Plant scientists have made great progress in understanding molecular mechanisms controlling root responses to nutrients of arabidopsis (Arabidopsis thaliana) plants under controlled conditions. ...Simultaneously, ecologists and agronomists have demonstrated that root–root interactions involve more than competition for nutrients. Here, we highlight the importance of both root exudates and soil microbes for root–root interactions, ubiquitous in natural and agricultural ecosystems. We argue that it is time to bring together the recent insights from both scientific disciplines to fully understand root functioning in the real world.
Many studies have shown the importance of nutrient competition for plant performance in natural and agricultural ecosystems. However, results of several recent experiments show that these interactions cannot be explained by competition for nutrients alone. The rhizosphere has a key role in mitigating root–root interactions.
Both the rhizobiome and root exudation affect root–root interactions either directly or via modification of resource availability to the plants. These pathways are closely connected because exudates affect different components of the rhizobiome, and vice versa.
Combining the toolbox from molecular biology with ecological complexity will elucidate the functional interplay among the rhizobiome, root exudates, and the root, and its importance for plant performance in the real world.
CONTENTS: Summary 30 I. Allocation in perspective 31 II. Topics of this review 32 III. Methodology 32 IV. Environmental effects 33 V. Ontogeny 36 VI. Differences between species 40 VII. Physiology ...and molecular regulation 41 VIII. Ecological aspects 42 IX. Perspectives 45 Acknowledgements 45 References 45 Appendices A1–A4 49 SUMMARY: We quantified the biomass allocation patterns to leaves, stems and roots in vegetative plants, and how this is influenced by the growth environment, plant size, evolutionary history and competition. Dose–response curves of allocation were constructed by means of a meta‐analysis from a wide array of experimental data. They show that the fraction of whole‐plant mass represented by leaves (LMF) increases most strongly with nutrients and decreases most strongly with light. Correction for size‐induced allocation patterns diminishes the LMF‐response to light, but makes the effect of temperature on LMF more apparent. There is a clear phylogenetic effect on allocation, as eudicots invest relatively more than monocots in leaves, as do gymnosperms compared with woody angiosperms. Plants grown at high densities show a clear increase in the stem fraction. However, in most comparisons across species groups or environmental factors, the variation in LMF is smaller than the variation in one of the other components of the growth analysis equation: the leaf area : leaf mass ratio (SLA). In competitive situations, the stem mass fraction increases to a smaller extent than the specific stem length (stem length : stem mass). Thus, we conclude that plants generally are less able to adjust allocation than to alter organ morphology.
Summary
The effects of plants on the biosphere, atmosphere and geosphere are key determinants of terrestrial ecosystem functioning. However, despite substantial progress made regarding plant ...belowground components, we are still only beginning to explore the complex relationships between root traits and functions. Drawing on the literature in plant physiology, ecophysiology, ecology, agronomy and soil science, we reviewed 24 aspects of plant and ecosystem functioning and their relationships with a number of root system traits, including aspects of architecture, physiology, morphology, anatomy, chemistry, biomechanics and biotic interactions. Based on this assessment, we critically evaluated the current strengths and gaps in our knowledge, and identify future research challenges in the field of root ecology. Most importantly, we found that belowground traits with the broadest importance in plant and ecosystem functioning are not those most commonly measured. Also, the estimation of trait relative importance for functioning requires us to consider a more comprehensive range of functionally relevant traits from a diverse range of species, across environments and over time series. We also advocate that establishing causal hierarchical links among root traits will provide a hypothesis‐based framework to identify the most parsimonious sets of traits with the strongest links on functions, and to link genotypes to plant and ecosystem functioning.
The search for a root economics spectrum (RES) has been sparked by recent interest in trait-based plant ecology. By analogy with the one-dimensional leaf economics spectrum (LES), fine-root traits ...are hypothesised to match leaf traits which are coordinated along one axis from resource acquisitive to conservative traits. However, our literature review and meta-level analysis reveal no consistent evidence of an RES mirroring an LES. Instead the RES appears to be multidimensional. We discuss three fundamental differences contributing to the discrepancy between these spectra. First, root traits are simultaneously constrained by various environmental drivers not necessarily related to resource uptake. Second, above- and belowground traits cannot be considered analogues, because they function differently and might not be related to resource uptake in a similar manner. Third, mycorrhizal interactions may offset selection for an RES. Understanding and explaining the belowground mechanisms and trade-offs that drive variation in root traits, resource acquisition and plant performance across species, thus requires a fundamentally different approach than applied aboveground. We therefore call for studies that can functionally incorporate the root traits involved in resource uptake, the complex soil environment and the various soil resource uptake mechanisms – particularly the mycorrhizal pathway – in a multidimensional root trait framework.
Summary
Feedback between plants and soil microbial communities can be a powerful driver of vegetation dynamics. Plants elicit changes in the soil microbiome that either promote or suppress ...conspecifics at the same location, thereby regulating population density‐dependence and species co‐existence. Such effects are often attributed to the accumulation of host‐specific antagonistic or beneficial microbiota in the rhizosphere. However, the identity and host‐specificity of the microbial taxa involved are rarely empirically assessed. Here we review the evidence for host‐specificity in plant‐associated microbes and propose that specific plant–soil feedbacks can also be driven by generalists. We outline the potential mechanisms by which generalist microbial pathogens, mutualists and decomposers can generate differential effects on plant hosts and synthesize existing evidence to predict these effects as a function of plant investments into defence, microbial mutualists and dispersal. Importantly, the capacity of generalist microbiota to drive plant–soil feedbacks depends not only on the traits of individual plants but also on the phylogenetic and functional diversity of plant communities. Identifying factors that promote specialization or generalism in plant–microbial interactions and thereby modulate the impact of microbiota on plant performance will advance our understanding of the mechanisms underlying plant–soil feedback and the ways it contributes to plant co‐existence.
The storage of carbon (C) and nitrogen (N) in soil is important ecosystem functions. Grassland biodiversity experiments have shown a positive effect of plant diversity on soil C and N storage. ...However, these experiments all included legumes, which constitute an important N input through N₂‐fixation. Indeed, the results of these experiments suggest that N₂ fixation by legumes is a major driver of soil C and N storage. We studied whether plant diversity affects soil C and N storage in the absence of legumes. In an 11‐year grassland biodiversity experiment without legumes, we measured soil C and N stocks. We further determined above‐ground biomass productivity, standing root biomass, soil organic matter decomposition and N mineralization rates to understand the mechanisms underlying the change in soil C and N stocks in relation to plant diversity and their feedbacks to plant productivity. We found that soil C and N stocks increased by 18% and 16% in eight‐species mixtures compared to the average of monocultures of the same species, respectively. Increased soil C and N stocks were mainly driven by increased C input and N retention, resulting from enhanced plant productivity, which surpassed enhanced C loss from decomposition. Importantly, higher soil C and N stocks were associated with enhanced soil N mineralization rates, which can explain the strengthening of the positive diversity–productivity relationship observed in the last years of the experiment. Synthesis. We demonstrated that also in the absence of legumes, plant species richness promotes soil carbon (C) and nitrogen (N) stocks via increased plant productivity. In turn, enhanced soil C and N stocks showed a positive feedback to plant productivity via enhanced N mineralization, which could further accelerate soil C and N storage in the long term.
The root economics spectrum (RES) hypothesis predicts that fast‐growing tree species have short‐lived roots with high specific root length (SRL) to allow rapid resource uptake, and opposite trait ...expressions for slow‐growing species. Yet, the mixed support for this hypothesis suggests that trees can adopt alternative strategies to increase resource uptake, besides an increase in SRL.
We combined a novel mechanistic whole‐tree model and empirical fine‐root data of 10 tree species to test the effects of one of these alternative strategies, notably increasing fine‐root mass, on the tree's net C gain (used here as a proxy for tree performance), and to assess how fine‐root life span influences the relative importance of SRL and fine‐root mass for the C balance of trees.
Our results indicate that accounting for the short life span of high‐SRL roots has important implications for explaining tree performance and the role of roots herein. Without considering their faster turnover, high‐SRL roots and low fine‐root mass resulted in the highest performance as predicted from the RES. Yet, when their higher turnover rates were accounted for, a high fine‐root mass and low SRL lead to the highest performance. Both our model outcomes and field data further show a negative relationship between SRL and fine‐root mass through which species aim to realize a similar root length density. This trade‐off further indicates how high a SRL and low fine‐root mass as well as opposite trait values can both lead to a positive C balance in a similar environment.
Our study may explain why high‐SRL roots do not necessarily lead to the fastest tree growth as often hypothesized and demonstrates the importance of fine‐root mass in combination with fine‐root life span for explaining interspecific differences in tree performance. More generally, our work demonstrates the value of identifying and investigating different below‐ground strategies across species from a whole‐plant modelling perspective, and identifies the relationship between SRL, fine‐root biomass and life span as an important functional dimension to variation in species’ performance.
A free Plain Language Summary can be found within the Supporting Information of this article.
A free Plain Language Summary can be found within the Supporting Information of this article.
Aims
Saprophytic fungi are important agents of soil mineralization and carbon cycling. Their community structure is known to be affected by soil conditions such as organic matter and pH. However, the ...effect of plant species, whose roots provide the litter input into the soil, on the saprophytic fungal community is largely unknown.
Methods
We examined the saprophytic fungi in a grassland biodiversity experiment with eight plant species belonging to two functional groups (grasses and forbs), combining DNA extraction from plant roots, next-generation sequencing and literature research.
Results
We found that saprophyte richness increased with plant species richness, but plant functional group richness was the best predictor. Plant functional group was also the main factor driving fungal saprophytic community structure. This effect was correlated with differences in root lignin content and C:N ratio between grasses and forbs. In monocultures, root traits and plant functional group type explained 16% of the variation in community structure. The saprophyte taxa detected in mixed plant communities were to a large extent subsets of those found in monocultures.
Conclusions
Our work shows that the richness and community structure of the root-associated saprophytic fungi can largely be predicted by plant functional groups and their associated root traits. This means that the effects of plant diversity on ecosystem functions such as litter decomposition may also be predictable using information on plant functional groups in grasslands.
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