Plant roots contribute substantially to the formation of stable soil organic matter (SOM), and there is evidence that species differ in their contribution to SOM stabilization. However, it remains ...unclear what specific root traits contribute to the three SOM stabilization mechanisms: recalcitrance against decomposition, occlusion in soil aggregates and interaction with soil minerals and metals. This is likely because research is highly fragmented and hampered by disciplinary barriers. By reviewing both plant functional ecology and soil science literature, we identified 18 different traits: architectural, morphological, physiological, symbiotic and chemical root characteristics, influencing the three SOM stabilization mechanisms. We found that traits increasing root recalcitrance promote short term stabilization by slowing decomposition, but that traits reducing recalcitrance contribute to long term stabilization by reaction of microbial products with mineral surfaces. Root length density, mycorrhizal association and rhizodeposition contribute to microaggregation. These and other traits, such as hemicellulose, soluble compounds, and high root branching index, favor macroaggregation. For stabilization by minerals and metals, those root traits promoting higher microbial activity: root nitrogen, hemicellulose and soluble compound concentrations are fundamental, while polyphenols, and litter Al and Mn also contribute to complexification and stabilization. Root depth distribution is the most important trait to control root C storage and stabilization in the subsoil; once roots have reached deeper soil layers, other traits, such as rhizodeposition and root chemistry, influence interaction with minerals and metals. Both mycorrhizal presence and root suberin promote SOC stabilization by means of all three mechanisms, indicating that these are important targets for continued work. Surprisingly, morphological traits commonly measured, namely specific root length and root diameter, poorly relate to stabilization mechanisms. Alternative traits such as chemical composition of the different root orders, root apex characteristics, quantity and quality of rhizodeposits as well as mycorrhizal fungal traits, should be further investigated. For future research, this review highlights the need to evaluate root decomposition and root-C stabilization concomitantly over the long-term, considering simultaneously root litter quality, estimated by root traits, the microbial products and properties of the soil matrix. The information accrued in this review can be used to evaluate the potential of plant species and cultivars to promote SOM stabilization, based on their root traits.
•We identified 18 root traits influencing three soil organic matter stabilization mechanisms.•Chemical traits reducing root recalcitrance promote interactions with mineral surfaces.•Mycorrhizae and root suberin concentration are important to the three mechanisms.•Specific root length and root diameter poorly relate to stabilization mechanisms.•Root depth distribution is the most important trait to control root C stabilization in subsoil.
Summary
Roots vary in anatomy, morphology and physiology, both spatially (different parts of the same root system) and temporally (plastic changes, root ageing), suggesting that root trait ...measurements are strongly affected by root sampling categories.
In this context, it is urgent to clarify the functional significance of current root sampling categories (e.g. fine roots of the first order, the first three orders, ≤1 mm or ≤2 mm), establish guidelines for choosing between sampling methods and revise root ontology to account for functional differences between traits measured on distinct root categories.
Here, we used a worldwide database of fine‐root traits to test the hypothesis that distinct fine‐root trait values – with link to fine‐root functions – were generally affected by different root sampling categories. We observed indeed a clear functional break between first‐order roots and roots of all three other sampling categories, and a smaller but substantial break between roots of the three first orders and the ≤2 mm category, demonstrating globally that different sampling methodologies capture different functional parts of roots.
Our synthesis suggests that all current root sampling categories present both advantages and pitfalls and that no single method can appropriately tackle the main current challenge of root functional ecology: i.e. linking fine roots to plant and ecosystem functions in a truly comparable way across all plants. We argue instead that a small set of complementary standardized sampling methods is necessary to capture the linkages between root forms and functions.
To assist experimenters selecting adequate sampling we developed a decision table following three logical questions: (i) what plant or ecosystem function must be addressed; (ii) what root categories are involved in this function and (iii) what traits should be measured on these root categories. Challenging, strengthening and expending such common reference framework would be a substantial step towards wider comparability of future functional trait datasets.
A lay summary is available for this article.
Lay Summary
Although fine roots are important components of the global carbon cycle, there is limited understanding of root structure–function relationships among species. We determined whether root respiration ...rate and decomposability, two key processes driving carbon cycling but always studied separately, varied with root morphological and chemical traits, in a coordinated way that would demonstrate the existence of a root economics spectrum (RES).
Twelve traits were measured on fine roots (diameter ≤ 2 mm) of 74 species (31 graminoids and 43 herbaceous and dwarf shrub eudicots) collected in three biomes.
The findings of this study support the existence of a RES representing an axis of trait variation in which root respiration was positively correlated to nitrogen concentration and specific root length and negatively correlated to the root dry matter content, lignin : nitrogen ratio and the remaining mass after decomposition. This pattern of traits was highly consistent within graminoids but less consistent within eudicots, as a result of an uncoupling between decomposability and morphology, and of heterogeneity of individual roots of eudicots within the fine-root pool.
The positive relationship found between root respiration and decomposability is essential for a better understanding of vegetation–soil feedbacks and for improving terrestrial biosphere models predicting the consequences of plant community changes for carbon cycling.
Background Roots play a pivotal role in defining plant ecological success and mediating terrestrial ecosystem functioning. However, roots are difficult to study as they are hidden in the soil matrix ...and express a plurality of forms and functions: nutrient acquisition, anchorage, nutrient and water transport. Scope In this special issue, we gather 20 articles from emerging topics in root ecology. We specifically focus on five frontiers that are essential to better capture plant, soil and ecosystem processes: (1) root structure and soil resource acquisition; (2) root dynamics; (3) root-root interactions; (4) root-microbe interactions and (5) the effects of roots on soil properties. For each of these sections we provide an overview of our current knowledge, highlight the contributions of this special issue, and pinpoint key remaining challenges for future work. Conclusions Root resource acquisition, root dynamics, root-root and root-microbes interaction and the relative importance of root vs. soil properties on soil aggregation were shown to vary among biomes, soil and climate conditions. Such variability gives a complex but realistic picture of root functioning in real ecosystems and lays an important foundation for future work identify broad patterns of root functioning.
A worldview of root traits Valverde‐Barrantes, Oscar J.; Freschet, Grégoire T.; Roumet, Catherine ...
New phytologist,
September 2017, Letnik:
215, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Fine-root traits play key roles in ecosystem processes, but the drivers of fine-root trait diversity remain poorly understood. The plant economic spectrum (PES) hypothesis predicts that leaf and root ...traits evolved in coordination. Mycorrhizal association type, plant growth form and climate may also affect root traits. However, the extent to which these controls are confounded with phylogenetic structuring remains unclear.
Here we compiled information about root and leaf traits for > 600 species. Using phylogenetic relatedness, climatic ranges, growth form and mycorrhizal associations, we quantified the importance of these factors in the global distribution of fine-root traits.
Phylogenetic structuring accounts for most of the variation for all traits excepting root tissue density, with root diameter and nitrogen concentration showing the strongest phylogenetic signal and specific root length showing intermediate values. Climate was the second most important factor, whereas mycorrhizal type had little effect. Substantial trait coordination occurred between leaves and roots, but the strength varied between growth forms and clades.
Our analyses provide evidence that the integration of roots and leaves in the PES requires better accounting of the variation in traits across phylogenetic clades. Inclusion of phylogenetic information provides a powerful framework for predictions of belowground functional traits at global scales.
Characterizing plant functional diversity is essential to decipher community assembly rules and ecosystem functioning. Most studies focused on above‐ground traits whereas the analysis of root ...diversity lags far behind. We analyzed the structure of fine root (< 2 mm) diameter distribution at the community level as an indicator of root morphological diversity, and hypothesized that the shape of the distribution provide insights on root types and root exploration strategies. We tested this hypothesis along a successional gradient (6–69 year‐old) with yearly mowing to better understand assembly rules regarding to belowground processes, and explored the relations between the parameters describing its modes and the vegetation composition and ecological properties of plant communities. Most communities showed a multimodal distribution, with two main modes corresponding to absorptive roots (thinner root mode) and transport roots (coarser root mode), and a third mode of lower importance corresponding to large transport roots. In early succession, the thinnest root mode was prominent, reflecting the dominance of thin absorptive roots and a low proportion of transport roots, resulting in a low root morphological diversity. As succession proceeds, the relative proportion of the second mode increased, and the proportions of the two main modes were more balanced, resulting in an increased variance and root morphological diversity. Furthermore, the first root mode (absorptive roots) became wider and shifted from very thin to thicker roots, suggesting the coexistence of various root strategies for resource exploration. Yearly mowing did not affect root diameter distribution, which may relate to the relative low mowing pressure that enabled woody species to remain, with stunted stature. Overall, our study demonstrates that the distribution of fine root diameters sheds light on root morphological diversity at the community level, and provide hints on the co‐existence of root types and strategies for resource use and exploration.
• Background and Aims Fine root decomposition is an important determinant of nutrient and carbon cycling in grasslands; however, little is known about the factors controlling root decomposition among ...species. Our aim was to investigate whether interspecific variation in the potential decomposition rate of fine roots could be accounted for by root chemical and morphological traits, life history and taxonomic affiliation. We also investigated the co-ordinated variation in root and leaf traits and potential decomposition rates. • Methods We analysed potential decomposition rates and the chemical and morphological traits of fine roots on 18 Mediterranean herbaceous species grown in controlled conditions. The results were compared with those obtained for leaves in a previous study conducted on similar species. • Key Results Differences in the potential decomposition rates of fine roots between species were accounted for by root chemical composition, but not by morphological traits. The root potential decomposition rate varied with taxonomy, but not with life history. Poaceae, with high cellulose concentration and low concentrations of soluble compounds and phosphorus, decomposed more slowly than Asteraceae and Fabaceae. Patterns of root traits, including decomposition rate, mirrored those of leaf traits, resulting in a similar species clustering. • Conclusions The highly co-ordinated variation of roots and leaves in terms of traits and potential decomposition rate suggests that changes in the functional composition of communities in response to anthropogenic changes will strongly affect biogeochemical cycles at the ecosystem level.
1. Ecosystem functioning relies heavily on below-ground processes, which are largely regulated by plant fine-roots and their functional traits. However, our knowledge of fine-root trait distribution ...relies to date on local- and regional-scale studies with limited numbers of species, growth forms and environmental variation. 2. We compiled a world-wide fine-root trait dataset, featuring 1115 species from contrasting climatic areas, phylogeny and growth forms to test a series of hypotheses pertaining to the influence of plant functional types, soil and climate variables, and the degree of manipulation of plant growing conditions on species fine-root trait variation. Most particularly, we tested the competing hypotheses that fine-root traits typical of faster return on investment would be most strongly associated with conditions of limiting versus favourable soil resource availability. We accounted for both data source and species phylogenetic relatedness. 3. We demonstrate that: (i) Climate conditions promoting soil fertility relate negatively to fine-root traits favouring fast soil resource acquisition, with a particularly strong positive effect of temperature on fine-root diameter and negative effect on specific root length (SRL), and a negative effect of rainfall on root nitrogen concentration; (ii) Soil bulk density strongly influences species fine-root morphology, by favouring thicker, denser fine-roots; (iii) Fine-roots from herbaceous species are on average finer and have higher SRL than those of woody species, and N2-fixing capacity positively relates to root nitrogen; and (iv) Plants growing in pots have higher SRL than those grown in the field. 4. Synthesis. This study reveals both the large variation in fine-root traits encountered globally and the relevance of several key plant functional types and soil and climate variables for explaining a substantial part of this variation. Climate, particularly temperature, and plant functional types were the two strongest predictors of fine-root trait variation. High trait variation occurred at local scales, suggesting that wide-ranging below-ground resource economics strategies are viable within most climatic areas and soil conditions.
There is a fundamental trade‐off between leaf traits associated with either resource acquisition or resource conservation. This gradient of trait variation, called the economics spectrum, also ...applies to fine roots, but whether it is consistent for coarse roots or at the plant community level remains untested. We measured a set of morphological and chemical root traits at a community level (functional parameters; FP) in 20 plant communities located along land‐use intensity gradients and across three climatic zones (tropical, mediterranean and montane). We hypothesized (i) the existence of a root economics spectrum in plant communities consistent within root types (fine, < 2 mm; coarse, 2–5 mm), (ii) that variations in root FP occur with soil depths (top 20 cm of soil and 100–150 cm deep) and (iii) along land‐use gradients. Root FP covaried, in line with the resource acquisition–conservation trade‐off, from communities with root FP associated with resource acquisition (e.g. high specific root length, SRL; thin diameters and low root dry matter contents, RDMC) to root FP associated with resource conservation (e.g. low SRL, thick diameters and high RDMC). This pattern was consistent for both fine and coarse roots indicating a strong consistency of a trade‐off between resource acquisition and conservation for plant roots. Roots had different suites of traits at different depths, suggesting a disparity in root function and exploitation capacities. Shallow, fine roots were thinner, richer in nitrogen and with lower lignin concentrations associated with greater exploitation capacities compared to deep, fine roots. Shallow, coarse roots were richer in nitrogen, carbon and soluble concentrations than deep, coarse roots. Fine root parameters of highly disturbed, herbaceous‐dominated plant communities in poorer soils were associated with foraging strategies, that is greater SRL and lower RDMC and lignin concentration than those from less disturbed communities. Coarse roots, however, were less sensitive to the land‐use gradient. Synthesis. This study demonstrates the existence of a general trade‐off in root construction at a community level, which operates within all root types, suggesting that all plant tissues are controlled by the trade‐off between resource acquisition and conservation.
Plant economics run on carbon and nutrients instead of money. Leaf strategies aboveground span an economic spectrum from "live fast and die young" to "slow and steady," but the economy defined by ...root strategies belowground remains unclear. Here, we take a holistic view of the belowground economy and show that root-mycorrhizal collaboration can short circuit a one-dimensional economic spectrum, providing an entire space of economic possibilities. Root trait data from 1810 species across the globe confirm a classical fast-slow "conservation" gradient but show that most variation is explained by an orthogonal "collaboration" gradient, ranging from "do-it-yourself" resource uptake to "outsourcing" of resource uptake to mycorrhizal fungi. This broadened "root economics space" provides a solid foundation for predictive understanding of belowground responses to changing environmental conditions.
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