when compared to molecular methods.

The plant organism and associated microbial communities can be seen as a sunlight driven hotspot for the turnover of organic chemicals. In such environments the fate of a chemical will not only depend on its intrinsic structural stability toward (bio-)chemical reactions and its bioavailability but also on the functional effectiveness and stability of natural microbial communities as main drivers of natural attenuation of chemicals. Recent research demonstrates that interactions between plants and microorganisms are crucial for the biotransformation of organic chemicals, for various processes affecting the bioavailability of such compounds, and for the stability of the affected ecosystem. Practical bioremediation approaches, therefore, should encompass integrated measures targeting functional vegetation as well as functional microbial communities. Good examples for a successful practical approach are constructed wetlands, where an artificial, simplified ecosystem is used for the detoxification of organic contaminants. While such systems have considerable practical success, they are often treated as a black box and a sound mechanistic understanding of functional resilience and of the ‘reactive power’ of such plant–microbe ecosystems is poor. This situation has to change, if progress in the application of bioremediation is to be made.

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The identification and quantification of cyclohexenone glycoside derivatives from the model legume Lotus japonicus revealed far higher levels than expected according to the stoichiometric relation to another, already determined carotenoid cleavage product, i.e., mycorradicin. Mycorradicin is responsible for the yellow coloration of many arbuscular mycorrhizal (AM) roots and is usually esterified in a complex way to other compounds. After liberation from such complexes it has been detected in AM roots of many, but not of all plants examined. The non-stoichiometric occurrence of this compound compared with other carotenoid cleavage products suggested that carotenoid biosynthesis might be activated upon mycorrhization even in plant species without detectable levels of mycorradicin. This assumption has been supported by inhibition of a key enzyme of carotenoid biosynthesis (phytoene desaturase) and quantification of the accumulating enzymic substrate (phytoene). Our observations suggest that the activation of carotenoid biosynthesis in AM roots is a general phenomenon and that quantification of mycorradicin is not always a good indicator for this activation. (c) 2005 Elsevier Ltd. All rights reserved