Root-soil Interface Research Articles

Fine root distribution and morphological characteristics of switchgrass under different row spacings on semi-arid Loess Plateau, China

ABSTRACT To change row spacing is a common agricultural practice to adjust plant density to enhance plant growth and development. Here we examined the fine root (≤1.0 mm in diameter) distribution and morphological characteristics of switchgrass (Panicum virgatum L.) in the 7th growth year under three row spacing settings (20 cm, 40 cm and 60 cm) on a terrace field in the semi-arid Loess Plateau region. Roots from 0 to 150 cm soil layers were utilized to evaluate the root biomass, distribution and morphological traits in April and October 2016. Results showed that fine root biomass (FRB) mainly distributed in the 0–40 cm soil layer, and it was higher under the 20 cm and 40 cm than that under the 60 cm row spacing treatment. FRB in October was lower than in April under all three row spacings. Compared with 20 cm and 60 cm row spacing, root surface area, root length density and specific root length under 40 cm row spacing were higher, while root diameter was smaller. These conferred switchgrass with larger soil exploration and root-soil interface. It implied that switchgrass grown under 40 cm row spacing had stronger ability on exploiting soil resources in the semi-arid area.

Living with Salt

Living with Salt

Root System Architecture, Copper Uptake and Tissue Distribution in Soybean (Glycine max (L.) Merr.) Grown in Copper Oxide Nanoparticle (CuONP)-Amended Soil and Implications for Human Nutrition.

Understanding the potential uptake and biodistribution of engineered nanoparticles (ENPs) in soil-grown plants is imperative for realistic toxicity and risk assessment considering the oral intake of edibles by humans. Herein, growing N-fixing symbiont (Bradyrhizobium japonicum) inoculated soybean (Glycine max (L.) Merr.) for a full lifecycle of 120 days, we assessed the potential influence of particle size (25, 50, and 250 nm) and concentration (0, 50, 100, 200, and 500 mg/kg soil) of Copper oxide nanoparticles (CuONPs) on: (1) root system architecture, (2) soil physicochemical attributes at the soil–root interface, and (3) Cu transport and accumulation in root, stem, leaf, and seed in soybean, and compared them with the soluble Cu2+ ions and water-only controls. Finally, we performed a comparative assessment of total seed Cu levels in soybean with other valuable food sources for Cu intake and discussed potential human health implications. Results showed particle size- and concentration-dependent influence of CuONPs on Cu uptake and distribution in root, stem, leaf, and seed. Alterations in root architecture (root biomass, length, volume, and area) were dependent on the Cu compound types, Cu concentrations, and their interactions. Concentration–response relationships for all three sizes of CuONPs and Cu2+ ions were found to be linear. Furthermore, CuONPs and Cu2+ ions had inhibitory effects on root growth and development. Overall, soybean responses to the smallest size of CuONPs–25 nm—were greater for all parameters tested compared to the two larger-sized CuONPs (50 nm, 250 nm) or Cu2+ ions. Results suggest that minor changes in soil-root physicochemical attributes may not be a major driver for Cu uptake in soybean. Cu bioaccumulation followed the order: root > leaf > stem > seed. Despite reduction in root architecture and seed yield, the smallest size CuONPs–25 nm led to increased total seed Cu uptake compared to the larger-sized CuONPs or Cu2+ ions. Our findings also suggest that soil amendment with CuONPs, and more so with the smallest size of CuONPs–25 nm—could significantly improve seed nutritional Cu value in soybean as reflected by the % Daily Values (DV) and are rated “Good” to “Very Good” according to the “World’s Healthiest Foods” rating. However, until the potential toxicity and risk from CuONP-fortified soybean seed ingestion is characterized in humans, we caution recommending such seeds for daily human consumption when addressing food Cu-deficiency and associated diseases, globally.

Physiological effects of mature tree transplanting characterize the roles of the soil-root interface in the field

The intimate connection between roots and soil particles is a prerequisite for the continuous flow of water and nutrients from soil to trees, and carbon flow from roots to the rhizosphere. The soil-root interface has been studied in multiple laboratory and greenhouse experiments. Yet its multiple roles and their differential contributions to tree health have rarely been experimentally studied on mature trees in the field.We took advantage of mature olive tree transplanting to test the physiological effects of breakage of the soil-root interface in situ. Eight olive trees were monitored following transplantation into a site located 4 km from their native site, along two years. Additional eight trees were monitored simultaneously at the native site, as control. To decrease mortality risk, transplanted trees were heavily pruned before transplanting, and were irrigated and fertilized in their new site.Transplanted trees had ~50% lower rates of leaf photosynthesis and transpiration; ~80% lower root starch content; and ~30% higher loss of xylem conductivity, than native trees. Leaf water potential (LWP) was similar across trees, becoming more negative in the transplanted trees only in the second year following transplanting. While starch content and xylem conductivity recovered in the second year, leaf gas exchange and LWP remained significantly lower than in native trees. Soil P and K were higher under transplanted trees that remained stressed than under trees that recovered.Breakage of the soil-root interface caused a multi-system stress to trees. The lack of persistent LWP response might indicate that loss of hydraulic conductivity was driven by root, rather than aboveground, embolisms. Degradation of starch in the roots indicates an increase in belowground sinks. In the long run, recovery of starch content means no carbon limitation; yet the prevailing effects on leaf activities suggest a long-term stress unrelated with water or carbon supply.

Understanding the Impact of Cultivar, Seed Origin, and Substrate on Bacterial Diversity of the Sugar Beet Rhizosphere and Suppression of Soil-Borne Pathogens.

The rhizosphere microbiome is crucial for plant health, especially for preventing roots from being infected by soil-borne pathogens. Microbiota-mediated pathogen response in the soil-root interface may hold the key for microbiome-based control strategies of phytopathogens. We studied the pathosystem sugar beet—late sugar beet root rot caused by Rhizoctonia solani in an integrative design of combining in vitro and in vivo (greenhouse and field) trials. We used five different cultivars originating from two propagation sites (France, Italy) with different degrees of susceptibility towards R. solani (two susceptible, one moderately tolerant and two cultivars with partial resistance). Analyzing bacterial communities in seeds and roots grown under different conditions by 16S rRNA amplicon sequencing, we found site-, cultivar-, and microhabitat-specific amplicon sequences variants (ASV) as well as a seed core microbiome shared between all sugar beet cultivars (121 ASVs representing 80%–91% relative abundance). In general, cultivar-specific differences in the bacterial communities were more pronounced in seeds than in roots. Seeds of Rhizoctonia-tolerant cultivars contain a higher relative abundance of the genera Paenibacillus, Kosakonia, and Enterobacter, while Gaiellales, Rhizobiales, and Kosakonia were enhanced in responsive rhizospheres. These results indicate a correlation between bacterial seed endophytes and Rhizoctonia-tolerant cultivars. Root communities are mainly substrate-derived but also comprise taxa exclusively derived from seeds. Interestingly, the signature of Pseudomonas poae Re*1-1-14, a well-studied sugar-beet specific biocontrol agent, was frequently found and in higher relative abundances in Rhizoctonia-tolerant than in susceptible cultivars. For microbiome management, we introduced microbial inoculants (consortia) and microbiome transplants (vermicompost) in greenhouse and field trials; both can modulate the rhizosphere and mediate tolerance towards late sugar beet root rot. Both, seeds and soil, provide specific beneficial bacteria for rhizosphere assembly and microbiota-mediated pathogen tolerance. This can be translated into microbiome management strategies for plant and ecosystem health.

Co-occurring increased phosphatase activity and labile P depletion in the rhizosphere of Lupinus angustifolius assessed with a novel, combined 2D-imaging approach

Innovative non-destructive 2D imaging methods such as zymography and diffusive gradients in thin films (DGT) have been developed recently to assess the distribution of phosphatase activity and labile solutes at the root-soil interface. We report on the first combination of these techniques for the spatial distribution and potential interaction of labile phosphorus (P) and associated acid phosphatase activity in the rhizosphere of blue lupin (Lupinus angustifolius) in two contrasting soils (‘Sand’ and ‘Loam’). Zymography, based on 4-methylumbelliferylphosphate (MU-P) for acid phosphatase activity mapping, and DGT gels capable of binding labile P were deployed to individual root axes to visualise P mobilisation and depletion in the rhizosphere of blue lupin grown in rhizotrons in glasshouse conditions for 45 days. Acid phosphatase activity was evidently higher in the rhizosphere and co-occurred with P-depletion zones around the roots in both soils. Lateral root profiles showed that elevated acid phosphatase activity as well as P-depletion extended up to 2 mm from the root centre into the rhizosphere. Despite larger total and organic P pools in the Loam, P was less plant available (DGT labile P) than in the Sand. However, phosphatase activities in the rhizosphere and plant P contents were similar in both soils. These results indicate that enzyme-catalysed hydrolysis of organic P was limited in the Loam, due to low P availability, hence explaining similar P contents in lupins from both soils. P did not accumulate in labile pools in the rhizosphere indicating that P supply via phosphatase could not compensate the plants demand for P resulting in P depletion zones. While our work demonstrates the applicability of combined zymography and DGT deployments to study rhizosphere processes at microscale, we also discuss limitations and perspectives of this approach.

A hyperaccumulator plant Sedum alfredii recruits Cd/Zn-tolerant but not Pb-tolerant endospheric bacterial communities from its rhizospheric soil

For a metal hyperaccumulator plant Sedum alfredii, the recruitment of unique rhizospheric bacterial communities from bulk soils has been well studied. However, in the root-soil interface, the knowledge on the establishment of endospheric microbiomes from rhizospheric soil is still scarce. In this study, we combined culture-independent that was 16S rRNA gene amplicon sequencing, and culture-dependent methods that included bacterial isolation, heavy metal tolerance and plant growth-promoting traits. The Cd/Zn concentrations in endosphere were significantly higher than in soil, while Pb concentration in endosphere was significantly lower than in soil. The α-diversity in rhizosphere soils was higher than in root endosphere, and the compartments as a major determinant revealed 85.9% of the taxa variations. The relative abundance of Proteobacteria increased in endosphere compared to rhizosphere. The difference of Cd/Zn tolerance between endospheric and rhizospheric isolates was not obvious, while the Pb tolerance of endospheric isolates significantly decreased compared to rhizosphere. The results suggest that S. alfredii recruits Cd/Zn-tolerant but not Pb-tolerant endospheric bacterial communities from its rhizospheric soil. The difference in the microbial structure and function in the root-soil interface might be related to the selective absorption of metals in S. alfredii.

Rhizosphere Spatiotemporal Organization–A Key to Rhizosphere Functions

Resilience of soils, i.e., their ability to maintain functions or recover after disturbance, is closely linked to the root-soil interface, the soil's power house. However, the limited observability of key processes at the root-soil interface has so far limited our understanding of how such resilience emerges. Here, we hypothesize that resilience emerges from self-organized spatiotemporal patterns which are the result of complex and dynamic feedbacks between physical, chemical, and biological processes occurring in the rhizosphere. We propose that the combination of modern experimental and modeling techniques, with a focus on imaging approaches, allows for understanding the complex feedbacks between plant resource acquisition, microbiome-related plant health, soil carbon sequestration, and soil structure development. A prerequisite for the identification of patterns, underlying processes, and feedback loops is that joint experimental platforms are defined and investigated in their true 2D and 3D geometry along time. This applies across different scientific disciplines from soil physics/chemistry/microbiology to plant genomics/physiology and across different scales from the nano/microscopic scale of the root soil interface, over the radial profiles around single roots, up to the root architecture and plant scale. Thus, we can move beyond isolated reductionist approaches which have dominated in rhizosphere research so far.

Radial oxygen loss is correlated with nitrogen nutrition in mangroves.

The present study aimed to explore the possible functions of radial oxygen loss (ROL) on mangrove nutrition. A field survey was conducted to explore the relations among ROL, root anatomy and leaf N in different mangrove species along a continuous tidal gradient. Three mangroves with different ROL (Avicennia marina [A. marina], Kandelia obovata and Rhizophora stylosa) were then selected to further explore the dynamics of N at the root-soil interface. The results showed that seaward pioneer mangrove species such as A. marina appeared to exhibit higher leaf N despite growing under poorer nutrient conditions. Greater leaf N in pioneer mangroves coincided with their special root structure (e.g., high porosity together with a thin lignified/suberized exodermis) and powerful ROL. An interesting positive relation was observed between ROL and leaf N in mangroves. Moreover, rhizo-box data further showed that soil nitrification was also strongly correlated with ROL. A. marina, which had the highest ROL among the three mangrove species studied, consistently possessed the highest levels of NO3-, nitrification and ammonia-oxidizing bacteria and archaea gene copies in the rhizosphere. Besides, both NO3- and NH4+ influxes were found to be higher in the roots of A. marina when compared to those of K. obovata and R. stylosa. In summary, greater N acquisition by pioneer mangroves such as A. marina was strongly correlated with ROL which would regulate N transformation and translocation at the root-soil interface. The implications of this study may be significant in mangrove nutrition and the mechanisms involved in mangrove zonation.

Community structure and diversity of the microbiomes of two microhabitats at the root-soil interface: implications of meta-analysis of the root-zone soil and root endosphere microbial communities in Xiong'an New Area.

The diversity of the microbial compositions of the root-zone soil (the rhizosphere-surrounding soil) and root endosphere (all inner root tissues) of Pinus tabulaeformis Carr. and Ginkgo biloba L. were evaluated in Xiong'an New Area using high-throughput sequencing; the influence of the soil edaphic parameters on microbial community compositions was also evaluated. Our results showed that both the taxonomic and phylogenetic diversities of the root endosphere were lower than those of the root-zone soil, but the variation in the endosphere microbial community structure was remarkably higher than that of the root-zone soil. Spearman correlation analysis showed that the soil organic matter, total nitrogen, total phosphate, total potassium, ratio of carbon to nitrogen, and pH significantly explained the α-diversity of the bacterial community and that total nitrogen differentially contributed to the α-diversity of the fungal community. Variation partitioning analysis showed that plant species had a greater influence on microbial composition variations than did any other soil property, although soil chemical parameters explained more variation when integrated. Together, our results suggest that both plant species and soil chemical parameters played a critical role in shaping the microbial community composition.

Arsenic redox transformations and cycling in the rhizosphere of Pteris vittata and Pteris quadriaurita

Pteris vittata (PV) and Pteris quadriaurita (PQ) are reported to hyperaccumulate arsenic (As) when grown in As-rich soil. Yet, little is known about the impact of their unique As accumulation mechanisms on As transformations and cycling at the soil-root interface. Using a combined approach of two-dimensional (2D), sub-mm scale solute imaging of arsenite (AsIII), arsenate (AsV), phosphorus (P), manganese (Mn), iron (Fe) and oxygen (O2), we found localized patterns of AsIII/AsV redox transformations in the PV rhizosphere (AsIII/AsV ratio of 0.57) compared to bulk soil (AsIII/AsV ratio of ≤0.04). Our data indicate that the high As root uptake, translocation and accumulation from the As-rich experimental soil (2080 mg kg−1) to PV fronds (6986 mg kg−1) induced As detoxification via AsV reduction and AsIII root efflux, leading to AsIII accumulation and re-oxidation to AsV in the rhizosphere porewater. This As cycling mechanism is linked to the reduction of O2 and MnIII/IV (oxyhydr)oxides resulting in decreased O2 levels and increased Mn solubilization along roots. Compared to PV, we found 4-fold lower As translocation to PQ fronds (1611 mg kg−1), 2-fold lower AsV depletion in the PQ rhizosphere, and no AsIII efflux from PQ roots, suggesting that PQ efficiently controls As uptake to avoid toxic As levels in roots. Analysis of root exudates obtained from soil-grown PV showed that As acquisition by PV roots was not associated with phytic acid release. Our study demonstrates that two closely-related As-accumulating ferns have distinct mechanisms for As uptake modulating As cycling in As-rich environments.

An approach for broad molecular imaging of the root-soil interface via indirect matrix-assisted laser desorption/ionization mass spectrometry

Understanding rhizospheric processes is limited by the need for imaging complex molecular transformations at relevant spatial scales within the root soil continuum. Here, we demonstrate a method to enable this analysis by first extracting organic compounds from the rhizosphere onto a PVDF membrane while maintaining their 2D distribution and then imaging the distribution of chemical compounds using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). This approach permitted us to visualize and identify compounds on the root surface and presumed root exudates in the rhizosphere. Within a 1.8 cm × 0.6 cm sampling area of a switchgrass rhizosphere, we could observe at least four chemically distinct zones. Using high performance Fourier transform ion cyclotron resonance MS, we were able to accurately annotate numerous molecules co-localized to each of these zones.

Influence of Polyurethane Polymer on the Strength and Mechanical Behavior of Sand-root Composite

Vegetation has good application in slope stabilization, but its beneficial effects on reinforcing topsoil are generally limited by the soil properties it was cultivated in. This study aims at evaluating the strength improvements of sand-root composite by treating with polyurethane polymer and hence investigating the mechanism of polymer-root-soil interactions. Vegetation roots were selected and mixed with dry sand and polymer solution to prepare remolded specimens. A series of experimental tests were then performed at different percentages of root content (0, 0.4, 0.8, 1.2, and 1.6 % by weight of dry sand) and polymer content (1, 2, and 4% by weight of dry sand) to evaluate the shear parameters and unconfined compressive strength (UCS). The combined mechanism was studied by scanning electron microscopy (SEM) images. The results showed that the strengthening effect has greater efficiency with higher polymer content. Through varying contents of vegetation root, it was found that low root content induced an undesirable weakening effect on the strength of the treated soil. However, this situation was somewhat improved with the increase in root content. The good flexibility of polymers not only promote the capacity of soil to energy absorption, but also impart good ductility to soil. The presence of polymers greatly strengthens soil stability due to its special network structure, by which the improved shear resistance at the root-soil interface provides sufficient anchorage effect for the tensile strength of roots to be fully mobilized. Overall, the synergistic effect of root reinforcement and polymer treatment has the potential for its use in soil stabilization.

How root nutrient uptake affects rhizosphere pH: A modelling study

Rhizosphere pH is the consequence of numerous, complex processes and is a key factor of biogeochemistry at the soil-root interface. This work aimed at evaluating the impact of nutrient root absorption on rhizosphere pH using a mechanistic model formalizing major nutrient cation and anion transport, under the influence of concentration, electric potential gradients and of convection. The electrical neutrality of the system was maintained through a H+ root efflux or influx (or equivalent opposite OH− fluxes). The model outputs were compared to the rhizosphere pH of maize (Zea mays L.), ryegrass (Lolium perenne L.) and Alpine pennycress (Noccaea caerulescens (J. & C. Presl) F. K. Mey.) grown in three different soils, determined with planar optodes. A global sensitivity analysis was carried out in order to highlight the dominating factors. The composition of 37 soil solutions from the literature was used to predict rhizosphere pH. The model most of the time predicts the alkalinisation of the rhizosphere by up to +3.3 pH units, given that in aerobic soils, nitrate is the most absorbed nutrient and its internalization is equilibrated by H+ root absorption (or equivalent OH− excretion). This alkalinisation is close to that observed in the rhizosphere of ryegrass and Alpine pennycress. The rhizosphere pH predictions are very sensitive to the initial composition of the soil solution and quite insensitive to root absorption kinetics and soil buffer power for the various nutrients. Because it favours nitrate diffusion, water content is positively correlated with rhizosphere alkalinisation. Moreover, the lower the root density, the lower the effect of the root uptake on both ion concentrations and on rhizosphere pH. Convection has little influence on rhizosphere pH. This study confirms the prominent role of mineral nutrition in the determination of rhizosphere pH. The model therefore constitutes a tool for understanding and predicting rhizosphere pH.

Optical coherence tomography imaging of plant root growth in soil.

Complex interactions between roots and soil provide the nutrients and physical support required for robust plant growth. Yet, visualizing the root-soil interface is challenged by soil's opaque scattering characteristics. Herein, we describe methods for using optical coherence tomography (OCT) to provide non-destructive 3D and cross-sectional root imaging not available with traditional bright-field microscopy. OCT is regularly used for bioimaging, especially in ophthalmology, where it can detect retinal abnormalities. Prior use of OCT in plant biology has focused on surface defects of above-ground tissues, predominantly in food crops. Our results show OCT is also viable for detailed, in situ study of living plant roots. Using OCT for direct observations of root growth in soil can help elucidate key interactions between root morphology and various components of the soil environment including soil structure, microbial communities, and nutrient patches. Better understanding of these interactions can guide efforts to improve plant nutrient acquisition from soil to increase agricultural efficiency as well as better understand drivers of plant growth in natural systems.

Root physiological and biochemical responses of seashore paspalum and centipedegrass exposed to iso‐osmotic salt and drought stresses

AbstractDrought and salt are two major stresses of turfgrass. Both damage the plant root system by disrupting osmotic balance at the root–soil interface. The objectives of our hydroponic experiments were to understand root physiological and biochemical responses in two turfgrass species (seashore paspalum [Paspalum vaginatum Swartz] and centipedegrass [Eremochloa ophiuroides (Munro) Hack.]) with distinct salinity tolerance exposed to iso‐osmotic drought and salt stresses. Two seashore paspalum genotypes, ‘Seastar’ and ‘UGP113’, and a centipedegrass genotype, ‘TifBlair’, were exposed to three treatments: control, salt stress (−0.4 MPa by adding NaCl), and drought stress (−0.4 MPa by adding polyethylene glycol). Although equal in osmotic potential, drought stress damage was more evident compared with salt stress in terms of lipid peroxidation and decreased root viability in all three turfgrass genotypes. Salt stress reduced root viability only in the centipedegrass genotype, indicating ion toxicity effects in this turfgrass genotype. Seashore paspalum genotypes had greater total accumulation of Na+ and K+ for greater root osmotic adjustment than centipedegrass under salt stress. Seashore paspalum genotypes also had greater activities of root antioxidant enzymes such as superoxide dismutase, peroxidase, and glutathione reductase compared with centipedegrass under salt stress. In contrast, centipedegrass had greater total accumulation of total soluble sugar and proline for greater osmotic adjustment compared with seashore paspalum genotypes under drought stress. Despite greater accumulation of organic osmolytes, activities of superoxide dismutase, catalase, peroxidase, and glutathione reductase in centipedegrass were either lower or similar to those in seashore paspalum genotypes, and therefore centipedegrass accumulated similar oxidative damage as seashore paspalum genotypes under drought stress.

Assessment of Rhizospheric Arbuscular Mycorrhizae Spores in Relation to Soil Characters in the Rice Fields of Malda District, India

Arbuscular mycorrhizal fungi (AMF) show symbiotic associations with the roots of plants and facilitates nutrients uptake through the hyphae-plant root-soil interface. Many edaphic and anthropogenic activities greatly influence this mutualistic association. Among which soil parameters are of great importance. The major objective of this study is to assess the soil colonization of AM fungi in different textural and nutritional conditions in agricultural rice fields of different locations of Malda district, West Bengal, India. Fifty rhizospheric soil samples of rice fields were collected from Jadupur (25.080° N, 88.1255° S) and Mohadipur (24.8525° N, 88.1255° E) areas of Malda district, India and the mycorrhizal spores were isolated and quantified according to adhesion and floatation techniques and identified morphometrically through the INVAM database. Soil texture was determined by conventional pipette method and soil pH, conductivity, nitrogen, potassium, phosphorus, and organic carbon contents were quantified according to standard methods. It was observed that the soil samples had a wide diversity of genera of mycorrhizal spores such as Aculospora sp., Entrophospora sp., Gigaspora sp., Glomus sp., and Scutellospora sp. The texture of the sample sites are mostly sandy loamy to loamy and have variations in soil chemical properties. The clay and phosphorous contents are inversely proportional to mycorrhizal spore content whereas it was directly proportional to the organic carbon and nitrogen content in the soil. The AM fungal diversity was more in Jadupur soil while it was less in Mohadipur. The study concludes that low nutrient content soil had more spore numbers than high nutrient content, reflecting their role in mineral acquisition for better plant nutrient supply.

The dominant runoff processes on grassland versus bare soil hillslopes in a temperate environment - An experimental study

Abstract This paper aimed to investigate the dominant runoff processes (DRP’s) at plot-scale in the Curvature Subcarpathians under natural rainfall conditions characteristic for Romania’s temperate environment. The study was based on 32 selected rainfall-runoff events produced during the interval April–September (2014–2017). By comparing water balance on the analyzed Luvisol plots for two types of land use (grassland vs. bare soil), we showed that DRP’s are mostly formed by Hortonian Overland Flow (HOF), 47% vs. 59% respectively. On grassland, HOF is followed by Deep Percolation (DP, 31%) and Fast Subsurface Flow (SSF, 22%), whereas, on bare soil, DP shows a higher percentage (38%) and SSF a lower one (3%), which suggests that the soil-root interface controls the runoff generation. Concerning the relationship between antecedent precipitation and runoff, the study indicated the nonlinearity of the two processes, more obvious on grassland and in drought conditions than on bare soil and in wet conditions (as demonstrated by the higher runoff coefficients). Moreover, the HOF appeared to respond differently to rainfall events on the two plots - slightly longer lag-time, lower discharge and lower volume on grassland - which suggests the hydrologic key role of vegetation in runoff generation processes.

Research progress of root exudates collection technology

Root exudates play an important role in mediating the exchange of substrates, energy and information within the rhizosphere in terrestrial ecosystems. Constructing accurate and applicable sampling system of root exudates to precisely collect the characters including the component and concentration of root exudates and their responses to changing environments are a critical link and prerequisite to understand ecological processes and information exchanges across the root-soil interface. While both traditional and novel techniques for collecting root exudates aim to explore the diversity and concentration of root exudates, current exudate-collection methods could hardly avoid the damage of root system, the adsorption and release of root exudates by soil particles, and distur-bance from microbial metabolism, largely because plant roots are highly associated with the surrounding substrates and environments supporting their growth. Therefore, all root exudate collection methods have their own merits and shortcomings. We systematically reviewed the widely-used traditional and novel root exudates collection techniques and compared their advantages and disadvantages. Furthermore, considering the significance and authenticity of field study on the rhizosphere ecological processes in forest ecosystems, we proposed three frontier research directions regarding the construction of techniques for collecting root exudates in forest ecosystems according to the limits of current studies, aiming to provide foundation for related studies in the future.

Rice root Fe plaque enhances paddy soil N2O emissions via Fe(II) oxidation-coupled denitrification

Iron (Fe) plaque, defined as a film of poorly crystalline Fe oxides deposited on the surface of rice roots, potentially mediates paddy-soil N2O emissions. The aims of this study were to test if, and how, Fe plaque affects N2O production and reduction within a rice paddy soil. Rice seedlings were grown so that Fe plaque was either present or absent. With Fe plaque present, emissions of both N2O and N2 doubled, with the abundance of both Fe-redox bacteria and denitrifying functional genes elevated at the root-soil interface. Under hydroponic conditions, Fe plaque promoted N2O emissions in the presence of NO3− but not NH4+. In addition, chelating the Fe(II) eliminated the promoting effects of Fe plaque on N2O emission while Fe(II) addition to the Fe plaque-free roots increased N2O emission. These results demonstrate that Fe plaque promotes soil N2O emission and N loss predominately via Fe(II) oxidation-coupled denitrification. Our results indicate that Fe plaque is a hotspot for both N2O emission and N loss from paddy soils. Mitigation of N2O emission and N loss from paddy soils should consider methods to limit Fe plaque effects.