Anika Lehmann

Mycorrhizal responsiveness trends in annual crop plants and their wild relatives—a meta-analysis on studies from 1981 to 2010

Background and aimsYear of release of a cultivar reflects the agricultural and breeding practices of its time; we hypothesize that there are differences in mycorrhizal responsiveness of new high yielding and old crop plants and landraces. We evaluated the importance of the year of release on mycorrhizal responsiveness, arbuscular mycorrhizal (AM) fungal root colonization and P efficiency. We also analyzed the effect of experimental treatments, P efficiency (P acquisition and P utilization efficiency) and AM fungal root colonization on a potential mycorrhizal responsiveness trend for year of release.MethodsWe conducted a meta-analysis on 39 publications working on 320 different crop plant genotypes.ResultsNew cultivars were less intensely colonized but were more mycorrhiza-responsive (and possibly dependent) compared to ancestral genotypes. This trend was potentially influenced by the moderator variables density, pre-germination, plant, plant type and AMF species. AM root colonization was also important for the mycorrhizal responsiveness trend for year of release, but P efficiency was not.ConclusionsWith the data available we could find no evidence that new crop plant genotypes lost their ability to respond to mycorrhiza due to agricultural and breeding practices.

Towards an Integrated Mycorrhizal Technology: Harnessing Mycorrhiza for Sustainable Intensification in Agriculture

BACKGROUND Sustainability in Agriculture In order to meet future needs of a growing human population and to achieve food security in the context of climate change, food production will likely need to increase—among other measures—while at the same time minimizing negative environmental impact (Foley et al., 2011). Sustainable intensification of agriculture (Garnett et al., 2013; Pretty and Bharucha, 2014; Andres and Bhullar, 2016; Gunton et al., 2016), sometimes also called ecological intensification, is likely to include key aspects of conservation agriculture (e.g., Hobbs et al., 2008; Giller et al., 2015). Pillars of conservation agriculture (FAO, 2015) are no-till practices (Pittelkow et al., 2015), continuous crop cover (by various means, for example cover crops) and diversification practices (multi-cropping and crop rotations; Ponisio et al., 2015).

Soil aggregates as massively concurrent evolutionary incubators

Soil aggregation, a key component of soil structure, has mostly been examined from the perspective of soil management and the mediation of ecosystem processes such as soil carbon storage. However, soil aggregation is also a major factor to consider in terms of the fine-scale organization of the soil microbiome. For example, the physico-chemical conditions inside of aggregates usually differ from the conditions prevalent in the bulk soil and aggregates therefore increase the spatial heterogeneity of the soil. In addition, aggregates can provide a refuge for microbes against predation since their interior is not accessible to many predators. Soil aggregates are thus clearly important for microbial community ecology in soils (for example, Vos et al., 2013; Rillig et al., 2016) and for microbially driven biogeochemistry, and soil microbial ecologists are increasingly appreciating these aspects of soil aggregation. Soil aggregates have, however, so far been neglected when it comes to evolutionary considerations (Crawford et al., 2005) and we here propose that the process of soil aggregation should be considered as an important driver of evolution in the soil microbial community. There are several features that make soil aggregates specifically interesting, and perhaps even unique, in terms of a setting for microbial evolution (Table 1). Soil aggregation is a continuous and dynamic process in which the formation and disintegration of individual microand macroaggregates are separated in time by periods of relative stability. Each individual soil aggregate may provide a unique environmental compartmentalization of the soil microbial community that is, to a large extent, isolated from its surroundings and that can be thought of as an ‘incubator’ for microbial evolutionary change. Because of their isolation, different aggregates can be regarded as ‘concurrent incubators’ that allow enclosed microbial communities to pursue their own independent evolutionary trajectories during their lifetime (‘incubation period’). The huge number of aggregates that exists at any moment in time validates their conceptualization as ‘massively concurrent incubators’ for microbial evolutionary change (Figure 1). Upon disintegration of soil aggregates (‘incubation cycle ends’), formerly enclosed microbial communities are released and allowed to interact with the microbial community of the soil at large. This combination of features (isolation, large number and relative stability) sets soil aggregates apart from other microbial habitats that may also provide temporary isolation of microbial communities, such as the animal intestinal tract and other parts of the animal body (see also Cordero and Datta, 2016), leaves, roots, and many aquatic habitats (Table 1). However, these habitats do not provide the same combination of extent of isolation, duration of isolation and number of concurrent ‘incubators’ as soil aggregates do. We discuss these specific characteristics of soil aggregates next, before describing how evolutionary change in aggregates can occur and explaining how this system can be tackled empirically.

Microbial stress priming – a meta‐analysis

Microbes have to cope with complex and dynamic environments, making it likely that anticipatory responses provide fitness benefits. Mild, previous stressors can prepare microbes (stress priming) to further and potentially damaging stressors (triggering). We here quantitatively summarize the findings from over 250 trials of 34 studies including bacteria and fungi, demonstrating that priming to stress has a beneficial impact on microbial survival. In fact, survival of primed microbes was about 10-fold higher compared with that in non-primed microbes. Categorical moderators related to microbial taxonomy and the kind of stress applied as priming or as triggering revealed significant differences of priming effect size among 14 different microbial species, 6 stress categories and stressor combination. We found that priming by osmotic, physiological and temperature stress had the highest positive effect sizes on microbial response. Cross-protection was evident for physiological, temperature and pH stresses. Microbes are better prepared against triggering by oxidative, temperature and osmotic stress. Our finding of an overall positive mean effect of priming regardless of the microbial system and particular stressor provides unprecedentedly strong evidence of the broad ecological significance of microbial stress priming. These results further suggest that stress priming may be an important factor in shaping microbial communities.

Soil biota contributions to soil aggregation

Humankind depends on the sustainability of soils for its survival and well-being. Threatened by a rapidly changing world, our soils suffer from degradation and biodiversity loss, making it increasingly important to understand the role of soil biodiversity in soil aggregation—a key parameter for soil sustainability. Here, we provide evidence of the contribution of soil biota to soil aggregation on macro- and microaggregate scales, and evaluate how specific traits, soil biota groups and species interactions contribute to this. We conducted a global meta-analysis comprising 279 soil biota species. Our study shows a clear positive effect of soil biota on soil aggregation, with bacteria and fungi generally appearing to be more important for soil aggregation than soil animals. Bacteria contribute strongly to both macro- and microaggregates while fungi strongly affect macroaggregation. Motility, body size and population density were important traits modulating effect sizes. Investigating species interactions across major taxonomic groups revealed their beneficial impact on soil aggregation. At the broadest level, our results highlight the need to consider biodiversity as a causal factor in soil aggregation. This will require a shift from the current management and physicochemical perspective to an approach that fully embraces the significance of soil organisms, their diversity and interactions.The structuring of soil into distinct aggregates is a key element in biogeochemical cycling. Here, a meta-analysis reveals a strong positive effect of soil biota on soil aggregation, with the largest influence coming from bacteria and fungi.

Impacts of domestication on the arbuscular mycorrhizal symbiosis of 27 crop species

The arbuscular mycorrhizal (AM) symbiosis is key to plant nutrition, and hence is potentially key in sustainable agriculture. Fertilization and other agricultural practices reduce soil AM fungi and root colonization. Such conditions might promote the evolution of low mycorrhizal responsive crops. Therefore, we ask if and how evolution under domestication has altered AM symbioses of crops. We measured the effect of domestication on mycorrhizal responsiveness across 27 crop species and their wild progenitors. Additionally, in a subset of 14 crops, we tested if domestication effects differed under contrasting phosphorus (P) availabilities. The response of AM symbiosis to domestication varied with P availability. On average, wild progenitors benefited from the AM symbiosis irrespective of P availability, while domesticated crops only profited under P-limited conditions. Magnitudes and directions of response were diverse among the 27 crops, and were unrelated to phylogenetic affinities or to the coordinated evolution with fine root traits. Our results indicate disruptions in the efficiency of the AM symbiosis linked to domestication. Under high fertilization, domestication could have altered the regulation of resource trafficking between AM fungi and associated plant hosts. Provided that crops are commonly raised under high fertilization, this result has important implications for sustainable agriculture.

Intransitive competition is common across five major taxonomic groups and is driven by productivity, competitive rank and functional traits

TRY is currently supported by DIVERSITAS/Future Earth and the German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig. S.S. was supported by the Spanish Government under a Ramon y Cajal contract (RYC-2016-20604). A.L. and M.C.R. acknowledge funding from Deutsche Forschungsgemeinschaft (DFG, grant no: RI 1815/16-1). F.A. has been supported by the Swiss National Science Foundation (grants no. 31003A_135622 and PP00P3_150698). M.D.-B. acknowledges support from the Marie Sklodowska-Curie Actions of the Horizon 2020 Framework Program H2020-MSCA-IF-2016 under REA grant agreement no. 702057. E.A. received financial support from the Swiss National Science Foundation (grant number 31003A_160212). S.B., E.A. and S.S. were partly funded by the DFG Priority Program 1374 “Infrastructure-Biodiversity-Exploratories” (Fi-1246/6-1). Fieldwork permits were issued by the responsible state environmental offices of Baden-Wurttemberg. B.K.S. is supported by Australian Research Council (DP170104634).

Impacts of Microplastics on the Soil Biophysical Environment

Soils are essential components of terrestrial ecosystems that experience strong pollution pressure. Microplastic contamination of soils is being increasingly documented, with potential consequences for soil biodiversity and function. Notwithstanding, data on effects of such contaminants on fundamental properties potentially impacting soil biota are lacking. The present study explores the potential of microplastics to disturb vital relationships between soil and water, as well as its consequences for soil structure and microbial function. During a 5-weeks garden experiment we exposed a loamy sand soil to environmentally relevant nominal concentrations (up to 2%) of four common microplastic types (polyacrylic fibers, polyamide beads, polyester fibers, and polyethylene fragments). Then, we measured bulk density, water holding capacity, hydraulic conductivity, soil aggregation, and microbial activity. Microplastics affected the bulk density, water holding capacity, and the functional relationship between the microbial activity and water stable aggregates. The effects are underestimated if idiosyncrasies of particle type and concentrations are neglected, suggesting that purely qualitative environmental microplastic data might be of limited value for the assessment of effects in soil. If extended to other soils and plastic types, the processes unravelled here suggest that microplastics are relevant long-term anthropogenic stressors and drivers of global change in terrestrial ecosystems.

Soil Biodiversity Effects from Field to Fork

Our knowledge of soil biodiversity in agriculture in general is currently increasing rapidly. However, almost all studies have stopped with the quantification of soil biodiversity effects on crops at harvest time, ignoring subsequent processes along the agrifood chain until food arrives on our plates. Here we develop a conceptual framework for the study of such postharvest effects. We present the main mechanisms (direct and indirect) via which soil biodiversity can influence crop quality aspects and give examples of how effects at harvest time may become attenuated through postharvest operations and how biodiversity may also affect some of these operations (i.e., storage) themselves. Future research with a broader focus has the potential to unveil how soil biodiversity may benefit from what ends up on our forks.

Chapter 14 – Mycorrhizas and Soil Aggregation

Soil aggregation is a key ecosystem service provided by a multitude of soil biota, among which mycorrhizal fungi can play a pivotal role. We review the evidence for mycorrhizal involvement in soil aggregation for different mycorrhizal types, including arbuscular mycorrhizas and ectomycorrhizas. Evidence for the importance of arbuscular mycorrhizas in the provision of soil aggregation is particularly strong and comes from a variety of sources, including observational and experimental studies. Various mechanisms are hypothesized to play a role in the mycorrhizal mediation of soil aggregation and its component processes. We critically discuss the evidence for these, highlighting that our mechanistic understanding of soil aggregation is still quite rudimentary. Another critical area of knowledge concerns the relative importance of mycorrhizas compared with other biota within a given setting, and also across different site characteristics and ecosystem types. We finish by pointing out high-priority areas in need of further research, among which adopting a trait-based approach appears particularly promising.