Rhizosphere processes are quantitatively important components of terrestrial carbon and nutrient cycles

Changes in soil and plant carbon pools after 9 years of experimental summer warming and increased snow depth

Climate change can have positive and negative effects on the carbon pools and budgets in soil and plant fractions, but net effects are unclear and expected to vary widely within the arctic. We report responses after nine years (2012-2021) of increased snow depth (snow fences) and summer warming (open top chambers) and the combination on soil and plant carbon pools within a tundra ecosystem in West Greenland. Data included characteristics of depth-specific soil samples, including the rhizosphere soil, as well as vegetation responses of NDVI-derived traits, plant species cover and aboveground biomass, litter and roots. Furthermore, natural vegetation growth through the study period was quantified based on time-integrated NDVI Landsat 8 satellite imagery. Our results showed that summer warming resulted in a significant and positive vegetation response driven by the deciduous low shrub Betula nana (no other vascular plant species), while snow addition alone resulted in a significant negative response for Betula. A significant positive effect of summer warming was also observed for moss biomass, possibly driven increasing shade by Betula. The aboveground effects cascaded to belowground traits. The rhizosphere soil characteristics differed from those of the bulk soil regardless of treatment. Only the rhizosphere fraction showed responses to treatment, as soil organic C stock increased in near-surface and top 20 cm with summer warming. We observed no belowground effects from snow addition. The study highlights the plant species response to treatment followed by impacts on belowground C pools, mainly driven by dead fine roots via Betula nana. We conclude that the summer warming treatment and snow addition treatment separately showed opposing effects on ecosystem C pools, with lack of interactive effects between main factors in the combination treatment. Furthermore, changes in soil C are more clearly observed in the rhizosphere soil fraction, which should receive more attention in the future.

A transition from arbuscular to ectomycorrhizal forests halts soil carbon sequestration during subtropical forest rewilding

Ecological succession and restoration rapidly promote multiple dimensions of ecosystem functions and mitigate global climate change. However, the factors governing the limited capacity to sequester soil organic carbon (SOC) in old forests are poorly understood. Ecological theory predicts that plants and microorganisms jointly evolve into a more mutualistic relationship to accelerate detritus decomposition and nutrient regeneration in old than young forests, likely explaining the changes in C sinks across forest succession or rewilding. To test this hypothesis, we conducted a field experiment of root-mycorrhizal exclusion in successional subtropical forests to investigate plant-decomposer interactions and their effects on SOC sequestration. Our results showed that SOC accrual rate at the 0-10 cm soil layer was 1.26 mg g-1 yr-1 in early-successional arbuscular mycorrhizal (AM) forests, which was higher than that in the late-successional ectomycorrhizal (EcM) forests with non-significant change. A transition from early-successional AM to late-successional EcM forests increase fungal diversity, especially EcM fungi. In the late-successional forests, the presence of ectomycorrhizal hyphae promotes SOC decomposition and nutrient cycle by increasing soil nitrogen and phosphorus degrading enzyme activity as well as saprotrophic microbial richness. Across early- to late-successional forests, mycorrhizal priming effects on SOC decomposition explain a slow-down in the capacity of older forests to sequester soil C. Our findings suggest that a transition from AM to EcM forests supporting greater C decomposition can halt the capacity of forests to provide nature-based global climate change solutions.

Managing tobacco black shank disease using biochar: direct toxicity and indirect ecological mechanisms

Black shank disease in tobacco, caused by Phytophthora nicotianae, can lead to yield losses of 30%-50% upon outbreak. Recently, biochar derived from agricultural waste has shown significant potential in controlling soil-borne diseases, though its mechanisms remain unclear. Over a 3-year observation period, we found that the incidence of black shank was significantly lower in plots amended with biochar compared with normal cultivation plots. To investigate the underlying mechanisms, we studied both the direct and indirect effects of biochar on black shank. Direct antifungal assays indicated that biochar reduced the total number of sporangia by 53.91%. Further pot experiments revealed a 62.34% reduction in the P. nicotianae population in the soil following biochar application. Additionally, biochar application led to notable changes in soil physicochemical properties and microbial community composition. Microbial species analysis showed that biochar promoted the aggregation of beneficial microbes such as Sphingomonas, Flavisolibacter, and Mucoromycota. Functional predictions using the PICRUSt 2 software revealed that biochar enhances bacterial functions related to antimicrobial substance synthesis (Tetracycline biosynthesis), detoxification metabolism (D-arginine and D-ornithine metabolism, arginine and proline metabolism), and lipid and fatty acid metabolism (Lipopolysaccharide biosynthesis, fatty acid biosynthesis), while fungal functions showed no significant changes. This suggests that rhizosphere bacteria play a more prominent role in the suppression of black shank by biochar, a finding supported by partial least squares path modeling analysis. Therefore, we hypothesize that biochar not only directly inhibits P. nicotianae growth but also regulates the composition of the rhizosphere microbial community, inducing the production of antimicrobial substances by rhizosphere bacteria, effectively preventing P. nicotianae invasion.IMPORTANCEBlack shank, a global soil-borne fungal disease in tobacco, currently lacks effective control methods. Notably, biochar derived from agricultural waste has shown significant potential in controlling soil-borne diseases. Over a 3-year observation period, we found that plots amended with biochar had a significantly lower incidence of black shank compared with normal cultivation plots. However, the mechanisms of disease suppression remained unclear. Through in vitro antifungal assays and pot experiments, we discovered that tobacco-derived biochar can directly inhibit the growth of the pathogen. Additionally, biochar regulates the composition of the rhizosphere microbial community, inducing rhizosphere bacteria to produce antimicrobial substances, effectively preventing pathogen invasion. This discovery reveals both the direct and indirect mechanisms by which biochar suppresses black shank in tobacco. It provides a scientific basis for developing green control technologies for black shank and offers theoretical support for the application of biochar in managing soil-borne diseases in tobacco cultivation areas.

Water Management-Mediated Changes in the Rhizosphere and Bulk Soil Microbial Communities Alter Their Utilization of Urea-Derived Carbon

As one of the most important fertilizers in agriculture, the fate of urea-derived nitrogen (urea-N) in agricultural ecosystems has been well documented. However, little is known about the function of urea-derived carbon (urea-C) in soil ecosystems, especially which soil microorganisms benefit most from the supply of urea-C and whether the utilization of urea-C by the rhizosphere and bulk soil microorganisms is affected by irrigation regimes. To address this, a soil pot experiment was conducted using 13C-labeled urea to investigate changes in the composition of the rhizosphere and bulk soil microbial communities and differences in the incorporation of urea-derived C into the rhizosphere and bulk soil phospholipid fatty acids (PLFA) pool under flooded irrigation (FI) and water-saving irrigation (CI). Our results suggest that the size and structure of the rhizosphere and bulk soil microbial communities were strongly influenced by the irrigation regime. The CI treatment significantly increased the total amount of PLFA in both the rhizosphere and bulk soil compared to the FI treatment, but it only significantly affected the abundance of Gram-positive bacteria (G+) in the bulk soil. In contrast, shifts in the microbial community structure induced by irrigation regimes were more pronounced in the rhizosphere soil than in the bulk soil. Compared to the FI treatment, the CI treatment significantly increased the relative abundances of the G+ and Actinobacteria in the rhizosphere soil (p < 0.05). According to the PLFA-SIP, most of the labeled urea-derived C was incorporated into 16:1ω7c, 16:0 and 18:1ω7c under both treatments. Despite these general trends, the pattern of 13C incorporation into the PLFA pool differed between the treatments. The factor loadings of individual PLFAs suggested that 18:1ω7c, 16:1ω7c and 16:1ω5c were relatively enriched in urea-C in the bulk soil, while 17:1ω8c, i16:0 and 16:0 were relatively enriched in urea-C in the rhizosphere soil under different irrigation regimes. The loadings also confirmed that 10-me16:0, cy17:0 and cy19:0 were relatively enriched in urea-C under the CI treatment, whereas 14:0, a15:0 and 15:0 were relatively enriched in urea-C under the FI treatment. These results are helpful not only in revealing the interception mechanism of urea-C in soil but also in understanding the functions of key microbes in element cycles.

The pore-rhizosheath shapes maize root architecture by enhancing root distribution in macropores

Pores and old root-channels are preferentially used by roots to allow them to penetrate hard soils. However, there are few studies that have accounted for the effects of pore-rhizosheath on root growth. In this study, we developed an approach by adding the synthetic root exudates using a porous stainless tube with 0.1-mm micropores through a peristaltic pump to reproduce the rhizosheath around the artificial pore, and investigated the effects of pores with and without rhizosheaths on maize root growth in a dense soil. The results indicated that the artificial rhizosheath was about 2.69 mm wide in the region surrounding the pores. The rhizosheath had a higher content of organic carbon, total nitrogen, and abundance of Actinobacteria than that of the bulk soil. Compared with the artificial macropores, the artificial root-pores with a rhizosheath increased the opportunities for root utilisation of the pores space, promoting steeper and deeper root growth. It is concluded that the pore-rhizosheath has a significant impact on root architecture by enhancing root distribution in macropores.

Rhizosphere effects and microbial N limitations drive the root N limitations in the rhizosphere during secondary succession in a Pinus tabuliformis forest in North China

Introduction

Rhizosphere effects (REs) have recently been identified as important regulators of root and microbial nutrient acquisition and are positively involved in nutrient cycling of belowground carbon (C), nitrogen (N), and phosphorus (P). Nutrient conditions of the fine roots and soil N are likely to influence REs. Still, it is unclear how REs of soil nutrients themselves variably impact the supply of nutrients to plants in terms of the responses to soil N due to succession.

Methods

In this study, we applied both fine roots and extracellular enzymes for vector analysis and stoichiometry of N:P to explore the metabolic limitations of roots and rhizospheric soil microbes and their relationships with REs across five levels of soil N (0, 5, 10, 15, and 20 kg N m-2 year-1) along successional age classes of 42, 55, and 65 years in a Pinus tabuliformis forest.

Results

Overall, the metabolism of root and rhizospheric soil microbes was mediated by soil N. N limitation of roots initially decreased before increasing, whereas that of microbes demonstrated opposite trends to the N levels owing to competition for inorganic N between them by REs of NO3 --N. However, N limitations of both roots and microbes were alleviated in young stands and increased with succession after the application of N. In addition, root N limitations were manipulated by REs of three different soil N-related indicators, i.e., total N, NH4 +-N, and NO3 --N. Rhizospheric soil microbial N limitation was almost unaffected by REs due to their strong homeostasis but was an important driver in the regulation of root N limitation.

Discussion

Our results indicated that successional age was the most critical driver that directly and indirectly affected root N metabolism. However, the level of N application had a slight effect on root N limitation. Microbial N limitation and variations in the REs of N indicators regulated root N limitation in the rhizosphere. As a result, roots utilized REs to sequester N to alleviate N limitations. These findings contribute to novel mechanistic perspectives on the sustainability of N nutrition by regulating N cycling in a system of plant-soil-microbes in the rhizosphere to adapt to global N deposition or the heterogeneous distribution of bioavailable soil N with succession.

Effects of different sizes of microplastic particles on soil respiration, enzyme activities, microbial communities, and seed germination

Microplastics (MPs) are emerging pollutants of terrestrial ecosystems. The impacts of MP particle size on terrestrial systems remain unclear. The current study aimed to investigate the effects of six particle sizes (i.e., 4500, 1500, 500, 50, 5, and 0.5 μm) of polyethylene (PE) and polyvinyl chloride (PVC) on soil respiration, enzyme activity, bacteria, fungi, protists, and seed germination. MPs significantly promoted soil respiration, and the stimulating effects of PE were the strongest for medium and small-sized (0.5-1500 μm) particles, while those of PVC were the strongest for small particle sizes (0.5-50 μm). Large-sized (4500 μm) PE and all sizes of PVC significantly improved soil urease activity, while medium-sized (1500 μm) PVC significantly improved soil invertase activity. MPs altered the soil microbial community diversity, and the effects were especially pronounced for medium and small-sized (0.5-1500 μm) particles of PE and PVC on bacteria and fungi and small-sized (0.5 μm) particles of PE on protists. The impacts of MPs on bacteria and fungi were greater than on protists. The seed germination rate of Brassica chinensis decreased gradually with the decrease in PE MPs particle size. Therefore, to reduce the impact of MPs on soil ecosystems, effective measures should be taken to avoid the transformation of MPs into smaller particles in soil environmental management.

Effects of Lake Erie dredged material on microbiomes in a farm soil of northwestern Ohio

Dredged materials are often considered as candidates for replenishing lost topsoils in the watersheds of rivers and lakes. This study aimed to investigate the impacts of Lake Erie dredged material on the microbial community in a farm soil of northwestern Ohio. Dredged material from the Toledo Harbor, OH was mixed with a local farm soil at ratios of 0:100, 10:90, 20:80, and 100:0 for soybean growth in a greenhouse for 123 days and was subject to 16S rRNA gene sequencing. α-Diversity analysis revealed that although the original dredged material hosted a highly diverse microbiome, soils blended with the dredged material had similar levels of bacterial diversity to 100% farm soil throughout the experiment. β-Diversity analysis demonstrated that, given the same plant status, that is, with or without soybean, blended soils had similar bacterial communities to 100% farm soil during the experiment. Furthermore, by the end of the experiment, all soils with soybeans merged into one cluster distinctive from those without the plants, indicating that the growth of plants played a dominating role in defining the structure of soil microbiomes. The majority (73.8%) of the operational taxonomy units that were unique to the original dredged material were not detected by the end of the experiment. This study demonstrates that up to 20% of the dredged material can be safely blended into the farm soil without distorting the microbial communities of the latter, implying a potential beneficial use of the dredged material for topsoil restoration.

Nutrient allocation patterns of Picea crassifolia on the eastern margin of the Qinghai-Tibet Plateau

It can provide a basis for decision making for the conservation and sustainable use of forest ecosystems in mountains to understand the stoichiometric properties and nutrient allocation strategies of major tree species. However, the plant nutrient allocation strategies under different environmental gradients in forest systems of arid and semi-arid mountains are not fully understand. Therefore, three typical regions in the Qilian Mountains on the eastern edge of the Qinghai-Tibet Plateau were selected based on precipitation and temperature gradients, and the stoichiometric characteristics and nutrient allocation strategies of Qinghai spruce (Picea crassifolia) of the dominant tree species under different environmental gradients were investigated. The results showed that (1) the stoichiometric characteristics of plant tissues were different in the three regions. (2) The importance of each tissue in the plant nutrient allocation varied in different regions, showing that the plant roots are more important in the warm-wet region, while the plant leaves, branches and trunks are more important in the transition and hot-dry regions. (3) The influencing factors affecting plant nutrient allocation strategies were inconsistent across regions, which showed that plant nutrient allocation strategies in the warm-wet and transition region were mainly influenced by soil factors, while they were more influenced by climatic factors in the hot-dry region. The patterns of plant nutrient allocation strategies and drivers under different environmental gradients could help us better understand the ecological adaptation mechanism and physiological adjustment mechanism of forest ecosystem in mountains.

Activity and community structure of nitrifiers and denitrifiers in nitrogen-polluted rivers along a latitudinal gradient

Nitrogen (N) cycling in rivers is particularly active and dynamic due to excess nutrient inputs worldwide. However, the multidimensional spatial patterns of the activity and community structure of N-cycling microorganisms in rivers remain unclear, limiting our understanding of river ecological functions, especially N removal capacity. Here, we measured the nitrification and denitrification rates and identified nitrifying and denitrifying microorganisms using high-throughput sequencing of archaeal amoA, bacterial amoA, nirK, and nirS genes in channel sediments, riparian rhizosphere soils, and riparian bulk soils of 30 N-polluted rivers across China. Results showed that in the lateral dimension, nitrification rates in sediments did not differ significantly from those in rhizosphere and bulk soils, but denitrification rates were higher in sediments than in bulk soils. However, the archaeal amoA gene abundance in sediments was considerably lower than that in rhizosphere and bulk soils, and bacterial amoA gene abundance in sediments was greater than that in rhizosphere soils. In the vertical dimension, both nitrification and denitrification rates in riparian bulk soils decreased with soil depth, and topsoils harbored more nitrifying and denitrifying microbes than subsoils. Denitrification but not nitrification rates increased with latitude and altitude but decreased with increasing mean annual temperature and precipitation. Overall, these results provide new insights into the multidimensional spatial patterns of river N cycling at a large scale, which is crucial to evaluating the N removal function of global rivers.

Metabolic niches in the rhizosphere microbiome: dependence on soil horizons, root traits and climate variables in forest ecosystems

Understanding belowground plant-microbial interactions is important for biodiversity maintenance, community assembly and ecosystem functioning of forest ecosystems. Consequently, a large number of studies were conducted on root and microbial interactions, especially in the context of precipitation and temperature gradients under global climate change scenarios. Forests ecosystems have high biodiversity of plants and associated microbes, and contribute to major primary productivity of terrestrial ecosystems. However, the impact of root metabolites/exudates and root traits on soil microbial functional groups along these climate gradients is poorly described in these forest ecosystems. The plant root system exhibits differentiated exudation profiles and considerable trait plasticity in terms of root morphological/phenotypic traits, which can cause shifts in microbial abundance and diversity. The root metabolites composed of primary and secondary metabolites and volatile organic compounds that have diverse roles in appealing to and preventing distinct microbial strains, thus benefit plant fitness and growth, and tolerance to abiotic stresses such as drought. Climatic factors significantly alter the quantity and quality of metabolites that forest trees secrete into the soil. Thus, the heterogeneities in the rhizosphere due to different climate drivers generate ecological niches for various microbial assemblages to foster beneficial rhizospheric interactions in the forest ecosystems. However, the root exudations and microbial diversity in forest trees vary across different soil layers due to alterations in root system architecture, soil moisture, temperature, and nutrient stoichiometry. Changes in root system architecture or traits, e.g. root tissue density (RTD), specific root length (SRL), and specific root area (SRA), impact the root exudation profile and amount released into the soil and thus influence the abundance and diversity of different functional guilds of microbes. Here, we review the current knowledge about root morphological and functional (root exudation) trait changes that affect microbial interactions along drought and temperature gradients. This review aims to clarify how forest trees adapt to challenging environments by leveraging their root traits to interact beneficially with microbes. Understanding these strategies is vital for comprehending plant adaptation under global climate change, with significant implications for future research in plant biodiversity conservation, particularly within forest ecosystems.

The proliferation of beneficial bacteria influences the soil C, N, and P cycling in the soybean-maize intercropping system

Soybean-maize intercropping system can improve the utilization rate of farmland and the sustainability of crop production systems. However, there is a significant gap in understanding the interaction mechanisms between soil carbon (C), nitrogen (N), and phosphorus (P) cycling functional genes, rhizosphere microorganisms, and nutrient availability. To reveal the key microorganisms associated with soil nutrient utilization and C, N, and P cycling function in the soybean-maize intercropping system, we investigated the changes in soil properties, microbial community structure, and abundance of functional genes for C, N, and P cycling under soybean-maize intercropping and monocropping at different fertility stages in a pot experiment. We found that there was no significant difference in the rhizosphere microbial community between soybean-maize intercropping and monocropping at the seeding stage. As the reproductive period progressed, differences in microbial community structure between intercropping and monocropping gradually became significant, manifesting the advantages of intercropping. During the intercropping process of soybean and maize, the relative abundance of beneficial bacteria in soil rhizosphere significantly increased, particularly Streptomycetaceae and Pseudomonadaceae. Moreover, the abundances of C, N, and P cycling functional genes, such as abfA, mnp, rbcL, pmoA (C cycling), nifH, nirS-3, nosZ-2, amoB (N cycling), phoD, and ppx (P cycling), also increased significantly. Redundancy analysis and correlation analysis showed that Streptomycetaceae and Pseudomonadaceae were significantly correlated with soil properties and C, N, and P cycling functional genes. In brief, soybean and maize intercropping can change the structure of microbial community and promote the proliferation of beneficial bacteria in the soil rhizosphere. The accumulation of these beneficial bacteria increased the abundance of C, N, and P cycling functional genes in soil and enhanced the ability of plants to fully utilize environmental nutrients and promoted growth.

Relationships between root exudation and root morphological and architectural traits vary with growing season

Plants allocate a substantial amount of C belowground for root exudates and for the construction and adjustment of root morphological and architectural traits. What relationships exist between root exudates and other root traits and these relationships change with growing season, however, remain unclear. We quantified the root exudation rate and root morphological traits, including total root length (RL), total root surface area (RS), root diameter (RD), specific root length (SRL), specific root area (SRA) and root tissue density (RTD), and architectural traits, such as branching intensity (BI), and investigated their associations during the rapidly growing season (April and August) and the slowly growing season (December) of three common native tree species, Liquidambar formosana, Michelia maudiae and Schima superba, in subtropical China. We found that the linkages of RD, SRL, SRA, RTD and BI did not change with the growing season, reflecting their highly conservative relationships. The root exudation rate varied significantly with growing season (P < 0.05) and produced various associations with other root traits at different growing seasons. During the rapidly growing season (i.e., April), the exudation rate was the highest and was positively correlated with RL. The exudation rate was the lowest during the slowly growing season (i.e., December) and was negatively associated with RL, RS and RTD. Our findings demonstrate the seasonality of the linkages of root exudation rate with other root traits, which highlights the highly plastic and complex associations of belowground root traits. These findings help to deepen our understanding of plant nutrient acquisition strategies.

Soil microbiome analysis reveals effects of periodic waterlogging stress on sugarcane growth

Sugarcane is one of the major agricultural crops with high economic importance in Thailand. Periodic waterlogging has a long-term negative effect on sugarcane development, soil properties, and microbial diversity, impacting overall sugarcane production. Yet, the microbial structure in periodically waterlogged sugarcane fields across soil compartments and growth stages in Thailand has not been documented. This study investigated soil and rhizosphere microbial communities in a periodic waterlogged field in comparison with a normal field in a sugarcane plantation in Ratchaburi, Thailand, using 16S rRNA and ITS amplicon sequencing. Alpha diversity analysis revealed comparable values in periodic waterlogged and normal fields across all growth stages, while beta diversity analysis highlighted distinct microbial community profiles in both fields throughout the growth stages. In the periodic waterlogged field, the relative abundance of Chloroflexi, Actinobacteria, and Basidiomycota increased, while Acidobacteria and Ascomycota decreased. Beneficial microbes such as Arthrobacter, Azoarcus, Bacillus, Paenibacillus, Pseudomonas, and Streptomyces thrived in the normal field, potentially serving as biomarkers for favorable soil conditions. Conversely, phytopathogens and growth-inhibiting bacteria were prevalent in the periodic waterlogged field, indicating unfavorable conditions. The co-occurrence network in rhizosphere of the normal field had the highest complexity, implying increased sharing of resources among microorganisms and enhanced soil biological fertility. Altogether, this study demonstrated that the periodic waterlogged field had a long-term negative effect on the soil microbial community which is a key determining factor of sugarcane growth.

Plant species identity and plant-induced changes in soil physicochemistry-but not plant phylogeny or functional traits - shape the assembly of the root-associated soil microbiome

The root-associated soil microbiome contributes immensely to support plant health and performance against abiotic and biotic stressors. Understanding the processes that shape microbial assembly in root-associated soils is of interest in microbial ecology and plant health research. In this study, 37 plant species were grown in the same soil mixture for 10 months, whereupon the root-associated soil microbiome was assessed using amplicon sequencing. From this, the contribution of direct and indirect plant effects on microbial assembly was assessed. Plant species and plant-induced changes in soil physicochemistry were the most significant factors that accounted for bacterial and fungal community variation. Considering that all plants were grown in the same starting soil mixture, our results suggest that plants, in part, shape the assembly of their root-associated soil microbiome via their effects on soil physicochemistry. With the increase in phylogenetic ranking from plant species to class, we observed declines in the degree of community variation attributed to phylogenetic origin. That is, plant-microbe associations were unique to each plant species, but the phylogenetic associations between plant species were not important. We observed a large degree of residual variation (> 65%) not accounted for by any plant-related factors, which may be attributed to random community assembly.

Soil phosphorus availability alters the correlations between root phosphorus-uptake rates and net photosynthesis of dominant C3 and C4 species in a typical temperate grassland of Northern China

Phosphorus (P) fertilization can alleviate a soil P deficiency in grassland ecosystems. Understanding plant functional traits that enhance P uptake can improve grassland management. We measured impacts of P addition on soil chemical and microbial properties, net photosynthetic rate (Pn ) and nonstructural carbohydrate concentrations ([NSC]), and root P-uptake rate (PUR), morphology, anatomy, and exudation of two dominant grass species: Leymus chinensis (C3 ) and Cleistogenes squarrosa (C4 ). For L. chinensis, PUR and Pn showed a nonlinear correlation. Growing more adventitious roots compensated for the decrease in P transport per unit root length, so that it maintained a high PUR. For C. squarrosa, PUR and Pn presented a linear correlation. Increased Pn was associated with modifications in root morphology, which further enhanced its PUR and a greater surplus of photosynthate and significantly stimulated root exudation (proxied by leaf [Mn]), which had a greater impact on rhizosheath micro-environment and microbial PLFAs. Our results present correlations between the PUR and the Pn of L. chinensis and C. squarrosa and reveal that NSC appeared to drive the modifications of root morphology and exudation; they provide more objective basis for more efficient P-input in grasslands to address the urgent problem of P deficiency.

Biochar Enhances Soil Resource Availability and Suppresses Microbial Metabolism Genes in the Rhizosphere of Wheat

Despite the well-documented role of biochar in promoting soil quality and crop productivity, the underlying biological mechanisms remain poorly understood. Here, we explored the effects of straw biochar on soil microbiome in the rhizosphere from wheat using metagenomic sequencing. Our results showed that straw return decreased the yields of wheat, while the straw biochar return increased the wheat yields. Further, both the richness and community composition confirmed different effects of the straw return and straw biochar return. The straw biochar return also resulted in greater rhizosphere effects from wheat, represented by resource availability, including soil organic carbon, soil total nitrogen, available phosphorus, and available potassium. The rhizosphere effects from wheat, represented by microbial metabolism genes involved in carbon, nitrogen, phosphorus, and potassium cycling, however, were decreased by straw biochar returning. In addition, the rhizosphere effects from nitrogen content and the nitrogen cycling genes showed negative relationships with wheat yields. Together, these results revealed that straw biochar enhanced soil resource availability but suppressed microbial metabolism genes in the rhizosphere from wheat, supporting the idea that straw biochar serves as a nutrient pool for crops.

Mineral reactivity determines root effects on soil organic carbon

Modern conceptual models of soil organic carbon (SOC) cycling focus heavily on the microbe-mineral interactions that regulate C stabilization. However, the formation of 'stable' (i.e. slowly cycling) soil organic matter, which consists mainly of microbial residues associated with mineral surfaces, is inextricably linked to C loss through microbial respiration. Therefore, what is the net impact of microbial metabolism on the total quantity of C held in the soil? To address this question, we constructed artificial root-soil systems to identify controls on C cycling across the plant-microbe-mineral continuum, simultaneously quantifying the formation of mineral-associated C and SOC losses to respiration. Here we show that root exudates and minerals interacted to regulate these processes: while roots stimulated respiratory C losses and depleted mineral-associated C pools in low-activity clays, root exudates triggered formation of stable C in high-activity clays. Moreover, we observed a positive correlation between the formation of mineral-associated C and respiration. This suggests that the growth of slow-cycling C pools comes at the expense of C loss from the system.

Microbial community in buckwheat rhizosphere with different nitrogen application rates

Microorganism plays a pivotal role in regulating sustainable development of agriculture. The excessive application of nitrogen fertilizer is considered to affect the microbial structure in many agricultural systems. The present study aimed to assess the impacts of nitrogen application rate on microbial diversity, community and functionality in rhizosphere of Tartary buckwheat in short-time. The nitrogen fertilizer was applied at rates of 90 kg (N90), 120 kg (N120) and 150 kg (N150) urea per hectare, respectively. The soil properties were measured chemical analysis and displayed no difference among treatments. Metagenome analysis results showed that the microbial diversity was not affected, but the microbial community and functionality were affected by the nitrogen application rate. According to the Linear discriminant analysis effect size (LEfSe) analysis, 15 taxa were significantly enriched in the N120 and N150 groups, no taxon was enriched in the N90 group. Kyoto Encyclopaedia of Genes and Genomes (KEGG) annotation results revealed that the genes related to butanoate and beta alanine metabolism were significantly enriched in the N90 group, the genes related to thiamine metabolism, lipopolysaccharide biosynthesis and biofilm formation were significantly enriched in the N120 group, and the genes related to neurodegenerative disease was significantly enriched in the N150 group. In conclusion, short-time nitrogen fertilizer application shifted the microbial community structure and functionality.

Plant growth strategy determines the magnitude and direction of drought-induced changes in root exudates in subtropical forests

Root exudates are an important pathway for plant-microbial interactions and are highly sensitive to climate change. However, how extreme drought affects root exudates and the main components, as well as species-specific differences in response magnitude and direction, are poorly understood. In this study, root exudation rates of total carbon (C) and its components (e.g., sugar, organic acid, and amino acid) were measured under the control and extreme drought treatments (i.e., 70% throughfall reduction) by in situ collection of four tree species with different growth rates in a subtropical forest. We also quantified soil properties, root morphological traits, and mycorrhizal infection rates to examine the driving factors underlying variations in root exudation. Our results showed that extreme drought significantly decreased root exudation rates of total C, sugar, and amino acid by 17.8%, 30.8%, and 35.0%, respectively, but increased root exudation rate of organic acid by 38.6%, which were largely associated with drought-induced changes in tree growth rates, root morphological traits, and mycorrhizal infection rates. Specifically, trees with relatively high growth rates were more responsive to drought for root exudation rates compared with those with relatively low growth rates, which were closely related to root morphological traits and mycorrhizal infection rates. These findings highlight the importance of plant growth strategy in mediating drought-induced changes in root exudation rates. The coordinations among root exudation rates, root morphological traits, and mycorrhizal symbioses in response to drought could be incorporated into land surface models to improve the prediction of climate change impacts on rhizosphere C dynamics in forest ecosystems.

Integrative transcriptome and metabolome analysis reveals the mechanism of exogenous melatonin alleviating drought stress in maize roots

Melatonin (MT) is essential for plant development and drought adaptation. However, the molecular and metabolic mechanisms underlying MT-induced drought tolerance in maize roots remain largely unclear. Herein, we investigated the effects of MT on drought tolerance in maize roots using integrated transcriptomic and metabolomic analyses, and identified MT-induced genes and metabolites associated with drought resistance. Compared with the untreated control plants, MT application alleviated the deleterious effects of drought on roots, by decreasing the malondialdehyde level and increasing the solute potential, eventually promoting root growth. Transcriptome and metabolome analysis demonstrated that MT significantly upregulates the expression of genes related to flavonoid biosynthesis (PAL, C4H, 4CL, HCT, CHS, CHI, F3'5'H, and DFR), activates drought-responsive transcription factors (ERFs, NACs, MYBs, and bHLHs), and regulates hormone signaling-related genes, especially ethylene response factors (ERF4, ERF81, and ERF110). Moreover, MT increased the accumulation of flavonoid metabolites, particularly apigenin, luteolin, and quercetin, under drought-stress conditions. These findings were further supported by quantitative real-time polymerase chain reaction analysis and total flavonoid measurements. Altogether, our findings suggest that MT promotes maize root growth during drought by regulating flavonoid synthesis pathways, transcription factors, and plant hormone signals. This study provides new insights into the complex mechanisms by which MT enhances crop resistance to drought damage.

Empirical and Mechanistic Modeling of Release Kinetics of Heavy Metals and Their Chemical Distribution in the Rhizosphere and Non-rhizosphere Soils Under Vegetable Cultivation

Biochemical processes in the rhizosphere affect the availability and distribution of heavy metals (HMs) in various forms. Rhizosphere soil (RS) and non-rhizosphere soil (NRS) samples were collected from 10 fields under tarragon (Artemisia dracunculus L.) cultivation to investigate the release kinetics and distribution of HMs including cadmium (Cd), cobalt (Co), copper (Cu), iron (Fe), and zinc (Zn) in five fractions. The cumulative amounts of Cu and Fe released after 88 h were in the following ranges, respectively: 1.31-2.76 and 3.24-6.35 mg kg^-1 in RS and 1.41-2.72 and 3.15-5.27 mg kg^-1 in NRS. The parabolic diffusion and pseudo-second-order equations provided the best fit to the release kinetics data of Cu and Fe, respectively. The cation exchange model (CEM) based on Gaines-Thomas selectivity coefficients implemented in the PHREEQC program could well simulate the release of Cu and Fe suggesting that cation exchange was the dominant mechanism in the release of Fe and Cu from soils by 0.01 M CaCl₂. Cadmium was predominantly found in fraction F2, while other HMs were mainly present in fraction F5. According to the risk assessment code, there was a very high risk for Cd, a medium risk for Co and Cu, a very low risk for Fe, and a low risk for Zn. Correlation analysis showed that soil physicochemical properties were effective in the distribution and transformation of HMs. Significant positive correlations between five fractions indicated that different forms of HMs can potentially transform into each other.

Root Exudates Mediate the Processes of Soil Organic Carbon Input and Efflux

Root exudates, as an important form of material input from plants to the soil, regulate the carbon input and efflux of plant rhizosphere soil and play an important role in maintaining the carbon and nutrient balance of the whole ecosystem. Root exudates are notoriously difficult to collect due to their underlying characteristics (e.g., low concentration and fast turnover rate) and the associated methodological challenges of accurately measuring root exudates in native soils. As a result, up until now, it has been difficult to accurately quantify the soil organic carbon input from root exudates to the soil in most studies. In recent years, the contribution and ecological effects of root exudates to soil organic carbon input and efflux have been paid more and more attention. However, the ecological mechanism of soil organic carbon input and efflux mediated by root exudates are rarely analyzed comprehensively. In this review, the main processes and influencing factors of soil organic carbon input and efflux mediated by root exudates are demonstrated. Soil minerals and soil microbes play key roles in the processes. The carbon allocation from plants to soil is influenced by the relationship between root exudates and root functional traits. Compared with the quantity of root exudates, the response of root exudate quality to environmental changes affects soil carbon function more. In the future, the contribution of root exudates in different plants to soil carbon turnover and their relationship with soil nutrient availability will be accurately quantified, which will be helpful to understand the mechanism of soil organic carbon sequestration.

Species of fast bulk-soil nutrient cycling have lower rhizosphere effects: A nutrient spectrum of rhizosphere effects

Tree roots not only acquire readily-usable soil nutrients but also affect microbial decomposition and manipulate nutrient availability in their surrounding soils, that is, rhizosphere effects (REs). Thus, REs challenge the basic understanding of how plants adapt to the environment and co-exist with other species. Yet, how REs vary among species in response to species-specific bulk soil nutrient cycling is not well-known. Here, we studied how plant-controlled microbial decomposition activities in rhizosphere soils respond to those in their corresponding bulk soils and whether these relations depend on species-specific nutrient cycling in the bulk soils. We targeted 55 woody species of different clades and mycorrhizal types in three contrasting biomes, namely a temperate forest, a subtropical forest, and a tropical forest. We found that microbial decomposition activities in rhizosphere soils responded linearly to those in their corresponding bulk soils at the species level. Thereafter, we found that REs (parameters in rhizosphere soils minus those in corresponding bulk soils) of microbial decomposition activities had negative linear correlations with microbial decomposition activities in corresponding bulk soils. A multiple factor analysis revealed that soil organic carbon, total nitrogen, and soil water content favored bulk soil decomposition activities in all three biomes, showing that the magnitude of REs varied along a fast-slow nutrient cycling spectrum in bulk soils. The species of fast nutrient cycling in their bulk soils tended to have smaller or even negative REs. Therefore, woody plants commonly utilize both positive and negative REs as a nutrient-acquisition strategy. Based on the trade-offs between REs and other nutrient-acquisition strategies, we proposed a push and pull conceptual model which can bring plant nutrient-acquisition cost and plant carbon economics spectrum together in the future. This model will facilitate not only the carbon and nutrient cycling but also the mechanisms of species co-existence in forest ecosystems.

Soil microbial community structure dynamics shape the rhizosphere priming effect patterns in the paddy soil

Microbial community structure plays a crucial part in soil organic carbon (SOC) decomposition and variation of rhizosphere priming effects (RPEs) during plant growth. However, it is still uncertain how bacterial community structure regulates RPEs in soil and how RPE patterns respond to plant growth. Therefore, we conducted an experiment to examine the RPE response to plant growth and nitrogen (N) addition (0 (N0), 150 (N150), and 300 (N300) kg N ha^-1) using the ¹³C natural abundance method in a C₃ soil (paddy soil) - C₄ plant (maize, Zea mays L.) system; we then explored the underlying biotic mechanisms using 16S rRNA sequencing techniques. Networks were constructed to identify keystone taxa and to analyze the correlations between network functional modules of bacterial community and C decomposition. The results indicated that negative and positive RPEs occurred on Day 30 and Day 75 after maize planting, respectively. Bacterial community structure significantly changed and tended to shift from r-strategists toward K-strategists with changing labile C: N stoichiometry and soil pH during plant growth stages. The different network modules of bacterial community were aggregated in response to RPE pattern variation. Caulobacteraceae, Bacillus, and Chitinophagaceae were keystone taxa on Day 30, while Gemmatimonas, Candidatus Koribacter, and Xanthobacteraceae were keystone taxa on Day 75. Moreover, keystone taxa with different C utilization strategies were significantly different between the two growth stages and related closely to different RPE patterns. This study provides deeper insights into the network structure of bacterial communities corresponding to RPE patterns and emphasizes the significance of keystone taxa in RPE variation.

Global magnitude of rhizosphere effects on soil microbial communities and carbon cycling in natural terrestrial ecosystems

The rhizosphere is one of the most dynamic interfaces on the Earth. Understanding the magnitudes of rhizosphere effects (RE, difference in bio-physicochemical properties between rhizosphere and bulk soils) on soil microbial communities and their moderators is important for studying on below-ground carbon (C) cycling. A comprehensive meta-analysis was conducted to quantify the REs on soil microbial biomass, community structure, respiration, and C-degrading enzymes. We found that REs on soil C and nutrients, total microbial biomass, the abundance of specific microbial groups, fungi to bacteria ratio, respiration, and C-degrading enzymes were positive, but the magnitudes were varied with biomes, plant functional types, and mycorrhizal types. REs on microbial biomass, respiration, and C-degrading enzymes increased with the increase of mean annual temperature and mean annual precipitation, but decreased with the increase of soil clay, C, nitrogen (N), and phosphorus (P) contents. The REs on microbial biomass and respiration also increased as the REs on soil C:N:P increased. Compared with bulk soil, per unit rhizosphere soil C supported more microbial biomass, per unit of which respired more C, leading to faster C decomposition in rhizosphere. Our findings indicate that the increase in microbial biomass, co-metabolism induced by labile and energy-rich organic C of root exudates, and overflow respiration induced by stoichiometric imbalance together contribute to the enhanced C decomposition in rhizosphere. The global pattern of REs on soil microbial communities is critical to revealing the plant-microbe-soil interactions in terrestrial ecosystems.

Quantification and uncertainty of root growth stimulation by elevated CO2 in a mature temperate deciduous forest

Increasing CO2 levels are a major global challenge, and the potential mitigation of anthropogenic CO2 emissions by natural carbon sinks remains poorly understood. The uptake of elevated CO2 (eCO2) by the terrestrial biosphere, and subsequent sequestration as biomass in ecosystems, remain hard to quantify in natural ecosystems. Here, we combine field observations of fine root stocks and flows, derived from belowground imaging and soil cores, with image analysis, stochastic modelling, and statistical inference, to elucidate belowground root dynamics in a mature temperate deciduous forest under free-air eCO2 to 150 ppm above ambient levels. eCO2 led to relatively faster root production (a peak volume fold change of 4.52 ± 0.44 eCO2 versus 2.58 ± 0.21 control), with increased root elongation relative to decay the likely causal mechanism for this acceleration. Physical analysis of 552 root systems from soil cores support this picture, with lengths and widths of fine roots significantly increasing under eCO2. Estimated fine root contributions to belowground net primary productivity increase under eCO2 (mean annual 204 ± 93 g dw m-2 yr-1 eCO2 versus 140 ± 60 g dw m-2 yr-1 control). This multi-faceted approach thus sheds quantitative light on the challenging characterisation of the eCO2 response of root biomass in mature temperate forests.

Critical transition of multifunctional stability induced by nitrogen enrichment in grasslands differing in degradation severity

Nitrogen (N) enrichment poses a severe threat to ecosystem multifunctionality. Given increasing variability of ecosystem functioning and uncertainty under global change, a pressing question is how N enrichment affects temporal stability of multiple functions (i.e., 'multifunctional stability'). Whether the responses of multifunctional stability to N enrichment change with external disturbance, such as grasslands with different degradation statuses, remains unclear. We conducted multi-level N enrichment experiments at four grassland sites with no, moderate, severe, and extreme degradation statuses in Inner Mongolia, China. We measured temporal stability of five functions, comprising aboveground net primary productivity, soil total carbon (C) and N storage, and soil microbial biomass C and N storage, to explore how multifunctional stability responded to N enrichment. The temporal stability of most individual functions and multifunctional stability decreased sharply when N input exceeded 20 g N m^-2 y^-1 in the non-, moderately, and severely degraded grasslands, whereas the threshold declined to 10 g N m^-2 y^-1 in the extremely degraded grassland. The relative importance of plant and soil microbes in regulating multifunctional stability varied along the degradation gradient. In particular, plant species asynchrony and species richness showed strong positive relationships with multifunctional stability in the non- and moderately degraded grasslands, whereas soil microbial diversity, especially bacterial diversity, was positively associated with multifunctional stability in the severely and extremely degraded grasslands. Overall, our findings identified a critical threshold for N-induced multifunctional stability and called for context-specific biodiversity conservation strategies to buffer the negative effect of N enrichment on grassland ecosystem stability.

Modelling the Rhizosphere Priming Effect in Combination with Soil Food Webs to Quantify Interaction between Living Plant, Soil Biota and Soil Organic Matter

A model of rhizosphere priming effect under impact of root exudate input into rhizosphere soil was developed as an important process of the plant-soil interaction. The model was based on the concept of nitrogen (N) mining, compensating for the N scarcity in exudates for microbial growth by accelerating SOM mineralisation. In the model, N deficiency for microbial growth is covered ("mined") by the increased SOM mineralisation depending on the C:N ratio of the soil and exudates. The new aspect in the model is a food web procedure, which calculates soil fauna feeding on microorganisms, the return of faunal by-products to SOM and mineral N production for root uptake. The model verification demonstrated similar magnitude of the priming effect in simulations as in the published experimental data. Model testing revealed high sensitivity of the simulation results to N content in exudates. Simulated CO₂ emission from the priming can reach 10-40% of CO₂ emission from the whole Ah horizon of boreal forest soil depending on root exudation rates. This modeling approach with including food web activity allows quantifying wider aspects of the priming effect functioning including ecologically important available N production.

14 year applications of chemical fertilizers and crop straw effects on soil labile organic carbon fractions, enzyme activities and microbial community in rice-wheat rotation of middle China

Traditional fertilization management can damage soil structure and lead to severe soil erosion. The practice of crop straw returning to the field reduces the negative impact of straw burning and improves soil quality. We investigated the effects of these agricultural practices on soil organic carbon components, enzyme activities, and soil microorganisms over 14 years of field experiments. Specifically, we studied four management strategies: no fertilizer or crop straw returning (CK), traditional chemical fertilization (NPK), crop straw returning (S), and crop straw returning with chemical fertilizer (NPKS). We found NPKS treatments significantly (P < 0.05) increased the dissolved organic carbon (DOC), microbial biomass carbon (MBC), particulate organic carbon (POC) and readily oxidized organic carbon (ROC) concentrations by 79.32 %, 82.16 %, 92.46 %, and 104.32 % relative to CK. Furthermore, under NPKS, the activities of soil enzymes related C, N, and P (α-glucosidase (αG), β-glucosidase (βG), cellulase (CBH), xylanase (βX), acetyl β-glucosaminidase (NAG), leucine aminopeptidase (LAP), and acid phosphate (AP)) were increased by 54.66 %, 113.26 %, 76.73 %, 52.41 %, 45.74 %, 56.69 %, and 68.92 % relative to CK, respectively. Redundancy analysis and structural equation modelling showed that straw returning had positive effects on soil microbial community diversity and richness, and also improved microbial activity which is favorable in the degradation of soil carbon. Furthermore, we found that soil fungi were more sensitive than bacteria to changes in soil carbon composition and enzyme activities following straw returning. These results suggest that straw returning combined with chemical fertilizer can be an effective strategy to improve soil labile organic carbon components, enzyme activities, and ecological function of microorganisms.

Effects of mycorrhiza and hyphae on the response of soil microbial community to warming in eastern Tibetan Plateau

The effects of mycorrhiza and its external hyphae on the response of soil microbes to global warming remain unclear. This study investigates the role of mycorrhiza and its hyphae in regulating soil microbial community under warming by examining the microbial biomass and composition in the ingrowth cores of arbuscular mycorrhiza (AM) plant, Fargesia nitida, and ectomycorrhiza (ECM) plant, Picea asperata, with/without mycorrhiza/hyphae and experimental warming. The results showed that warming significantly increased the biomass of all soil microbes (by 19.89%-137.48%) and altered the microbial composition in both plant plots without mycorrhiza/hyphae. However, this effect was weakened in the presence of mycorrhiza or hyphae. In F. nitida plots, warming did not significantly affect biomass and composition of most soil microbial groups when mycorrhiza or hyphae were present. In P. asperata plots, warming significantly increased the total and ECM fungi (ECMF) biomass in the presence of hyphae (p < 0.05) and the total, Gn, and AM fungi (AMF) biomass in the presence of mycorrhiza (p < 0.05). Meanwhile, the response of enzyme activities to warming was also altered with mycorrhiza or hyphae. Additionally, soil microbial community composition was mainly influenced by soil available phosphorus (avaP), while enzyme activities depended on soil avaP, dissolved organic carbon (DOC), and nitrate concentrations. Our results indicate that mycorrhiza and its hyphae are essential in regulating the response of microbes to warming.

Younger carbon dominates global soil carbon efflux

Soil carbon (C) is comprised of a continuum of organic compounds with distinct ages (i.e., the time a C atom has experienced in soil since the C atom entered soil). The contribution of different age groups to soil C efflux is critical for understanding soil C stability and persistence, but is poorly understood due to the complexity of soil C pool age structure and potential distinct turnover behaviors of age groups. Here, we build upon the quantification of soil C transit times to infer the age of C atoms in soil C efflux (a_efflux ) from seven sequential soil layer depths down to 2 m at a global scale, and compare this age with radiocarbon-inferred ages of C retained in corresponding soil layers (a_soil ). In the whole 0-2 m soil profile, the mean a_efflux is 194 21 1021 (mean with 5%-95% quantiles) year and is just about one-eighth of a_soil ( 1476 717 2547 year), demonstrating that younger C dominates soil C efflux. With increasing soil depth, both a_efflux and a_soil are increased, but their disparities are markedly narrowed. That is, the proportional contribution of relatively younger soil C to efflux is decreased in deeper layers, demonstrating that C inputs (new and young) stay longer in deeper layers. Across the globe, we find large spatial variability of the contribution of soil C age groups to C efflux. Especially, in deep soil layers of cold regions (e.g., boreal forests and tundra), a_efflux may be older than a_soil , suggesting that older C dominates C efflux only under a limited range of conditions. These results imply that most C inputs may not contribute to long-term soil C storage, particularly in upper layers that hold the majority of new C inputs.

Exogenous Melatonin Reprograms the Rhizosphere Microbial Community to Modulate the Responses of Barley to Drought Stress

The rhizospheric melatonin application-induced drought tolerance has been illuminated in various plant species, while the roles of the rhizosphere microbial community in this process are still unclear. Here, the diversity and functions of the rhizosphere microbial community and related physiological parameters were tested in barley under the rhizospheric melatonin application and drought. Exogenous melatonin improved plant performance under drought via increasing the activities of non-structural carbohydrate metabolism enzymes and activating the antioxidant enzyme systems in barley roots under drought. The 16S/ITS rRNA gene sequencing revealed that drought and melatonin altered the compositions of the microbiome. Exogenous melatonin increased the relative abundance of the bacterial community in carbohydrate and carboxylate degradation, while decreasing the relative abundance in the pathways of fatty acid and lipid degradation and inorganic nutrient metabolism under drought. These results suggest that the effects of melatonin on rhizosphere microbes and nutrient condition need to be considered in its application for crop drought-resistant cultivation.

Impact of Leaf Litter and Fine Roots in the Pool of Carbon, Nitrogen and Phosphorus Accumulated in Soil in Various Scenarios of Regeneration and Reconstruction of Forest Ecosystems

This study determined the rate of decomposition of fine roots and leaf litter from birch, larch, and pine, and compared the impact of fine root decomposition and leaf litter on carbon, nitrogen, and phosphorus accumulation in various regenerated and reconstructed forest ecosystems. The control plots were located on podzol soils in managed forest non-degraded habitats. Over a one-year experimental season, the decomposition of birch and larch fine roots released less carbon in comparison to leaf litter. The carbon mass-loss rates were 16% for birch roots and 15% for larch roots, while for birch and larch litter, the rates were 36% and 27%, respectively. For nitrogen, mass-loss rates were 48% for birch fine roots and 60% for larch and pine fine roots, whereas for pine and birch litter the rates were 14%, and 33% for larch litter. The results of our study prove the important role of fine root input to the soil’s carbon, nitrogen, and phosphorus pool and additionally their significance for CO2 sequestration within the studied regenerated terrestrial ecosystems.

Carbon allocation to root exudates is maintained in mature temperate tree species under drought

Carbon (C) exuded via roots is proposed to increase under drought and facilitate important ecosystem functions. However, it is unknown how exudate quantities relate to the total C budget of a drought-stressed tree, that is, how much of net-C assimilation is allocated to exudation at the tree level. We calculated the proportion of daily C assimilation allocated to root exudation during early summer by collecting root exudates from mature Fagus sylvatica and Picea abies exposed to experimental drought, and combining above- and belowground C fluxes with leaf, stem and fine-root surface area. Exudation from individual roots increased exponentially with decreasing soil moisture, with the highest increase at the wilting point. Despite c. 50% reduced C assimilation under drought, exudation from fine-root systems was maintained and trees exuded 1.0% (F. sylvatica) to 2.5% (P. abies) of net C into the rhizosphere, increasing the proportion of C allocation to exudates two- to three-fold. Water-limited P. abies released two-thirds of its exudate C into the surface soil, whereas in droughted F. sylvatica it was only one-third. Across the entire root system, droughted trees maintained exudation similar to controls, suggesting drought-imposed belowground C investment, which could be beneficial for ecosystem resilience.

Eco-Stoichiometric Characteristics of Rhizosphere and Bulk Soils of Smilax china L. along Vertical Zone Spectrum of Fanjing Mountain

To explore the correlations between nutrients and stoichiometric characteristics in the rhizosphere and bulk soils of understory Smilax china L. in forest ecosystems at different altitudes and to clarify the rhizosphere effect of understory vegetation in forest ecosystems and its response strategy to altitude, providing a theoretical basis for better forest ecological environment protection and high-quality development in Fanjing Mountain. Understory Smilax china L. at four different altitudes were selected, with the differences and influencing factors of carbon (C), nitrogen (N), phosphorus (P) and potassium (K) mass fractions and stoichiometric ratios in their rhizosphere and bulk soils analyzed. The average mass fractions of total C, total N and alkali-hydrolyzed N in the rhizosphere and bulk soils of Smilax china L. at different altitudes were 224.43 and 181.55 g·kg⁻¹; 9.56 and 6.81 g·kg⁻¹; and 648.19 and 600.70 g·kg⁻¹, respectively. The rhizosphere effect of Smilax china L. was significant at altitudes of 500 m and 1000 m but became not so prominent with the rise of altitude. The C:N ratio in the rhizosphere and bulk soils ranged from 19.51 to 39.75 and the C:P ratio ranged from 225.29 to 543.05. C accumulation is greater than N accumulation in the rhizosphere and bulk soils of Smilax china L., and both present P limitation. Based on the comprehensive analysis of the mass fractions and eco-stoichiometric ratios of soil nutrients, the P limitation in Fanjing Mountain forest ecosystem is commonly seen and should be addressed.

Rhizosphere Effects along an Altitudinal Gradient of the Changbai Mountain, China

Rhizosphere effects (REs) play important roles in regulating carbon (C) and nutrient cycling in terrestrial ecosystems. However, little is known about the REs of mature trees in the field, especially at the ecosystem scale. This study aimed to explore the variation and patterns of REs in natural ecosystems. Here, combining soil monoliths with an adhering soil (shaking fine roots) method was adopted to sample paired rhizosphere soil and bulk soil along an altitudinal gradient. Based on the relative REs and the percentage of rhizosphere soil mass, the REs on soil C and net nitrogen mineralization rates (Cmin and net Nmin) at the ecosystem scale were estimated. Our results showed that the REs on soil processes, soil microbial biomass C and extracellular enzyme activities (β-glucosidase and N-acetyl-glucosaminidase activities), and soil chemical properties (total C, total N, inorganic N, extractable P, K, Ca, Mg, Fe, and Mn) were significantly positive across altitudinal sites, while soil pH was significantly negative. Although the relative REs on investigated variables varied significantly among altitudes, the relative REs did not show a clear trend with the increased altitudes. Across altitudes, the mean magnitude of ecosystem-level REs on Cmin and net Nmin were 19% (ranging from 4% to 48%) and 16% (ranging from 3% to 34%), respectively. Furthermore, the magnitude of ecosystem-level rhizosphere effects increased linearly with the increased altitudes. The altitudinal patterns of ecosystem-level RE mainly depend on the percentage of rhizosphere soil mass. In conclusion, our results provided a set of new evidence for the REs, and highlighted the need to incorporate REs into land C and N models.

A Stronger Rhizosphere Impact on the Fungal Communities Compared to the Bacterial Communities in Pecan Plantations

Understanding microbial communities associated with bulk and rhizosphere soils will benefit the maintenance of forest health and productivity and the sustainable development of forest ecosystems. Based on MiSeq sequencing, we explored the differences between the bulk soil and the rhizosphere soil on bacterial and fungal communities of pecan plantation. Results suggested that rhizosphere-associated fungal rather than bacterial community structures differed from bulk soil, and rhizosphere soil had lower fungal diversity than bulk soil. Actinobacteria and Cantharellales were the bacterial and fungal biomarkers of the rhizosphere soil of pecan plantation, respectively. In addition, Pleosporales, which are mainly involved in saprophylaxis and plant pathogenic processes, was identified as one of the most important fungal biomarkers for the bulk soil, and the FunGuild predicted a higher relative abundance of pathogenic fungi in bulk soil compared to rhizosphere soil. The pH, ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N), and total carbon (TC) contents drove microbial community structure and composition. The bacterial network was simpler in the rhizosphere soil than in the bulk soil. However, fungi showed the opposite network pattern. Keystone species in bacterial and fungal networks were mostly involved in nutrient cycling and the C cycling, and were found to be enriched in the rhizosphere soil. Overall, in terms of bacterial and fungal communities, the rhizosphere soil behaves more healthily than the bulk soil and has a higher potential for nutrient cycling.

Harnessing belowground processes for sustainable intensification of agricultural systems

Increasing food demand coupled with climate change pose a great challenge to agricultural systems. In this review we summarize recent advances in our knowledge of how plants, together with their associated microbiota, shape rhizosphere processes. We address (molecular) mechanisms operating at the plant-microbe-soil interface and aim to link this knowledge with actual and potential avenues for intensifying agricultural systems, while at the same time reducing irrigation water, fertilizer inputs and pesticide use. Combining in-depth knowledge about above and belowground plant traits will not only significantly advance our mechanistic understanding of involved processes but also allow for more informed decisions regarding agricultural practices and plant breeding. Including belowground plant-soil-microbe interactions in our breeding efforts will help to select crops resilient to abiotic and biotic environmental stresses and ultimately enable us to produce sufficient food in a more sustainable agriculture in the upcoming decades.

Multifarious Responses of Forest Soil Microbial Community Toward Climate Change

Forest soils are a pressing subject of worldwide research owing to the several roles of forests such as carbon sinks. Currently, the living soil ecosystem has become dreadful as a consequence of several anthropogenic activities including climate change. Climate change continues to transform the living soil ecosystem as well as the soil microbiome of planet Earth. The majority of studies have aimed to decipher the role of forest soil bacteria and fungi to understand and predict the impact of climate change on soil microbiome community structure and their ecosystem in the environment. In forest soils, microorganisms live in diverse habitats with specific behavior, comprising bulk soil, rhizosphere, litter, and deadwood habitats, where their communities are influenced by biotic interactions and nutrient accessibility. Soil microbiome also drives multiple crucial steps in the nutrient biogeochemical cycles (carbon, nitrogen, phosphorous, and sulfur cycles). Soil microbes help in the nitrogen cycle through nitrogen fixation during the nitrogen cycle and maintain the concentration of nitrogen in the atmosphere. Soil microorganisms in forest soils respond to various effects of climate change, for instance, global warming, elevated level of CO₂, drought, anthropogenic nitrogen deposition, increased precipitation, and flood. As the major burning issue of the globe, researchers are facing the major challenges to study soil microbiome. This review sheds light on the current scenario of knowledge about the effect of climate change on living soil ecosystems in various climate-sensitive soil ecosystems and the consequences for vegetation-soil-climate feedbacks.

Root exudation in a sloping Moso bamboo forest in relation to fine root biomass and traits

Exudation by fine roots generally varies with their morphological traits, but the effect of belowground resource availability on the root exudation via root morphological traits and biomass remains unknown. We aimed to determine the effects of morphological and physiological traits on root exudation rates and to estimate stand-scale exudation (E_stand) by measuring the mass, length, and surface area of fine roots in a Moso bamboo forest. We measured root exudation as well as morphological and physiological traits in upper and lower plots on a slope with different belowground resource availability. The mean (± S.D.) root exudation rates per mass in the upper and lower slope were 0.049 ± 0.047 and 0.040 ± 0.059 mg C g^-1 h^-1, respectively, which were in the range of exudation found in woody forest ecosystems. We observed significant relationships between root exudation per mass and root respiration, as well as specific root length and surface area. In contrast, exudation per length and area did not correlate with morphological traits. The morphological traits did not differ between slope positions, resulting in no significant difference in root exudation per mass. Fine root biomass, length, and surface area on a unit ground basis were much higher in the lower than those in the upper slope positions. E_stand was higher when estimated by mass than by length and area because the morphological effect on exudation was ignored when scaled using mass. E_stand was 1.4–2.0-fold higher in the lower than that in upper slope positions, suggesting that the scaling parameters of mass, length, and area determined the E_stand estimate more than the exudation rate per mass, length, and area. Regardless of scaling, E_stand was much higher in the Moso bamboo forest than in other forest ecosystems because of a large fine-root biomass.

Three source-partitioning of CO2 fluxes based on a dual-isotope approach to investigate interactions between soil organic carbon, glucose and straw

Inputs of available organic materials into soil alter the decomposition of soil organic matter (SOM), a process called priming effect. Organic carbon (C) inputs in terrestrial ecosystems are common from various sources (e.g. rhizodeposits, plant residues, microbial necromass) simultaneously, but their interactions as well as mutual effects on SOM decomposition are unknown because multisource partitioning of pools and fluxes was not available. A dual-isotope approach (identical materials except for straw being possessed two ¹³C abundances) was adopted to partition total CO₂ emission from three C sources: SOM, glucose and straw. Cumulative CO₂ efflux was quantified into straw-derived (558 μg C g^-1), glucose-derived (480 μg C g^-1) and SOM-derived (58 μg C g^-1) CO₂ during the first 7 days of incubation. Glucose or straw addition induced positive SOM priming, whereas glucose combined with straw resulted in higher SOC loss than that induced by single addition of glucose or straw after day 7. The Spearman's correlation showed that the interactions between glucose and straw shifted from increased CO₂ evolved during their intensive decomposition (days 1 to 3) to mutual constraint on mineralization during the late stage (days 5 to 7). This study provides evidences for the suitability of the dual-isotope approach to partition multiple sources of CO₂ fluxes and C pools, and evaluates their individual or mutual contributions to SOM priming, thus, implicating C sequestration in terrestrial ecosystems.

Sugarcane cultivar-dependent changes in assemblage of soil rhizosphere fungal communities in subtropical ecosystem

Sugarcane cultivars (Saccharum officinarum L.) are widely cultivated for both sugar and renewable energy in China. The response of rhizosphere fungal composition and diversity to different emerging sugarcane cultivars is limited. Therefore, utilizing high-throughput sequencing, we explored fungal communities' structure in soils adhering to six sugarcane cultivars' roots (Guitang 08-120, Regan14-62, Guitang 08-1180, Haizhe 22, Liucheng 05-136, Taitang 22) in Guangxi Province, China. Our results suggested that sugarcane varieties significantly altered rhizosphere soil attributes, with Haizhe 22 having substantially lower soil pH, organic matter (OM), available phosphorus (AP), and soil water contents (SWC) than others cultivars. Different sugarcane varieties did not substantially affected the Shannon fungal diversity index, but the apparent effect on fungal richness was significant. Beta diversity analysis revealed that "Haizhe 22" distinguished the fungal community from the other five cultivars. Soil pH, OM, cultivars, and soil moisture were crucial determinants in shaping soil fungal composition. The Haizhe 22 rhizosphere significantly enriched the operational taxonomic units (OTUs) assigned to two fungal genera (Cephalotheca and Sagenomella), while rhizosphere of other verities significantly enriched the OTUs assigned to four fungal genera (Chaetomium, Chaetosphaeria, Mortierella, and Talaromyces), suggesting their essential role in plant development, disease tolerance, and bioremediation. These findings may help in selecting or breeding innovative genotypes capable of supporting abundant rhizosphere fungi beneficial to plants that would likely improve crops' agronomic potential and maintain soil ecosystem sustainability.