Ecologists are increasingly adopting trait-based approaches to understand how community change influences ecosystem processes. However, most of this research has focussed on aboveground plant traits, whereas it is becoming clear that root traits are important drivers of many ecosystem processes, such as carbon (C) and nutrient cycling, and the formation and structural stability of soil. Here, we synthesise emerging evidence that illustrates how root traits impact ecosystem processes, and propose a pathway to unravel the complex roles of root traits in driving ecosystem processes and their response to global change. Finally, we identify research challenges and novel technologies to address them.

Extramatrical mycelia (EMM) of ectomycorrhizal (ECM) fungi are potentially extensive in soil and receive significant allocations of plant-derived carbon. Although losses from living EMM occur via respiration and exudation, EMM represents a considerable biomass component and potential carbon sink in many forest soils. ECM root tips and rhizomorphs may persist in soil for many months, but interactions between grazing arthropods and decomposers probably facilitate more rapid turnover of diffuse EMM. Elevated atmospheric CO2 concentration [CO2] is likely to increase carbon allocation to ECM fungi by their tree hosts. This will probably increase root colonization by ECM fungi and drive changes in their communities in soil. The likely effects of elevated [CO2] and other climate change factors on the production and turnover of EMM production are difficult to predict from current evidence, and this hampers our understanding of their potential value as future carbon sinks. Responses of grazing soil arthropods to future climate change will have a strong influence on EMM turnover, along with the abilities of ECM fungi to store carbon in below-ground, and this should be seen as a priority area for future research.

I.II.III.IV.V.VI.VII. SummaryThe rhizosphere priming effect (RPE) is a mechanism by which plants interact with soil functions. The large impact of the RPE on soil organic matter decomposition rates (from 50% reduction to 380% increase) warrants similar attention to that being paid to climatic controls on ecosystem functions. Furthermore, global increases in atmospheric CO2 concentration and surface temperature can significantly alter the RPE. Our analysis using a game theoretic model suggests that the RPE may have resulted from an evolutionarily stable mutualistic association between plants and rhizosphere microbes. Through model simulations based on microbial physiology, we demonstrate that a shift in microbial metabolic response to different substrate inputs from plants is a plausible mechanism leading to positive or negative RPEs. In a case study of the Duke Free-Air CO2 Enrichment experiment, performance of the PhotoCent model was significantly improved by including an RPE-induced 40% increase in soil organic matter decomposition rate for the elevated CO2 treatment demonstrating the value of incorporating the RPE into future ecosystem models. Overall, the RPE is emerging as a crucial mechanism in terrestrial ecosystems, which awaits substantial research and model development.

Nitrogen (N) and phosphorus (P) availability regulate plant productivity throughout the terrestrial biosphere, influencing the patterns and magnitude of net primary production (NPP) by land plants both now and into the future. These nutrients enter ecosystems via geologic and atmospheric pathways and are recycled to varying degrees through the plant-soil-microbe system via organic matter decay processes. However, the proportion of global NPP that can be attributed to new nutrient inputs versus recycled nutrients is unresolved, as are the large-scale patterns of variation across terrestrial ecosystems. Here, we combined satellite imagery, biogeochemical modeling, and empirical observations to identify previously unrecognized patterns of new versus recycled nutrient (N and P) productivity on land. Our analysis points to tropical forests as a hotspot of new NPP fueled by new N (accounting for 45% of total new NPP globally), much higher than previous estimates from temperate and high-latitude regions. The large fraction of tropical forest NPP resulting from new N is driven by the high capacity for N fixation, although this varies considerably within this diverse biome; N deposition explains a much smaller proportion of new NPP. By contrast, the contribution of new N to primary productivity is lower outside the tropics, and worldwide, new P inputs are uniformly low relative to plant demands. These results imply that new N inputs have the greatest capacity to fuel additional NPP by terrestrial plants, whereas low P availability may ultimately constrain NPP across much of the terrestrial biosphere.

A quick,convenient and robust method is presented for measuring the effective diffusion coefficients of non-sorbing solutes in soil. The method estimates the effective diffusion coefficient from a measured diffusion profile by optimizing the solution of a numerical simulation model describing the experimental system. The method was used to measure the effective diffusion coefficients of compounds found in root exudates.

The story of this “Rhizosphere book project” startedabout 3 years ago, as the three of us were discussingthe organization of the “International Rhizosphere 2Conference” held in Montpellier in 2007 (Hartmannet al. 2008a; Jones and Hinsinger 2008).

Rhizosphere priming is the change in decomposition of soil organic matter (SOM) caused by root activity. Rhizosphere priming plays a crucial role in soil carbon (C) dynamics and their response to global climate change. Rhizosphere priming may be affected by soil nutrient availability, but rhizosphere priming itself can also affect nutrient supply to plants. These interactive effects may be of particular relevance in understanding the sustained increase in plant growth and nutrient supply in response to a rise in atmospheric CO2concentration. We examined how these interactions were affected by elevated CO2in two similar semiarid grassland field studies. We found that an increase in rhizosphere priming enhanced the release of nitrogen (N) through decomposition of a larger fraction of SOM in one study, but not in the other. We postulate that rhizosphere priming may enhance N supply to plants in systems that are N limited, but that rhizosphere priming may not occur in systems that are phosphorus (P) limited. Under P limitation, rhizodeposition may be used for mobilization of P, rather than for decomposition of SOM. Therefore, with increasing atmospheric CO2concentrations, rhizosphere priming may play a larger role in affecting C sequestration in N poor than in P poor soils.

Plant roots release a wide range of chemicals into soils. This process, termed root exudation, is thought to increase the activity of microbes and the exoenzymes they synthesize, leading to accelerated rates of carbon (C) mineralization and nutrient cycling in rhizosphere soils relative to bulk soils. The nitrogen (N) content of microbial biomass and exoenzymes may introduce a stoichiometric constraint on the ability of microbes to effectively utilize the root exudates, particularly if the exudates are rich in C but low in N. We combined a theoretical model of microbial activity with an exudation experiment to test the hypothesis that the ability of soil microbes to utilize root exudates for the synthesis of additional biomass and exoenzymes is constrained by N availability. The field experiment simulated exudation by automatically pumping solutions of chemicals often found in root exudates ("exudate mimics") containing C alone or C in combination with N (C : N ratio of 10) through microlysimeter "root simulators" into intact forest soils in two 50-day experiments. The delivery of C-only exudate mimics increased microbial respiration but had no effect on microbial biomass or exoenzyme activities. By contrast, experimental delivery of exudate mimics containing both C and N significantly increased microbial respiration, microbial biomass, and the activity of exoenzymes that decompose low molecular weight components of soil organic matter (SOM, e.g., cellulose, amino sugars), while decreasing the activity of exoenzymes that degrade high molecular weight SOM (e.g., polyphenols, lignin). The modeling results were consistent with the experiments; simulated delivery of C-only exudates induced microbial N-limitation, which constrained the synthesis of microbial biomass and exoenzymes. Exuding N as well as C alleviated this stoichiometric constraint in the model, allowing for increased exoenzyme production, the priming of decomposition, and a net release of N from SOM (i.e., mineralization). The quantity of N released from SOM in the model simulations was, under most circumstances, in excess of the N in the exudate pulse, suggesting that the exudation of N-containing compounds can be a viable strategy for plant-N acquisition via a priming effect. The experimental and modeling results were consistent with our hypothesis that N-containing compounds in root exudates affect rhizosphere processes by providing substrates for the synthesis of N-rich microbial biomass and exoenzymes. This study suggests that exudate stoichiometry is an important and underappreciated driver of microbial activity in rhizosphere soils.

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...

Abstract While there is an emerging view that roots and their associated microbes actively alter resource availability and soil organic matter (SOM) decomposition, the ecosystem consequences of such rhizosphere effects have rarely been quantified. Using a meta-analysis, we show that multiple indices of microbially mediated C and nitrogen (N) cycling, including SOM decomposition, are significantly enhanced in the rhizospheres of diverse vegetation types. Then, using a numerical model that combines rhizosphere effect sizes with fine root morphology and depth distributions, we show that root-accelerated mineralization and priming can account for up to one-third of the total C and N mineralized in temperate forest soils. Finally, using a stoichiometrically constrained microbial decomposition model, we show that these effects can be induced by relatively modest fluxes of root-derived C, on the order of 4% and 6% of gross and net primary production, respectively. Collectively, our results indicate that rhizosphere processes are a widespread, quantitatively important driver of SOM decomposition and nutrient release at the ecosystem scale, with potential consequences for global C stocks and vegetation feedbacks to climate.

090004 Fine roots constitute a large and dynamic component of the carbon cycles of terrestrial ecosystems. The reported fivefold discrepancy in turnover estimates between median longevity (ML) from minirhizotrons and mean residence time (MRT) using carbon isotopes may have global consequences. 090004 Here, a root branch order-based model and a simulated factorial experiment were used to examine four sources of error. 090004 Inherent differences between ML, a number-based measure, and MRT, a mass-based measure, and the inability of the MRT method to account for multiple replacements of rapidly cycling roots were the two sources of error that contributed more to the disparity than did the improper choice of root age distribution models and sampling bias. Sensitivity analysis showed that the rate at which root longevity increases as order increases was the most important factor influencing the disparity between ML and MRT. 090004 Assessing root populations for each branch order may substantially reduce the errors in longevity estimates of the fine root guild. Our results point to the need to acquire longevity estimates of different orders, particularly those of higher orders.

Plant–microbe interactions actively control nitrogen (N) cycling in the ecosystem. We hypothesize that the investment into exudation and the coupling of plant–microbe N cycling will be larger in competitive plants compared to the more conservative species. Root exudation of competitive (Glyceria maxima) and conservative (Carex acuta) plants was estimated by 13C-CO2 labeling. Seasonal changes... [Show full abstract]

61Plant–microbe interactions are mediated by root exudation of different secondary metabolites.61Flavonoides and strigolactones exudation serve as signal molecules for symbiosis establishment.61Root exudates contain antimicrobial compounds to cope with pathogens.61Border cells protect root apex from pathogens and parasitic nematodes.61Nutrients from root exudates are assimilated by root associated bacteria.

The rhizosphere differs from the bulk soil in a range of biochemical, chemical and physical processes that occur as a consequence of root growth, water and nutrient uptake, respiration and rhizodeposition. These processes also affect microbial ecology and plant physiology to a considerable extent. This review concentrates on two features of this unique environment: rhizosphere geometry and heterogeneity in both space and time. Although it is often depicted as a soil cylinder of a given radius around the root, drawing a boundary between the rhizosphere and bulk soil is an impossible task because rhizosphere processes result in gradients of different sizes. For instance, because of diffusional constraints, root uptake can result in a depletion zone extending <1 mm for phosphate to several centimetres for nitrate, while respiration may affect the bulk of the soil. Rhizosphere processes are responsible for spatial and temporal heterogeneities in the soil, although these are sometimes difficult to distinguish from intrinsic soil heterogeneity. A further complexity is that these processes are regulated by plants, microbial communities and soil constituents, and their many interactions. Novel in situ techniques and modelling will help in providing a holistic view of rhizosphere functioning, which is a prerequisite for its management and manipulation.

Background: Recent advances in imaging techniques now make it possible to visualize the biogeochemical and physical environment around the roots, the rhizosphere. Detailed images of pore space geometry and water content dynamics around roots have demonstrated the heterogeneity of the rhizosphere compared with the soil far from the roots. These findings have inspired new models of root water... [Show full abstract]

Plants associate—analogous to animals or us humans—with a multitude of microorganisms, which collectively function as a microbiome. A major discovery of the last decade is that numerous organisms of a microbiome (aka microbiota) are not unpretentious background actors. Instead, some microbiota members influence host processes including behavior, appetite, and health in animals (1) and... [Show full abstract]

Despite decades of research progress, ecologists are still debating which pools and fluxes provide nitrogen (N) to plants and soil microbes across different ecosystems. Depolymerization of soil...

Multiple lines of existing evidence suggest that climate change enhances root exudation of organic compounds into soils. Recent experimental studies show that increased exudate inputs may cause a net loss of soil carbon. This stimulation of microbial carbon mineralization (`priming’) is commonly rationalized by the assumption that exudates provide a readily bioavailable supply of energy for the decomposition of native soil carbon (co-metabolism). Here we show that an alternate mechanism can cause carbon loss of equal or greater magnitude. We find that a common root exudate, oxalic acid, promotes carbon loss by liberating organic compounds from protective associations with minerals. By enhancing microbial access to previously mineral-protected compounds, this indirect mechanism accelerated carbon loss more than simply increasing the supply of energetically more favourable substrates. Our results provide insights into the coupled biotic-abiotic mechanisms underlying the `priming’ phenomenon and challenge the assumption that mineral-associated carbon is protected from microbial cycling over millennial timescales.

Soil organic matter is extensively humified; some fractions existing for more than 1000 years. The soil microbial biomass is surrounded by about 50 times its mass of soil organic matter, but can only metabolize it very slowly. Paradoxically, even if more than 90% of the soil microbial biomass is killed, the mineralization of soil organic matter proceeds at the same rate as in an unperturbed soil. Here we show that soil organic matter mineralization is independent of microbial biomass size, community structure or specific activity. We suggest that the rate limiting step is governed by abiological processes (which we term the Regulatory Gate hypothesis), which convert non-bioavailable soil organic matter into bioavailable soil organic matter, and cannot be affected by the microbial population. This work challenges one of the long held theories in soil microbiology proposed by Winogradsky, of the existence of autochthonous and zymogenous microbial populations. This has significant implications for our understanding of carbon mineralization in soils and the role of soil micro-organisms in the global carbon cycle. Here we describe experiments designed to determine if the Regulatory Gate operates. We conclude that there is sufficient experimental evidence for it to be offered as a working hypothesis.

Forest trees compete for light and soil resources, but photoassimilates, once produced in the foliage, are not considered to be exchanged between individuals. Applying stable carbon isotope labeling at the canopy scale, we show that carbon assimilated by 40-meter-tall spruce is traded over to neighboring beech, larch, and pine via overlapping root spheres. Isotope mixing signals indicate that the interspecific, bidirectional transfer, assisted by common ectomycorrhiza networks, accounted for 40% of the fine root carbon (about 280 kilograms per hectare per year tree-to-tree transfer). Although competition for resources is commonly considered as the dominant tree-to-tree interaction in forests, trees may interact in more complex ways, including substantial carbon exchange.

Abstract Contents 1597 I. 1597 II. 1597 III. 1598 IV. 1598 V. 1600 VI. 1601 VII. 1601 VIII. 1601 1602 References 1602 SUMMARY: Trait-based approaches have led to significant advances in plant ecology, but are currently biased toward above-ground traits. It is becoming clear that a stronger emphasis on below-ground traits is needed to better predict future changes in plant biodiversity and their consequences for ecosystem functioning. Here I propose six 'below-ground frontiers' in trait-based plant ecology, with an emphasis on traits governing soil nutrient acquisition: redefining fine roots; quantifying root trait dimensionality; integrating mycorrhizas; broadening the suite of root traits; determining linkages between root traits and abiotic and biotic factors; and understanding ecosystem-level consequences of root traits. Focusing research efforts along these frontiers should help to fulfil the promise of trait-based ecology: enhanced predictive capacity across ecological scales. 0008 2016 The Authors. New Phytologist 0008 2016 New Phytologist Trust.

Aims Nitrogen supply and atmospheric CO2 concentration ([CO2]) could influence root exudates directly by altering compound concentrations in roots and indirectly by regulating root morphology. This...

Plants form beneficial associations with arbuscular mycorrhizal fungi, which facilitate nutrient acquisition from the soil. In return, the fungi receive organic carbon from the plants. The transcription factor RAM1 (REQUIRED FOR ARBUSCULAR MYCORRHIZATION 1) is crucial for this symbiosis, and we demonstrate that it is required and sufficient for the induction of a lipid biosynthetic pathway that is expressed in plant cells accommodating fungal arbuscules. Lipids are transferred from the plant to mycorrhizal fungi, which are fatty acid auxotrophs, and this lipid export requires the glycerol-3-phosphate acyltransferase RAM2, a direct target of RAM1. Our work shows that in addition to sugars, lipids are a major source of organic carbon delivered to the fungus, and this is necessary for the production of fungal lipids.

Abstract This corrects the article DOI: 10.1038/nature25783.

plants expressing stable membrane-anchored ratiometric fluorescent sensors based on pHluorin. These sensors enabled noninvasive pH-specific measurements in mature root cells from the medium-epidermis interface up to the inner cell layers that lie beyond the Casparian strip. The membrane-associated apoplastic pH was much more alkaline than the overall apoplastic space pH. Proton concentration associated with the plasma membrane was very stable, even when the growth medium pH was altered. This is in apparent contradiction with the direct connection between root intercellular space and the external medium. The plasma membrane-associated pH in the stele was the most preserved and displayed the lowest apoplastic pH (6.0 to 6.1) and the highest transmembrane delta pH (1.5 to 2.2). Both pH values also correlated well with optimal activities of channels and transporters involved in ion uptake and redistribution from the root to the aerial part. In growth medium where ionic content is minimized, the root plasma membrane-associated pH was more affected by environmental proton changes, especially for the most external cell layers. Calcium concentration appears to play a major role in apoplastic pH under these restrictive conditions, supporting a role for the cell wall in pH homeostasis of the unstirred surface layer of plasma membrane in mature roots.

Abstract Fine roots acquire essential soil resources and mediate biogeochemical cycling in terrestrial ecosystems. Estimates of carbon and nutrient allocation to build and maintain these structures remain uncertain because of the challenges of consistently measuring and interpreting fine-root systems. Traditionally, fine roots have been defined as all roots 0909¤ 2 mm in diameter, yet it is now recognized that this approach fails to capture the diversity of form and function observed among fine-root orders. Here, we demonstrate how order-based and functional classification frameworks improve our understanding of dynamic root processes in ecosystems dominated by perennial plants. In these frameworks, fine roots are either separated into individual root orders or functionally defined into a shorter-lived absorptive pool and a longer-lived transport fine-root pool. Using these frameworks, we estimate that fine-root production and turnover represent 22% of terrestrial net primary production globally - a c. 30% reduction from previous estimates assuming a single fine-root pool. Future work developing tools to rapidly differentiate functional fine-root classes, explicit incorporation of mycorrhizal fungi into fine-root studies, and wider adoption of a two-pool approach to model fine roots provide opportunities to better understand below-ground processes in the terrestrial biosphere. 0008 2015 The Authors. New Phytologist 0008 2015 New Phytologist Trust.

Summary While multiple experiments have demonstrated that trees exposed to elevated CO2 can stimulate microbes to release nutrients from soil organic matter, the importance of root- versus mycorrhizal-induced changes in soil processes are presently unknown. We analyzed the contribution of roots and mycorrhizal activities to carbon (C) and nitrogen (N) turnover in a loblolly pine ( Pinus taeda ) forest exposed to elevated CO2 by measuring extracellular enzyme activities at soil microsites accessed via root windows. Specifically, we quantified enzyme activity from soil adjacent to root tips (rhizosphere), soil adjacent to hyphal tips (hyphosphere), and bulk soil. During the peak growing season, CO2 enrichment induced a greater increase of N-releasing enzymes in the rhizosphere (215% increase) than in the hyphosphere (36% increase), but a greater increase of recalcitrant C-degrading enzymes in the hyphosphere (118%) than in the rhizosphere (19%). Nitrogen fertilization influenced the magnitude of CO2 effects on enzyme activities in the rhizosphere only. At the ecosystem scale, the rhizosphere accounted for c . 50% and 40% of the total activity of N- and C-releasing enzymes, respectively. Collectively, our results suggest that root exudates may contribute more to accelerated N cycling under elevated CO2 at this site, while mycorrhizal fungi may contribute more to soil C degradation.

Summary Plant roots, their associated microbial community and free-living soil microbes interact to regulate the movement of carbon from the soil to the atmosphere, one of the most important and least understood fluxes of terrestrial carbon. Our inadequate understanding of how plant–microbial interactions alter soil carbon decomposition may lead to poor model predictions of terrestrial carbon feedbacks to the atmosphere. Roots, mycorrhizal fungi and free-living soil microbes can alter soil carbon decomposition through exudation of carbon into soil. Exudates of simple carbon compounds can increase microbial activity because microbes are typically carbon limited. When both roots and mycorrhizal fungi are present in the soil, they may additively increase carbon decomposition. However, when mycorrhizas are isolated from roots, they may limit soil carbon decomposition by competing with free-living decomposers for resources. We manipulated the access of roots and mycorrhizal fungi to soil in02situ in a temperate mixed deciduous forest. We added 13C-labelled substrate to trace metabolized carbon in respiration and measured carbon-degrading microbial extracellular enzyme activity and soil carbon pools. We used our data in a mechanistic soil carbon decomposition model to simulate and compare the effects of root and mycorrhizal fungal presence on soil carbon dynamics over longer time periods. Contrary to what we predicted, root and mycorrhizal biomass did not interact to additively increase microbial activity and soil carbon degradation. The metabolism of 13C-labelled starch was highest when root biomass was high and mycorrhizal biomass was low. These results suggest that mycorrhizas may negatively interact with the free-living microbial community to influence soil carbon dynamics, a hypothesis supported by our enzyme results. Our steady-state model simulations suggested that root presence increased mineral-associated and particulate organic carbon pools, while mycorrhizal fungal presence had a greater influence on particulate than mineral-associated organic carbon pools. Synthesis . Our results suggest that the activity of enzymes involved in organic matter decomposition was contingent upon root–mycorrhizal–microbial interactions. Using our experimental data in a decomposition simulation model, we show that root–mycorrhizal–microbial interactions may have longer-term legacy effects on soil carbon sequestration. Overall, our study suggests that roots stimulate microbial activity in the short term, but contribute to soil carbon storage over longer periods of time.

Key message The quantity of root exudate carbon produced by Quercus crispula Blume was strongly influenced by the amount of solar radiation 102day before collection.

This review summarizes and discusses methodological approaches for studies on the impact of plant roots on the surrounding rhizosphere and for elucidation of the related mechanisms, covering a range from simple model experiments up to the field scale. A section on rhizosphere sampling describes tools and culture systems employed for analysis of root growth, root morphology, vitality testing and for monitoring of root activity with respect to nutrient uptake, water, ion and carbon flows in the rhizosphere. The second section on rhizosphere probing covers techniques to detect physicochemical changes in the rhizosphere as a consequence of root activity. This comprises compartment systems to obtain rhizosphere samples, visualisation techniques, reporter gene approaches and remote sensing technologies for monitoring the conditions in the rhizosphere. Approaches for the experimental manipulation of the rhizosphere by use of molecular and genetic methods as tools to study rhizosphere processes are discussed in a third section. Finally it is concluded that in spite of a wide array of methodological approaches developed in the recent past for studying processes and interactions in the rhizosphere mainly under simplified conditions in model experiments, there is still an obvious lack of methods to test the relevance of these findings under real field conditions or even on the scale of ecosystems. This also limits reliable data input and validation in current rhizosphere modelling approaches. Possible interactions between different environmental factors or plantmicrobial interactions (e.g. mycorrhizae) are frequently not considered in model experiments. Moreover, most of the available knowledge arises from investigations with a very limited number of plant species, mainly crops and studies considering also intraspecific genotypic differences or the variability within wild plant species are just emerging.

Accurate information about the quantity, quality and spatiotemporal dynamics of metabolite release from plant roots is vital to understanding the functional significance of root exudates in biogeochemical processes occurring at the root-microbe-soil-interface. Significant progress in analytical techniques nowadays allows us to gain a much better picture of the rich diversity of compounds that are present in root exudates, but ultimately the choice of exudation sampling strategy will determine the ecological significance of obtained exudation results. Unfortunately, in the past, little consideration has been given to the experimental strategy used to sample root exudates. To date, our knowledge on root exudation is mainly based on plants grown and sampled in nutrient solution culture (hydroponics). Despite the operational benefit of hydroponic systems, the question remains as to how ecologically relevant exudation results obtained under these artificial conditions are compared to soil environments, particularly in the context of exudate driven rhizosphere processes. The quantitative and qualitative measurement of root exudation in soil, however, is fraught with problems due to: (i) continual removal of exudates from solution by the microbial community; (ii) loss of exudates from solution due to their sorption to the solid phase; and (iii) simultaneous release of compounds from soil organic matter breakdown. While a perfect method for sampling root exudates does not exist, soil based approaches, if appropriately applied and interpreted, may still provide more realistic insights into exudation dynamics in natural soil environments. This review aims to provide an overview of different root exudation sampling approaches and their advantages and limitations to support the selection of the most suitable experimental procedure for any specific research question. We address critical methodological aspects that need to be considered in the choice of experimental approach, like growth and sampling medium (soil, hydroponic), sterility, sampling location (whole root system, individual root segments) as well as plant age, daytime, re-uptake of metabolites affecting duration and timing of the sampling event and data presentation. In addition, we summarize the main analytical approaches to analyze root exudates, ranging from liquid sample analysis to isotope tracking and imaging techniques.

Plant functional traits are the features (morphological, physiological, phenological) that represent ecological strategies and determine how plants respond to environmental factors, affect other trophic levels and influence ecosystem properties. Variation in plant functional traits, and trait syndromes, has proven useful for tackling many important ecological questions at a range of scales, giving rise to a demand for standardised ways to measure ecologically meaningful plant traits. This line of research has been among the most fruitful avenues for understanding ecological and evolutionary patterns and processes. It also has the potential both to build a predictive set of local, regional and global relationships between plants and environment and to quantify a wide range of natural and human-driven processes, including changes in biodiversity, the impacts of species invasions, alterations in biogeochemical processes and vegetation-atmosphere interactions. The importance of these topics dictates the urgent need for more and better data, and increases the value of standardised protocols for quantifying trait variation of different species, in particular for traits with power to predict plant-and ecosystem-level processes, and for traits that can be measured relatively easily. Updated and expanded from the widely used previous version, this handbook retains the focus on clearly presented, widely applicable, step-by-step recipes, with a minimum of text on theory, and not only includes updated methods for the traits previously covered, but also introduces many new protocols for further traits. This new handbook has a better balance between whole-plant traits, leaf traits, root and stem traits and regenerative traits, and puts particular emphasis on traits important for predicting species' effects on key ecosystem properties. We hope this new handbook becomes a standard companion in local and global efforts to learn about the responses and impacts of different plant species with respect to environmental changes in the present, past and future.

1. Soluble root exudates are notoriously difficult to collect in non-hydroponic systems because they are released in a narrow zone around roots and are rapidly assimilated by rhizosphere microbes. This has substantially limited our understanding of their rates of release and chemical composition in situ , and by extension, their ecological significance. 2. Here we describe the advantages and limitations of several commonly employed methods for measuring exudation with respect to their potential adaptability for field use in forest ecosystems. Then, we introduce a novel in situ method for measuring exudation in forest soils, and present preliminary results of the spatial and temporal dynamics of loblolly pine ( Pinus taeda L.) exudation at the Duke Forest FACTS-1 site, North Carolina, USA from April 2007 to July 2008. 3. Exudation rates varied by an order of magnitude, with the highest rates occurring in late-June 2007 and mid-July 2008, and the lowest rates occurring during late-August 2007. On an annual basis, we estimate pine roots in the upper 15 cm of soil release c. 9 g C m 0908082 year 0908081 via this flux, which represents 10900092% of net primary productivity at the site. 4. The magnitude of exudation rates did not differ across an N availability gradient but did track general patterns of below-ground C allocation at the site. Exudation was well-predicted by root morphological characteristics such as surface area and the number of root and mycorrhizal tips, further supporting a possible link between root C allocation and exudation. 5. Because all methods for estimating exudates introduce experimental artefacts, we suggest that only a limited amount of ecologically relevant information is probably gleaned from a single method. Thus, a complementary suite of experimental approaches will best enable researchers to understand consequences of changing patterns of exudation in the wake of global environmental change.

The degree to which rising atmospheric CO(2) will be offset by carbon (C) sequestration in forests depends in part on the capacity of trees and soil microbes to make physiological adjustments that can alleviate resource limitation. Here, we show for the first time that mature trees exposed to CO(2) enrichment increase the release of soluble C from roots to soil, and that such increases are coupled to the accelerated turnover of nitrogen (N) pools in the rhizosphere. Over the course of 3 years, we measured in situ rates of root exudation from 420 intact loblolly pine (Pinus taeda L.) roots. Trees fumigated with elevated CO(2) (200 p.p.m.v. over background) increased exudation rates ( g C cm(-1) root h(-1) ) by 55% during the primary growing season, leading to a 50% annual increase in dissolved organic inputs to fumigated forest soils. These increases in root-derived C were positively correlated with microbial release of extracellular enzymes involved in breakdown of organic N (R(2) = 0.66; P = 0.006) in the rhizosphere, indicating that exudation stimulated microbial activity and accelerated the rate of soil organic matter (SOM) turnover. In support of this conclusion, trees exposed to both elevated CO(2) and N fertilization did not increase exudation rates and had reduced enzyme activities in the rhizosphere. Collectively, our results provide field-based empirical support suggesting that sustained growth responses of forests to elevated CO(2) in low fertility soils are maintained by enhanced rates of microbial activity and N cycling fuelled by inputs of root-derived C. To the extent that increases in exudation also stimulate SOM decomposition, such changes may prevent soil C accumulation in forest ecosystems.

The relationships of monoterpene emission with temperature, light, photosynthesis and stomatal conductance (gs) were studied in Quercus ilex L. trees throughout the four annual seasons under field conditions. The highest monoterpene emission was measured in spring and summer (midday average of 11 g [g DW]611 h611), whereas the lowest rates were found in autumn and winter (midday averages of... [Show full abstract]

The fine roots of trees are concentrated on lateral branches that arise from perennial roots. They are important in the acquisition of water and essential nutrients, and at the ecosystem level, they make a significant contribution to biogeochemical cycling. Fine roots have often been studied according to arbitrary size classes, e.g., all roots less than 1 or 2 mm in diameter. Because of the size class approach, the position of an individual root on the complex lateral branching system has often been ignored, and relationships between the form of the branching root system and its function are poorly understood. The fine roots of both gymnosperms and angiosperms, which formed ectomycorrhizae (EM) and arbuscular mycorrhizae (AM) fungal associations, were sampled in 1998 and 1999. Study sites were chosen to encompass a wide variety of environments in four regions of North America. Intact lateral branches were collected from each species and 18561 individual roots were dissected by order, with distal roots numbered as first-order roots. This scheme is similar to the one commonly used to number the order of streams. Fine root diameter, length, specific root length (SRL; m/g), and nitrogen (N) concentration of nine North American tree species (Acer saccharum, Juniperus monosperma, Liriodendron tulipifera, Picea glauca, Pinus edulis, Pinus elliottii, Pinus resinosa, Populus balsamifera, and Quercus alba) were then compared and contrasted. Lateral roots <0.5 mm in diameter accounted for >75% of the total number and length of individual roots sampled in all species except Liriodendron tulipifera. Both SRL and N concentration decreased with increasing root order in all nine species, and this pattern appears to be universal in all temperate and boreal trees. Nitrogen concentrations ranged from 8.5 to 30.9 g/kg and were highest in the first-order "root tips." On a mass basis, first-order roots are expensive to maintain per unit time (high tissue N concentration). Tissue N appears to be a key factor in understanding the C cost of maintaining first- and second-order roots, which dominate the display of absorbing root length. There were many significant differences among species in diameter, length, SRL, and N concentration. For example, two different species can have similar SRL but very different tissue N concentrations. Our findings run contrary to the common idea that all roots of a given size class function the same way and that a common size class for fine roots works well for all species. Interestingly, fine root lateral branches are apparently deciduous, with a distinct lateral branch scar. The position of an individual root on the branching root system appears to be important in understanding the function of fine roots.

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 ≤02202mm) 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, lignin02:02nitrogen 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.

Here we report on low molecular weight organic acids in root exudates and soil solutions of Norway spruce and silver birch grown in rhizoboxes, sterile microcosms and the field. Monocarboxylic acids dominated in all three experimental systems. Formic, shikimic and oxalic acids were found in both spruce and birch microcosms. Fumaric acid was exclusive for spruce, while lactic, malonic, butyric and phthalic acids were only found in the birch microcosms. In spruce rhizoboxes oxalic, lactic, formic, butyric and pthalic acids were found. In addition, citric, adipic, propionic, succinic and acetic acids were observed in the rhizosphere of birch. Behind root windows in the field, only oxalic and lactic acids were found in the rhizosphere of spruce fine roots, whereas also formic and phthalic were observed close to birch fine roots, all at low concentrations. The rhizosphere of mycorrhizal short roots of birch contained butyric acid along with the acids observed for birch fine roots. Our results emphasise that characteristics of both the trees e.g. species, developmental stage, root density, mycorrhizal status, and the experimental system, i.e. growth conditions are important for the composition and the amount of organic acids. We conclude that the rhizosphere of birch contains more organic acids at higher concentrations than spruce.

Climate and land-use changes modify plant rooting depth, signifying that organic matter with long residence times in deep soil layers can be exposed to rhizospheres and associated microbial activities. The presence of roots in soils stimulates mineralization of native soil C, via a process termed the rhizosphere priming effect (RPE), which may in consequence lead to loss of soil C. By growing a deep rooting grass, Festuca arundinacea, on soil columns and under continuous dual labelling (13C- & 14C-CO2), we show that root penetration up to 8062cm into a soil profile stimulated mineralization of 6515,000 year-old soil C. The RPE, after normalization with root biomass, was similar along the soil profile indicating that deep C is as vulnerable to priming as surface C. The RPE was strongly correlated with respiration of plant-derived C, and a PLFA marker representative of saprophytic fungi (18:2046c) across all soil layers. Moreover, experimental disruption of soil structure further stimulated soil C mineralization. These findings suggest that the slow soil C mineralization in deep layers results from an impoverishment of energy-rich plant C for microorganisms (especially for saprophytic fungi), combined with a physical disconnection between soil C and microorganisms. Based on our results, we anticipate higher mineralization rates of deep millennia-old SOM in response to deeper root penetration which could be induced by changes in agricultural practices and climate.

The rhizosphere is a biologically active zone of the soil around plant roots that contains soil-borne microbes including bacteria and fungi. Plant–microbe interactions in the rhizosphere can be beneficial to the plant, the microbes or to neither of them. One of the major difficulties that plant biologists and microbiologists face when studying these interactions is that many groups of microbes that inhabit this zone are not cultivable in the laboratory. Recent developments in molecular biology methods are shedding some light on rhizospheric microbial diversity. This review discusses recent findings and future challenges in the study of plant–microbe interactions in the rhizosphere.

土壤激发效应是指由各种有机物质添加等处理所引起的土壤有机质周转强烈的短期改变.根际是激发效应最主要也是最重要的发生部位.根际激发效应能够反映生态系统土壤碳氮周转的速度,并影响植物、土壤微生物等对养分的获取和竞争,维持生态系统各组分间的养分平衡.虽然对根际激发效应的产生机制已取得一定程度的认知,但是对根际激发效应在土壤碳氮转化过程中的作用机理及其生态重要性依然缺乏足够的理解.该文在论述激发效应的研究历史和主要发生部位的基础上对最新研究进展进行了综合分析,提出了一个具体的根际激发效应的发生机制,深入剖析了影响根际激发效应的生物与非生物因素,并阐释了根际激发效应的生态重要性,对未来根际激发效应的研究方向进行了展望.

土壤激发效应是指由各种有机物质添加等处理所引起的土壤有机质周转强烈的短期改变.根际是激发效应最主要也是最重要的发生部位.根际激发效应能够反映生态系统土壤碳氮周转的速度,并影响植物、土壤微生物等对养分的获取和竞争,维持生态系统各组分间的养分平衡.虽然对根际激发效应的产生机制已取得一定程度的认知,但是对根际激发效应在土壤碳氮转化过程中的作用机理及其生态重要性依然缺乏足够的理解.该文在论述激发效应的研究历史和主要发生部位的基础上对最新研究进展进行了综合分析,提出了一个具体的根际激发效应的发生机制,深入剖析了影响根际激发效应的生物与非生物因素,并阐释了根际激发效应的生态重要性,对未来根际激发效应的研究方向进行了展望.

Summary Microbial nitrification in soils is a major contributor to nitrogen (N) loss in agricultural systems. Some plants can secrete organic substances that act as biological nitrification inhibitors (BNIs), and a small number of BNIs have been identified and characterized. However, virtually no research has focused on the important food crop, rice ( Oryza sativa ). Here, 19 rice varieties were explored for BNI potential on the key nitrifying bacterium Nitrosomonas europaea . Exudates from both indica and japonica genotypes were found to possess strong BNI potential. Older seedlings had higher BNI abilities than younger ones; Zhongjiu25 (ZJ25) and Wuyunjing7 (WYJ7) were the most effective genotypes among indica and japonica varieties, respectively. A new nitrification inhibitor, 1,9-decanediol, was identified, shown to block the ammonia monooxygenase (AMO) pathway of ammonia oxidation and to possess an 80% effective dose (ED80) of 9002ng02μl611. Plant N-use efficiency (NUE) was determined using a 15N-labeling method. Correlation analyses indicated that both BNI abilities and 1,9-decanediol amounts of root exudates were positively correlated with plant ammonium-use efficiency and ammonium preference. These findings provide important new insights into the plant–bacterial interactions involved in the soil N cycle, and improve our understanding of the BNI capacity of rice in the context of02NUE.

Abstract The heterogeneous responses of soil organic carbon (SOC) decomposition in different soil fractions to nitrogen (N) addition remain elusive. In this study, turnover rates of SOC in different aggregate fractions were quantified based on changes in 13 C following the conversion of C 3 to C 4 vegetation in a temperate agroecosystem. The turnover of both total organic matter and specific organic compound classes within each aggregate fraction was inhibited by N addition. Moreover, the intensity of inhibition increases with decreasing aggregate size and increasing N addition level, but does not vary among chemical compound classes within each aggregate fraction. Overall, the response of SOC decomposition to N addition is dependent on the physico-chemical protection of SOC by aggregates and minerals, rather than the biochemical composition of organic substrates. The results of this study could help to understand the fate of SOC in the context of increasing N deposition. Copyright 2017 Elsevier B.V. All rights reserved.

Abstract Contents Summary I. 'Introduction' II. 'The return on investment approach' III. 'CO 2 response spectrum' IV. 'Discussion' 'Acknowledgements' References Summary Land ecosystems sequester on average about a quarter of anthropogenic CO 2 emissions. It has been proposed that nitrogen (N) availability will exert an increasingly limiting effect on plants ability to store additional carbon (C) under rising CO 2 , but these mechanisms are not well understood. Here, we review findings from elevated CO 2 experiments using a plant economics framework, highlighting how ecosystem responses to elevated CO 2 may depend on the costs and benefits of plant interactions with mycorrhizal fungi and symbiotic N-fixing microbes. We found that N-acquisition efficiency is positively correlated with leaf-level photosynthetic capacity and plant growth, and negatively with soil C storage. Plants that associate with ectomycorrhizal fungi and N-fixers may acquire N at a lower cost than plants associated with arbuscular mycorrhizal fungi. However, the additional growth in ectomycorrhizal plants is partly offset by decreases in soil C pools via priming. Collectively, our results indicate that predictive models aimed at quantifying C cycle feedbacks to global change may be improved by treating N as a resource that can be acquired by plants in exchange for energy, with different costs depending on plant interactions with microbial symbionts.

61Specific root exudation decreased in the subsoil to less than a fifth.61Root morphology changed from fibrous-type roots in the topsoil to pioneer-type roots in the subsoil.61Root exudation rate was positively related to EOC and ETN in the topsoil.61Exudation was particularly low in subsoil poor in SOM where positive priming effects are unlikely.

Metabolomics is becoming accepted as a tool for the (untargeted) analysis of metabolites in root exudates. The importance of the rhizosphere microbiome for the functioning of plants is becoming increasingly clear. Statistical analysis and genetic mapping are used to establish relations between metabolites and (other) traits. The increased understanding of metabolic engineering in plants will allow the critical assessment of the role of individual compounds in plant hizosphere communication. Novel chemical-analytical platforms, such as laser-assisted electrospray ionisation (LAESI), will allow for spatial metabolomics on the scale of roots and interacting organisms.

A substantial portion of the carbon (C) fixed by the trees is allocated belowground to ectomycorrhizal (EM) symbionts, but this fraction usually declines after fertilization. The aim of the present study was to estimate the effect of optimal fertilization (including all the necessary nutrients) on the growth of EM fungi in young Norway spruce forests over a three year period. In addition, the amount of carbon sequestered by EM mycelia was estimated using a method based on the difference in δ 13C between C 3 and C 4 plants. Sand-filled ingrowth mesh bags were used to estimate EM growth, and similar bags amended with compost made from maize leaves (a C 4 plant) were used to estimate C sequestration. Fertilizers had been applied either every year or every second year since 2002 and the estimates of EM growth started in 2007. The application of fertilizer reduced EM growth to between 0% and 40% of the growth in the control plots at one site (Ebbeg01rde), while no significant effect was found at the other three sites studied. The effect of the fertilizer was similar in sand-filled and maize-compost-amended mesh bags, but the total production of EM fungi was 3–4 times higher in maize-compost-amended mesh bags. The fertilizer tended to reduce EM growth more when applied every year than when applied every second year. The amount of C sequestered in maize-compost-amended mesh bags collected from unfertilized treatments was estimated to be between 0.2 and 0.7 mg C g sand 611 at Ebbeg01rde and between 0.2 and 0.5 mg C g sand 611 at Gr01ngshammar. This corresponds to between 300 and 1100 kg C per ha, assuming a similar production in the soil as in the mesh bags. Fertilization at the Ebbeg01rde site reduced carbon sequestration, which confirmed the results based on estimates of fungal growth (ergosterol levels). A correlation was found between fungal biomass and δ 13C in mesh bags amended with maize compost. Based on this, it was estimated that a fungal production of 1 μg ergosterol corresponded to 0.33 mg of sequestered carbon. In conclusion, the effect of the fertilizer on EM growth seemed to be dependent on the effect of the fertilizer on tree growth. Thus, at Ebbeg01rde, were tree growth was less stimulated by the fertilizer, EM growth was reduced upon fertilization. At other sites, where tree growth was more stimulated, the fertilizer did not influence EM growth. The large amounts of carbon sequestered during the experiment may be a result of fungal residues remaining in the soil after the death of the hyphae.

Mycorrhizal fungi constitute a considerable sink for carbon in most ecosystems. This carbon is used for building extensive mycelial networks in the soil as well as for metabolic activity related to nutrient uptake. A number of methods have been developed recently to quantify production, standing biomass and turnover of extramatrical mycorrhizal mycelia (EMM) in the field. These methods include minirhizotrons, in-growth mesh bags and cores, and indirect measurements of EMM based on classification of ectomycorrhizal fungi into exploration types. Here we review the state of the art of this methodology and discuss how it can be developed and applied most effectively in the field, Furthermore, we also discuss different ways to quantify fungal biomass based on biomarkers such as chitin, ergosterol and PLFAs, as well as molecular methods, such as qPCR. The evidence thus far indicates that mycorrhizal fungi are key components of microbial biomass in many ecosystems. We highlight the need to extend the application of current methods to focus on a greater range of habitats and mycorrhizal types enabling incorporation of mycorrhizal fungal biomass and turnover into biogeochemical cycling models. (C) 2012 Elsevier Ltd. All rights reserved.

Abstract Background & aims Some metabolites (e.g. amino acids) present in root exudates can be taken up by roots, but we do not know if this ability extends to the broader suite of metabolites found in exudates. The aim of this study was to examine the ability of wheat (Triticum aestivum L.) to efflux and take up a broad suite of small metabolites. Methods Four-week-old plants were placed into an uptake solution that contained a broad suite of 13C-labelled metabolites. Uptake was estimated from the decrease in concentration of the 13C isotopologue in the solution; and from the appearance of 13C isotopologues within roots. Efflux was estimated from appearance of 12C isotopologues in solution. Results When wheat plants were placed in 13C-metabolite solutions the concentration of U-13C isotopologues of 37 metabolites decreased as would be expected if plants were taking up the U-13C metabolites. After 4 h immersion in 13C metabolite solution, roots contained detectable amounts of U-13C isotopologues of 55 metabolites. U-13C isotopologues of organic acids were not detected within roots. Conclusions These findings indicate that wheat can take up a broad suite of N-containing metabolites and some sugars, but there was no evidence for uptake of organic acids.

根系分泌物是植物与土壤进行物质交换和信息传递的重要载体物质,是植物响应外界胁迫的重要途径,是构成植物不同根际微生态特征的关键因素,也是根际对话的主要调控者。根系分泌物对于生物地球化学循环、根际生态过程调控、植物生长发育等均具有重要功能,尤其是在调控根际微生态系统结构与功能方面发挥着重要作用,调节着植物-植物、植物-微生物、微生物-微生物间复杂的互作过程。植物化感作用、作物间套作、生物修复、生物入侵等都是现代农业生态学的研究热点,它们都涉及十分复杂的根际生物学过程。越来越多的研究表明,不论是同种植物还是不同种植物之间相互作用的正效应或是负效应,都是由根系分泌物介导下的植物与特异微生物共同作用的结果。近年来,随着现代生物技术的不断完善,有关土壤这一"黑箱"的研究方法与技术取得了长足的进步,尤其是各种宏组学技术(meta-omics technology),如环境宏基因组学、宏转录组学、宏蛋白组学、宏代谢组学等的问世,极大地推进了人们对土壤生物世界的认知,尤其是对植物地下部生物多样性和功能多样性的深层次剖析,根际生物学特性的研究成果被广泛运用于指导生产实践。深入系统地研究根系分泌物介导下的植物-土壤-微生物的相互作用方式与机理,对揭示土壤微生态系统功能、定向调控植物根际生物学过程、促进农业生产可持续发展等具有重要的指导意义。该文综述了根系分泌物的概念、组成及功能,论述了根系分泌物介导下植物与细菌、真菌、土壤动物群之间的密切关系,总结了探索根际生物学特性的各种研究技术及其优缺点,并对该领域未来的研究方向进行了展望。

根系分泌物是植物与土壤进行物质交换和信息传递的重要载体物质,是植物响应外界胁迫的重要途径,是构成植物不同根际微生态特征的关键因素,也是根际对话的主要调控者。根系分泌物对于生物地球化学循环、根际生态过程调控、植物生长发育等均具有重要功能,尤其是在调控根际微生态系统结构与功能方面发挥着重要作用,调节着植物-植物、植物-微生物、微生物-微生物间复杂的互作过程。植物化感作用、作物间套作、生物修复、生物入侵等都是现代农业生态学的研究热点,它们都涉及十分复杂的根际生物学过程。越来越多的研究表明,不论是同种植物还是不同种植物之间相互作用的正效应或是负效应,都是由根系分泌物介导下的植物与特异微生物共同作用的结果。近年来,随着现代生物技术的不断完善,有关土壤这一"黑箱"的研究方法与技术取得了长足的进步,尤其是各种宏组学技术(meta-omics technology),如环境宏基因组学、宏转录组学、宏蛋白组学、宏代谢组学等的问世,极大地推进了人们对土壤生物世界的认知,尤其是对植物地下部生物多样性和功能多样性的深层次剖析,根际生物学特性的研究成果被广泛运用于指导生产实践。深入系统地研究根系分泌物介导下的植物-土壤-微生物的相互作用方式与机理,对揭示土壤微生态系统功能、定向调控植物根际生物学过程、促进农业生产可持续发展等具有重要的指导意义。该文综述了根系分泌物的概念、组成及功能,论述了根系分泌物介导下植物与细菌、真菌、土壤动物群之间的密切关系,总结了探索根际生物学特性的各种研究技术及其优缺点,并对该领域未来的研究方向进行了展望。

Carbohydrates are good source of drugs and play important roles in metabolism processes and cellular interactions in organisms. Distinguishing monosaccharide isomers in saccharide derivates is an...

Understanding soil organic matter (SOM) decomposition and its interaction with rhizosphere processes is a crucial topic in soil biology and ecology. Using a natural 13C tracer method to separately measure SOM-derived CO2 from root-derived CO2, this study aims to connect the level of rhizosphere-dependent SOM decomposition with the C and N balance of the whole plant–soil system, and to... [Show full abstract]

Despite the perceived importance of exudation to forest ecosystem function, few studies have attempted to examine the effects of elevated temperature and nutrition availability on the rates of root exudation and associated microbial processes. In this study, we performed an experiment in which in situ exudates were collected from Picea asperata seedlings that were transplanted in disturbed soils exposed to two levels of temperature (ambient temperature and infrared heater warming) and two nitrogen levels (unfertilized and 25 g N m612 a611). Here, we show that the trees exposed to an elevated temperature increased their exudation rates I (μg C g611 root biomass h611), II (μg C cm611 root length h611) and III (μg C cm612 root area h611) in the unfertilized plots. The altered morphological and physiological traits of the roots exposed to experimental warming could be responsible for this variation in root exudation. Moreover, these increases in root-derived C were positively correlated with the microbial release of extracellular enzymes involved in the breakdown of organic N (R2 = 0.790; P = 0.038), which was coupled with stimulated microbial activity and accelerated N transformations in the unfertilized soils. In contrast, the trees exposed to both experimental warming and N fertilization did not show increased exudation rates or soil enzyme activity, indicating that the stimulatory effects of experimental warming on root exudation depend on soil fertility. Collectively, our results provide preliminary evidence that an increase in the release of root exudates into the soil may be an important physiological adjustment by which the sustained growth responses of plants to experimental warming may be maintained via enhanced soil microbial activity and soil N transformation. Accordingly, the underlying mechanisms by which plant root-microbe interactions influence soil organic matter decomposition and N cycling should be incorporated into climate-carbon cycle models to determine reliable estimates of long-term C storage in forests.

Root exudation is increasingly being recognized as an important driver of ecosystem processes; however, few studies have examined the degree to which variations in exudate stoichiometry and soil resources affect microbial controls of nutrient availability and decomposition. We added root exudate mimics of varying chemical quality to soils collected from two adjacent forest stands (one a 70 year-old spruce plantation, the other a 200 year-old spruce-fir forest) that differ strongly in N availability. The exudate treatments were added for 50 consecutive days, and included water (control), C alone, N alone, and three combinations of C and N that varied stoichiometrically (i.e., C:N ratio of 10, 50 and 100). Exudate additions containing little or no N promoted the greatest losses of soil C in two soils, with the greatest losses occurring in the moderately labile (i.e., acid-extractable) fraction of the low N plantation soils. However, despite the uniformity of priming effects between sites ( 7% loss of soil C for both), there was little congruence in exudate-induced effects on microbial biomass and activity. In the plantation soils, exudates generally increased microbial biomass (especially fungi), accelerated N cycling and increased lignin-degrading enzyme activities relative to controls. In contrast, exudate additions to spruce-fir soils mostly decreased microbial biomass, decelerated N cycling, and had variable impacts on lignin-degrading enzyme activities (decreased phenol oxidase but increased peroxidase). Collectively, this study suggests that while root exudates with low C and N have the potential to accelerate soil C losses by stimulating microbes to mine N from soil organic matter, the consequences of exudate inputs on nutrient fluxes are less predictable, and may hinge on the recalcitrance of (soil organic matter) SOM, N availability and microbial communities.

61Mycorrhizal type influences annual exudation rates in hardwood forests.61The magnitude of rhizosphere effects is positively correlated to exudation rates.61Modest fluxes of carbon can disproportionately affect ecosystem nitrogen cycling.

Root exudates can accelerate nitrogen (N) cycling by stimulating the decomposition of soil organic matter (SOM); however, it remains unclear how inputs of individual exudate components affect the biotic and abiotic processes that drive the transformation and release of inorganic N. In a well-controlled rhizosphere system, we added two exudate chemicals (i.e., glucose and oxalic acid) through artificial roots to soils collected from two forests (an 70-year-old spruce plantation and an 200-year-old spruce-fir forest) over a period of 50 days. The results showed that oxalic acid significantly accelerated both N mineralization and availability, which are mechanisms involved in abiotic process that disrupt previously protected mineral-organic associations (e.g., metal-organic complexes (MOCs) and short-range order phases (SROs)) and biotic process that accelerate microbial processes. Glucose also enhanced N decomposition, but this enhancement was significantly smaller than that obtained with oxalic acid, presumably because glucose addition increased the formation of mineral-organic associations of MOCs and SROs, thereby limiting microbial access. In addition, the preferential utilization of glucose by microbes can also lead to lower N decomposition. Our results also showed that the component-induced changes in N mineralization were lower in the established spruce plantation (e.g., from 45% to 89%) than in the spruce-fir forest (e.g., from 105% to 150%), potentially due to the decresed biotic processes observed after exudate additions in the spruce plantation compared with the spruce-fir forest. The decreased changes in N mineralization in the spruce plantation were also related to component-induced abiotic processes. These observations suggested that oxalic acid and glucose differentially impact rhizosphere N transformation in forest soils, but the impacts are mediated by soil type.

简要介绍了根系分泌物的种类、数量;综合论述了根系分泌物的分泌 特性、生理及分子生物学基础和根系特定分泌物对土壤养分有效性的影响;讨论了根系分泌物的影响因素和根系分泌的部位及根系分泌物在土壤中扩散的范围.并阐 明了根系分泌物与根际养分的活化及吸收、异株克生的关系,以及与土壤固氮微生物及连作障碍的关系.

简要介绍了根系分泌物的种类、数量;综合论述了根系分泌物的分泌 特性、生理及分子生物学基础和根系特定分泌物对土壤养分有效性的影响;讨论了根系分泌物的影响因素和根系分泌的部位及根系分泌物在土壤中扩散的范围.并阐 明了根系分泌物与根际养分的活化及吸收、异株克生的关系,以及与土壤固氮微生物及连作障碍的关系.

本文简述了根际研究的历史进程及根际微生态学的发展概况,讨论了根际微生态系统的基本概念和理论模型,初步构建了其理论框架,明确了其结构、边界、特征、研究范畴及内容,提出了令后个体、群体水平根际微生态系统理论研究的思路和重点.

本文简述了根际研究的历史进程及根际微生态学的发展概况,讨论了根际微生态系统的基本概念和理论模型,初步构建了其理论框架,明确了其结构、边界、特征、研究范畴及内容,提出了令后个体、群体水平根际微生态系统理论研究的思路和重点.

While multiple lines of evidence suggests that root carbon (C) can significantly impact the soil organic C (SOC) pool and subsequent C cycles via rhizosphere priming effects, the relative magnitude of the effects that root- and fungal-derived C inputs have in driving the priming of SOC decomposition are currently unknown. In this study, we used ingrowth cores and stable C isotope analyses to quantify root- and mycelium-derived C sequestered in soil labile and recalcitrant C pools and their relative contributions to the decomposition of native SOC in a spruce-fir-dominated coniferous forest on the eastern Tibetan Plateau, China. The results showed that new C sequestered in the soil labile C pool was primarily (77%) from mycelium-derived C, while 60% of the root-derived C sequestered in the soil was incorporated into the recalcitrant pool. Furthermore, although the total native SOC pool was not significantly influenced by new C derived from both roots and mycelia, mycelium-derived C induced a remarkably greater negative priming effect ( 12.0%) on the native labile C pool than did root-derived C ( 5.8%); in contrast, mycelium-derived C induced a greater positive priming effect (13.8%) than root-derived C (7.1%) on the native recalcitrant C pool. Collectively, our findings suggest that mycelium-derived C make a greater contribution to the newly sequestered C in the soil labile C pool than root-derived C, thereby inducing a remarkably greater positive priming effect on the decomposition of native soil recalcitrant C. Therefore, mycelium-derived C inputs may play a dominant role in soil C dynamics and long-term C storage, at least in alpine forest ecosystems where ectomycorrhizal mutualisms dominate.

The phenomenon that rhizosphere processes significantly control soil organic matter (SOM) decomposition, also termed rhizosphere priming effect (RPE), is now increasingly recognized as significant as the effects of soil temperature and moisture on SOM decomposition. However, the exact mechanisms responsible for RPE remain largely unknown. Particularly, some reports have suggested that the quality of rhizodeposits may play a significant role in causing different levels of RPE among various plant species. However, direct evidence for the “rhizodeposit quality hypothesis” has been lacking. Here we tested the hypothesis by investigating RPE on soil carbon (C) and nitrogen (N) mineralization of two soybean (Glycine max L. Merr.) isolines differing only in their ability to form nodules and to fix N2, and thus differing in tissue N concentration and rhizodeposit quality. We used a continuous 13C-labeling method for measuring RPE on soil organic C decomposition, and employed an N-budgeting method for quantifying RPE on soil net N mineralization. We found that the rhizodeposits from nodulated soybean produced a stronger RPE (53% vs. 26%) on soil organic C decomposition than the rhizodeposits from non-nodulated soybean at the maturity stage when nodulated soybean had significantly higher plant tissue N concentration but similar plant biomass, while both soybean isolines produced a similar RPE (33–34%) at the vegetative stage when there was no difference in plant tissue N concentration or plant biomass. The levels of RPE on soil net N mineralization were similar between the two isolines, ranging from 25% at the vegetative stage to 38–46% at the maturity stage. Moreover, RPE on soil organic C decomposition was not linearly proportional to RPE on soil net N mineralization. These results indicate that higher rhizodeposit quality is one of the most likely causes to the higher RPE of the nodulated soybean compared to the non-nodulated soybean. Further investigations of rhizodeposit quality and quantity between the two soybean isolines are warranted to further test this rhizodeposit quality hypothesis.

61Living roots increased soil C decomposition by 27–245%.61Living roots enhanced soil gross N mineralization by up to 62%.61Living roots led to higher microbial biomass and extracellular enzyme activity.61Results supported the microbial activation hypothesis for rhizosphere priming effect.61Rhizosphere priming was correlated with root biomass and rhizosphere respiration.