Root water uptake is an essential part of tree water relations and plays a crucial role in tree physiological activities. Water resource in deep soil is relatively abundant and can provide plenty of water to trees to guarantee their survival and healthy growth during dry seasons. Thus, a good comprehension of the characteristics and underlying mechanisms of deep soil water uptake and utilization by trees will deepen the understanding of the interaction between trees and the environment, tree survival and growth strategies, coexistence and competition among different species, etc. This knowledge is important in establishing green cultivation schemes for plantations, which depend less on the external water resources input and avoid the adverse effects on the water ecological environment. From existing studies, the characteristics and underlying mechanisms of deep water uptake and utilization by trees are reviewed. Firstly, the definition of deep roots and deep soil is discussed, and 1 m depth is recommended as the average (reference) definition standard in main forest vegetation types except the boreal forest. The reasons for the formation of deep tree roots around the globe were also determined. Secondly, the observed deep soil water uptake characteristics of trees and their influencing factors are summarized. Then, from the aspects of the adjustment of deep root traits and the coordination of hydraulic traits of different organs, the mechanisms of deep water uptake by trees are discussed. For example, the spatial, temporal and efficiency adjustment strategies of deep roots can be used to facilitate the absorption of deep soil water. Finally, some implications of deep soil water uptake for the cultivation of plantations are proposed, such as “for water management in plantations, trees should be induced to moderately utilize some deep soil water and an appropriate irrigation frequency should be selected”, “appropriate mixed planting of different tree species can facilitate the buffering effect of deep soil water storage”, “developing techniques of selecting trees for thinning based on the water uptake depths of different species”, etc. Deficiencies of existing studies and some future research directions were also pointed out.

Aims Accumulating evidence suggests that arbuscular mycorrhizal fungi (AMF) can promote the growth of plant roots. However, the effects of AMF on the root growth of dioecious plants, particularly those grown under different sexual combination patterns, remain largely unknown, this study therefore aimed at improving our understanding of the roles of AMF in these systems.
Methods In the present study, homogenized soil (river sand:surface soil:vermiculite = 1:1:1, volume ratio) was used as growth substrate. The Populus cathayana saplings uninoculated and inoculated with AMF under three sex combination patterns (male vs. male, MM; female vs. female, FF; male vs. female, MF) were defined as control (CK) and AMF treatment group, respectively. Subsequently, we compared the differences in colonization rate, root dry mass, root morphology, carbon (C) content and nitrogen (N) content between CK and AMF treatments under different sexual combination patterns.
Important findings Our results indicated that colonization rate, root dry mass, root morphology (except root branching intensity, specific root surface area) and C, N content were remarkably altered upon inoculation with AMF in comparison to uninoculated controls. Furthermore, the sexual combination patterns were shown to significantly affect root dry mass, root morphology and C, N content of male and female P. cathayana. After inoculation with AMF, root dry mass, root morphology and N content of female individuals were increased whereas these parameters of males were decreased or slightly increased in inter-sexual groups compared with the respective intra-sexual groups. Collectively, our data demonstrate the growth-promoting effects of AMF on the roots of P. cathayana individuals grown under different sexual combination patterns, and such beneficial effects are most pronounced in females grown under inter-sexual combination patterns.

Nitrogen (N) deposition has profound impacts on the phosphorus (P) cycling in forest ecosystems. Especially, the aggravated P limitation on tree growth under N addition has caused much attention to researchers. This article reviews the effects of N addition on plant P content in forest ecosystems. The result showed that N addition increased soil available P and facilitated the absorption of P by plants by promoting soil phosphatase activity, thereby increasing plant P content. Furthermore, changes in tree P content following N addition were also affected by species, life forms as well as experimental duration. Due to the inconsistency, the underlying mechanisms of changes in P content under N addition were further summarized as follows: 1) changes in soil available P content induced by exogenous N input affected the source of plant P; 2) N input affected the P uptake capacity of plants by affecting plant root exudates, mycorrhizal symbiosis and root morphological structure; 3) plant P utilization efficiency was also influenced with changes of P re-distribution and P re-absorption. Overall, for the changes in plant P under increasing exogenous N inputs, alterations of soil available P under N addition was the primary factor, while changes in plant P uptake capacity and P utilization efficiency ulteriorly regulated plant P content.

Aims Nitrogen use efficiency (NUE) is a key functional trait in plants, which closely relates to ecosystem functions. However, it is still unclear about the regional patterns and affecting factors of plant NUE.
Methods This study quantified leaf and root NUE in 139 grassland plant species and explored their relationships with environmental factors and plant functional groups across 82 sampling sites in Nei Mongol and Qinghai-‌Xizang Plateau.
Important findings 1) We found that leaf NUE (53 g·g ^-1) in meadow steppe was significantly greater than those in alpine meadow (46 g·g ^-1), desert steppe (41 g·g ^-1) and typical steppe (39 g·g ^-1). Root NUE (108 g·g ^-1) in alpine meadow was higher than those in other ecosystems. 2) Leaf NUE was more sensitive to temperature than root NUE, but with increasing drought index they all showed a significant decrease. 3) Leaf and root NUE in forbs were significantly lower than sedges and grasses. In addition, leaf and root NUE of legume were 48% and 60% lower than those of non-legume, respectively. 4) Plant NUE did not show any significant relationship with soil nitrogen content. Overall, there was significant difference between leaf and root NUE in their spatial patterns in the Nei Mongol and Qinghai-Xizang Plateau grasslands. The main impacting factors were plant functional group and drought index. The findings are helpful for better understanding the mechanisms underlying the variation of grassland productivity in China, and also provide more scientific basis for grassland management.

Aims The microstructure of root in cross section is composed of connected skeleton and interconnected or closed holes, which is closely related to the tensile mechanical properties of root system.
Methods The relationship of the microstructure of root in cross section and the tensile performances of root, in this study, was discussed by means of single root drawing test and scanning electron microscope (SEM).
Important findings The main results showed that: 1) the ability of root in cross section to bear tension and tensile strain decreased with the increase of root diameter in per unit area; 2) the tensile strength and ductile behaviors of root decreased with the increase of average pore size, and the uniformity of pore size also had a certain influence on them; with the increase of root diameter, the arrangement mode of vessels presents: single vessel → multiple vessel → chain of vessel → cluster of vessel. The arrangement mode of vessel and the uniformity of distribution have influences on the tensile strength characteristics of root, the influence of the arrangement mode has a greater influence than the uniformity distribution; 4) the influence of area ratio of root in cross section on root tensile properties, is also influenced by xylem, root bark and vessel characteristics. In conclusion, this study revealed that the increase of root diameter influenced the microstructure in cross section and affected the tensile properties of roots from the perspective of root microstructure, which provided a certain theoretical basis to further analysis of the soil fixation mechanism of shrub root systems.

Aims The diameter variation of fine roots plays an important role for the study of fine root variation. Phylogeny is a significant factor. In order to examine the diameter variation of the first-order roots in subtropical evergreen broadleaved forests, we investigated 89 woody plant species from a natural evergreen broadleaved forest in Wanmulin Nature Reserve, Jianou, Fujian Province.Methods We selected three trees of each species with similar diameters at breast height or ground diameters, and sampled the root system with intact soil block method. We classed fine root with root order method. One-way ANOVA was used to test the first-order root diameter difference among the life forms (evergreen and deciduous trees), growth forms (tree, semi-tree or shrub and shrub) and the taxonomic classes. Then the Blomberg’s K value was calculated to determine phylogenetic signal. We analyzed the correlation between divergence time and first-order root diameter by using linear regression from family perspective.Important findings 1) The coefficient of variation for the first-order root diameter was 23% in this subtropical evergreen broad-leaved forest. 2) There were no differences in first-order root diameter between evergreen and deciduous trees, but that of the shrubs was significantly different from that of the semi-tree, shrub and tree species. 3) Phylogenetic signal in first-order root diameter was not significant. In addition, the divergence time was positively correlated with the first-order root diameter in the family-level. These results showed that, the variations for first-order root diameter in the tested subtropical woody species was little affected by phylogenetic structure.

Aims In this study, changes in growth, photosynthesis and root structure in response to drought were tested in transgenic poplar 84K (Populus alba × P. glandulosa) seedlings with different expression levels of aquaporin gene (PtPIP2;8). The function of aquaporin gene PtPIP2;8 and its response to drought stress were analyzed.
Methods We selected PtPIP2;8 silencing line of poplar 84K, PtPIP2;8 overexpressing line of poplar 84K and wildtype (WT) as the experimental materials. The Real-time fluorescence quantitative PCR technique was used to detect the PtPIP2;8 expression in roots, stems and leaves. Root hydraulic conductance was measured by high pressure liquid flow meter. The photosynthetic light-response curve, and gas exchange parameters were measured by a LI-6400 photosynthetic system. Growth indexes were determined, and the root length, root surface area, root volume and total root tips were scanned and then analyzed with the root analysis software.
Important findings The results showed that: (1) The gene PtPIP2;8 was mainly expressed in the root system in WT, while its significant expression occurs not only in roots, but also in stems and leaves in PtPIP2;8 overexpressing poplar lines. The PtPIP2;8 RNAi-silence poplar lines only showed weak expression of PtPIP2;8 in the root, and the expression level were 1/20 and 1/80 of WT and overexpression line, respectively. (2) The root structure analysis showed that overexpression lines had significantly lower total root length, total root surface area, total root volume and total root tips than RNAi-silence line and WT, but higher root hydraulic conductance compared with RNAi-silence line and WT. These results showed that the aquaporin gene PtPIP2;8 participated in plant water transport and improved water transport efficiency. (3) Under normal water conditions, RNAi-silence lines showed lower plant height and leaf area but higher root-shoot ratio compared with overexpression line and WT. After drought stress, RNAi-silence lines only slightly decreased the net photosynthetic rate (P_n) and stomatal conductance (G_s), and maintained a relatively high P_n. Diurnal changes of P_n and G_s in RNAi-silence lines showed a single-peak pattern, in which the decrease of photosynthesis was caused by stomatal limitation. Diurnal changes of P_n in both overexpression lines and WT had a two-peak pattern, indicating the non-stomatal limitation of photosynthesis. Drought stress slightly decreased P_n of RNAi-silence lines, while largely decreased P_n of overexpression line and WT decreased, especially at 13:00 and 15:00, indicating that overexpression line and WT were more sensitive to drought stress compared with the RNAi-silence lines. (4) Under drought stress, RNAi-silence line showed the least decline in relative growth rate and total biomass, and the highest root-shoot ratio among the three poplar lines. The total root surface area, total root volume and total root tips of RNAi-silence line were significantly higher than those of WT. The results suggest that aquaporin gene PtPIP2;8 directly participates in the water transport and helps to improve the water transport efficiency, thus the transformation of aquaporin PtPIP2;8 gene may affect root development and growth of plants. Overexpression lines weaken their drought resistance with decreased root development and increased leaf area, while RNAi-silence line increases its drought resistance with reduced leaf area, increased root growth and root-shoot ratio. The results of this study indicate that aquaporin improves the efficiency of water transport across membranes, while the non-aquaporin water-conducting mechanism has greater tolerance to drought.

Aims Arbuscular mycorrhizal fungi (AMF) form symbiotic relationships with most terrestrial plants, contributing to the nutrient uptake of host plants. While little is known on how rhizospheric microorganisms affect the relationships between AMF and host plants under nutritional stress. We hypothesize that AMF may compete for nutrients with host plants in extremely nutrient-limited environments, such as nitrogen deficient habitats, and nitrogen-fixing bacteria will alleviate the competition.
Methods In order to test our hypotheses, we grew Solidago canadensis plants under nitrogen deficient treatments. We inoculated plants with AMF and a nitrogen-fixing bacterium to test the relationships among the host plant and microorganisms.
Important findings Under the lowest nitrogen level (0.025 mmol·L^-1 N of ammonium nitrogen), the growth of S. canadensis was more restricted with AMF colonization, suggesting competition between AMF and the host. However, with the inoculation of nitrogen-fixing bacterium, AMF colonization was promoted and plant growth was increased. These results indicate that nitrogen-fixing bacteria could moderate the competition for nutrients between AMF and their host under nitrogen deficiency. This study improves our understanding of the invasion mechanisms of alien plants, where they interact with different microorganisms under extreme nutrient stress.

Global change has exerted profound impacts on ecosystem function, such as variations in plant productivity and imbalances in nutrient cycling. Previous studies mostly focused on the impacts of global change on individual functions. However, ecosystems have multiple functions, known as ecosystem multifunctionality (EMF), such that the evaluation based on a single functionality is inappropriate to reflect the overall performance of ecosystems due to the occurrence of trade-offs or synergies among the differential functions. This imposes limitation to our understanding of the effects of global change on ecosystems. Since the initial quantitative study of EMF by Hector and Bagchi in 2007, this field of research has undergone rapid development and the environmental impacts on EMF have received wide attention with intensification of global change. In order to gain systematic understanding of the progress in EMF studies, we conducted a bibliometric analysis for the period 2007-2020 based on CNKI and ISI Web of Science databases. This paper provides a brief description of the development in EMF research and summary of studies concerning the impacts of land use change, warming, changes in precipitation, and nitrogen deposition on EMF. We raised six issues of further attention in future studies of EMF in the context of global change, including (1) requirement of consensus in EMF indices and evaluation method; (2) consideration on the interactive effects among different factors on EMF; (3) elucidation of EMF responses to global change across various temporal scales; (4) understanding of the relationships between multi-dimensional, multi-scale biodiversity and EMF; (5) understanding of the relationships between multiple trophic diversity and EMF; and (6) understanding of the relationships between root functional traits and EMF.

Aims It is very important to investigate the relationships between litter decomposition characteristics and plant functional traits in understanding the maintenance mechanism of ecosystem functions.

Methods In order to study the main driving factors that affect the leaf litters and root decomposition of different species, this study took the leaf litters and roots of six main plant species Stipa grandis, Cleistogenes squarrosa, Anemarrhena asphodeloides, Leymus chinensis, Convolvulus ammannii and Carex korshinskyi in S. grandis steppe. The litter bag method was used to study the decomposition rate constant of both leaf litters and root through 501 days of field incubation. Plant functional traits including leaf dry matter content, root specific surface area, root tissue density, contents of C, N and different cellulose components of the leaf and root litters were determined and the relationships between decomposition characteristics and functional traits of leaf litters and root across six plant species were examined.

Important findingsThe results showed that there were significant interspecific differences in leaf and root traits of six plant species. The ratios of maximum to minimum values for most traits were between 1 and 2, while the difference in some traits, such as C:N and specific surface area of roots between species was nearly 4 times. For the six plant species, the overall trend of the mass residue and decomposition rate constant of the leaf litter and root during 501 days of decomposition all showed the rapid decomposition in the early stage, relatively slow decomposition in the middle stage and the slowest decomposition in the later stage. During the decomposition process of leaf litters and roots, Cleistogenes squarrosa showed the slowest one, while the leaf litter decomposition of Anemarrhena asphodeloides was the fastest, and the root decomposition of Convolvulus ammannii was the fastest. Through the correlation analysis and stepwise regression analysis, it was found that the decomposition process of leaf litters and roots was affected by different traits in different decomposition periods. The structural carbohydrate content was the main factor affecting the early and late decomposition of litters and the early decomposition of roots, while the non-structural carbohydrate content was the main factor affecting the middle and late decomposition of roots. In addition, the decomposition rate of leaf litters in the middle stage of decomposition was mainly affected by leaf dry matter content, while the decomposition rates of roots in the middle and late stages of decomposition were also significantly affected by C:N and N content, respectively. Our results present the important guide for the prediction of carbon and nutrient cycling process in the S. grandis steppe.

Aims Plants absorb mineral nutrients such as nitrogen (N) mainly through their roots. The nutrient uptake of plants with different root morphologies differs. Many studies have shown that arbuscular mycorrhizal fungi (AMF) can help their symbiotic associates absorb mineral N. However, there is little research on whether the effect of AMF on nutrient uptake of plant roots is affected by root morphology.

Methods In this study, we selected three rice mutants and one wild type (root hairless (rhl1), lateral rootless (iaa11), adventitious rootless (arl1) and wild type (Kas)) to investigate the role of root morphology in plant nutrient uptake. Subsequently, we used the ¹⁵N isotope labeling method to explore the effects of arbuscular mycorrhizal fungi and N addition (low N: 20 mg·kg^-1 NH₄⁺-N; high N: 100 mg·kg^-1 NH₄⁺-N) on N uptake of rice mutants with different root morphologies.

Important findings The results show that the leaf ¹⁵N concentrations of rhl1,Kas, iaa11 and arl1 were increased by 60%, 72%, 128% and 118%, respectively, under the high N compared to the low N treatment. This result indicates that the addition of N significantly promoted rice N uptake with the most evident effect occurring in iaa11 and arl1. The average effect sizes of AMF on rhl1, Kas, iaa11 and arl1 were 17%, 31%, 42% and 51% under the low N level, indicating that root morphology can alter the effect of AMF on plant N uptake. Compared to the low N treatment, high N significantly downregulated the AMF effect on N uptake by rice plants with different root morphologies, indicating that N addition may mediate the complementary effect of AMF and root morphology on plant nutrient uptake. In conclusion, our data provide direct experimental evidence of funcitonal complementarity of mycrrohzal fungi and their associated roots with different root morphogy.

Changes in soil nutrient availability and primary succession of vegetation often co-occur during the processes of natural soil development. A low availability of nitrogen (N) and phosphorus (P) resources is common in the very early and late stage of soil development, respectively. Plants have evolved different nutrient-acquisition strategies (NASs) in response to low nutrient availability. Although the changes and responses of plant NASs to soil nutrients may affect primary succession and species diversity, the temporal trends and underlying mechanisms of plant NASs with soil development remain unknown. We reviewed 104 studies mainly carried out on soil chronosequences to clarify changes in plant NASs with soil age and its ecological significance. We classify plant NASs into Fine root, Microbial, Specialized root, Carnivorous and Parasitic strategies. We argue that the diversity of plant NASs changes with increasing soil age following a dumbbell-pattern, while reaching the maximum in the late stage of soil development. The role of Microbial and Fine root strategies in plants acquiring nutrients gradually decreases with increasing soil age, while the minimum and maximum role of Specialized root strategies in plants acquiring P is in the intermediate and late stages of soil development, respectively. The effects of NASs on interspecific relationships of plants vary with soil age. Specifically, pioneer plants with biological N fixation and specialized root strategies usually increase available soil N and regolith-derived nutrients to facilitate the colonization of subsequent plants in the early stage of soil development. During the early-intermediate stage, NASs mainly affect plant competitiveness in acquiring relatively abundantly available nutrients from soil. The facilitation and competition affected by NASs contribute to plant species turnover in the first two stages. In the late stage, diverse NASs enable plants to acquire distinct forms of nutrients from different soil spaces and complementary NASs enable plants to take up soil nutrients mobilized by their neighbors. Together with the interactions between NASs and soil pathogens, these processes contribute the coexistence and diversity of plant species in this stage when most soil nutrients have a very low availability. We propose that it is necessary to quantify the relationships between changes in soil nutrient availability (including concentrations and fractions) and plant NASs with soil age. More studies are also needed to quantify contributions of NASs to primary succession, diversity of plant species and soil development.

Aims The Longdong Loess Plateau in Gansu Province is one of the main apple producing areas in China. Plastic film-mulching is often applied to maintain soil moisture as well as water-saving in apple orchards. It is reported that long-term film mulching may cause the degradation of soil physical properties and inhibition of root growth. The objective of this study were to explore the effect of long-term mulching on the physical properties, stability of the surface (0-20 cm) and subsurface (20-40 cm) layer soil, and to investigate the changes of apple fine root growth characteristics in quantity, morphology, configuration and anatomical traits.

Methods Using soil profile and stratified sampling method, the changes of the physical properties and soil structural stability of the surface and subsurface layer soil was analyzed under film-mulching 2 years (2Y), film-mulching 4 years (4Y) and film-mulching 6 years (6Y), conventional tillage (CK) treatments, and roots of 18-year-old apple trees were collected at rapid growing period (days after fruit harvest and before defoliation) to investigate the spatial distribution by measuring the root length, surface area, specific root length, catheter diameter and catheter density. Principal component analysis was used to extract the main factors of root and soil changes under the condition of plastic film mulching, and to analyze the adaptation strategies for fine root growth of apple trees to the physical degradation of rhizosphere soil.

Important findings Short-term film mulching (2Y) treatment significantly improved the soil water content and total porosity in the subsurface soil layer, increased by 18.04%, 4.53%, respectively, and reduced the soil density by 2.36% than that of conventional tillage (CK) treatments. Growth of fine roots increased in subsurface soil, and the specific surface area was 151% of CK. Film mulching promoted the movement of clay particles to the subsurface soil resulting in obvious deposition and cementation. The physical clay in subsurface soil was higher than that of surface soil. The physical clay in subsurface soil under 2Y, 4Y and 6Y mulching were 115.64%, 115.58% and 114.21% of those in surface soil, which led to soil compaction. Soil texture, aggregate characteristics and organic matter content were selected as the main load factors, which dominated degradation process of subsurface soil, and inhibited the number and configuration characteristics of roots, apple fine roots of long-term film mulching (4Y or 6Y) concentrated in the surface layer of the soil. In the subsurface soil, fine roots were found to be shortened and coarsened with inhibiting elongation growth and increasing catheter diameter, indicating the “intensive” root construction strategy the offset the weakening of absorption function caused by the fine root quantity and weakening of morphological characteristics. In conclusion, the ‘invisible' degradation of subsurface soil physical property occurred in long-term film mulching orchard will have an influence on healthy roots growth and sustainable soil utilization. It is recommended that 2-year was the suitable for continuous film mulching years in Longdong area, and the mulching film should be removed periodically to promote root growth and optimize soil structure.

Interactions between plants and coexisting microorganisms have significant impacts on plant growth, development, and health. Human domestication has resulted in significant differences between modern crops and their wild ancestors in physiological and genetic characteristics and growth environment, which will inevitably affect the interaction between crops and their microbiomes. Understanding the impact of domestication on the diversity and community structure of microbiome and the mechanisms involved is an important theoretical basis for application of microbiome during crop improvement and breeding. In this review, we summarize the research progress of the effects of domestication on the community composition and diversity of root and shoot microbiome (bacteria and fungi) in crops. We also analyze the involved action pathways in shaping crop microbiomes by domestication, considering the domestication effect on crop morphology, root configuration, exudates and other physiological characteristics, and the change in growth environment. The research directions that need to be focused on in this field were proposed.

Aims The study aims to find the relationship between root interspecific interactions and root morphological characteristics, plant height and ground diameter accumulations. The effects of different division methods for the root interaction and interspecific interaction dynamics of chestnut (Castanea mollissima)/tea (Camellia sinensis) agroforestry systems were investigated, in order to provide the scientific basis for the sustainable development in chestnut/tea agroforestry systems. Methods The potting experiment set three cropping system, chestnut/tea agroforestry system, monocropping chestnut and monocropping tea as research objects, and three division methods with solid, mesh, and without root barrier under chestnut and tea roots. Plant height and ground diameter data were fitted to logistic growth models to investigate the temporal dynamics of plant growth, and the allometric relationship between plant height and ground diameter of chestnut and tea were fitted to power function. The relationships between plant growth and root interspecific interaction were analyzed in terms of the fine roots developments. Important findings The results showed that the aboveground dry mass, belowground dry mass, total plant dry mass, root length, root surface area, root volume, fractal abundance and root length of fine roots (0.2-1.0 mm) of intercropped tea with plastic separation were significantly increased by 357.1%, 281.8%, 345.2%, 74.3%, 273.9%, 244.8%, 42.0% and 382.4%, respectively, compared with those of the corresponding sole tea. The asymptotic value of plant height of the intercropped tea with plastic separation was 30.9% higher than the monoculture tea. The asymptotic value of plant height and ground diameter of the chestnut with nylon separation was 21.9% and 28.2% higher than the monoculture chestnut, respectively. In the division methods with plastic, intercropped tea significantly postponed the timing to reach the maximum daily plant height growth rates about 14 days, and intercropped chestnut markedly postponed the timing to reach the maximum daily ground diameter growth rates about 15 days compared with corresponding monocultures. There was significantly positive relationship between plant height and ground diameter of both chestnut and tea in the different treatments. In addition, the slopes of the growth equations about both intercropped chestnut and tea without separation were flatter than the other treatments, which were both lower than 1. Therefore, when tea is intercropped with chestnut trees, chestnut tree shading enhanced the growth of lateral root branches, fine root length and plant height, and facilitated the intercropped tea seedling dry mass accumulation. However, with the growing stresses of underground interspecific competition for intercropped chestnut, the aboveground interspecific facilitation was overridden gradually by the interspecific underground competition, and the final net outcome was manifested as neutral effects.

Aims Roots are an important organ for plants to absorb soil water and nutrients and can drive multiple ecosystem processes. This study disentangles the effects and underlying mechanisms of experimental warming on root biomass in terrestrial ecosystems, aiming to better understand soil carbon dynamics and inform the changes in ecosystem processes under climate warming.
Methods In this study we compiled data on 611 paired observations from 151 published peer-reviewed articles, and analyzed the responses of several plant root biomass variables, including total root biomass, fine root biomass, coarse root biomass, and root:shoot ratio, to warming using meta-analysis. The responses of root biomass to the magnitude, duration and method of warming treatments, and the warming responses of root biomass in relation to background environmental conditions (i.e. ecosystem types, mean annual air temperature, mean annual precipitation, and aridity index) were examined.
Important findings Simulated warming significantly increased fine root biomass by 8.87%, but had no significant effects on total root biomass, coarse root biomass, and root:shoot ratio. Moderate magnitude of warming (1-2 °C) significantly increased fine root biomass and root:shoot ratio by 14.57% and 23.63%, respectively. While the short- to medium-term (<5 years) warming enhanced fine root biomass, a long-term warming (≥5 years) had a tendency to decrease it. Both open-top chamber and infrared radiators significantly increased fine root biomass by 17.50% and 12.16%, respectively; whilst heating cables significantly decreased fine root biomass by 23.44% and coarse root biomass by 43.23%. The warming responses of root biomass were inconsistent across different ecosystem types. Notably, warming significantly increased fine root biomass by 21.03% in tundra ecosystems. The response of fine root biomass to simulated warming had significant and negative correlations with the background mean annual air temperature, mean annual precipitation, and aridity index.

Arbuscular mycorrhizal (AM) fungi are a group of soil-dwelling fungi that can form symbiotic associations with most terrestrial plants. The extraradical mycelium can colonize different plant roots in addition to hyphal fusion, thus form extensive arbuscular mycorrhizal networks (AMNs) underground. AMNs can transport and recycle water and nutrients including carbon, nitrogen, phosphorus among plants, recent evidences show that AMNs can also transfer defensive signals to neighboring plants when plants suffer environmental stresses, thus providing early warning to surrounding neighbors. However, the research on AMNs-mediated signal transfer is still in its infancy. Here, we firstly reviewed current research progresses in this research area, then proposed the unanswered questions that worth exploration in the future, including the possible pathways and mechanisms of signal transfer via AMNs among plants, the possible regulation of mycorrhizal symbionts by AMNs-mediated signal transfer, and the common techniques and their development used in the study of AMNs. Finally, we discussed about the possible ecological applications of AMNs such as filed crop protection.

Aims Phosphorus is one of the major limiting nutrients for plant growth in subtropical areas, whereas increasing nitrogen deposition may be a limiting factor in determining the availability of soil phosphorus. Here, focusing on soil microorganisms and plant fine roots, we explored the transformation of soil phosphorus to unravel the maintenance of soil phosphorus supply and plant productivity with low availability under nitrogen deposition.

Methods At the Fuzhou Changʼan Mountain in Fujian Province, China, control (0 kg·hm^-2·a^-1), low nitrogen (40 kg·hm^-2·a^-1), and high nitrogen (80 kg·hm^-2·a^-1) treatments were set up to simulate nitrogen addition. Soil and root samples of Cunninghamia lanceolata seedlings were then collected to comprehensively analyze soil phosphorus and nutrient contents as well as microbiological-plant root characteristics.

Important findings The results showed that the contents of soil labile organic phosphorus, moderately labile inorganic phosphorus and occluded phosphorus were significantly increased, whereas those of primary mineral phosphorus and moderately labile organic phosphorus decreased under the low nitrogen treatment as compared to the control treatment. However, there were no significant changes under the high nitrogen treatment. Redundancy analysis indicated that soil acid phosphatase activity, relative abundance of mycorrhizal fungi, soil microbial biomass phosphorus content, and root biomass were important soil microbiological-plant root characteristics factors that could explain the changes in soil phosphorus components. Variance partitioning analysis revealed that the soil microbiological-plant root characteristics synergy explained 57% of the alternations in soil phosphorus components, whereas correlation analysis showed a significant positive correlation between the relative abundance of mycorrhizal fungi and root biomass. Overall, these results suggest that mycorrhizal colonization is promoted under a low level of nitrogen input and the synergistic action of mycorrhizal fungi and C. lanceolata fine roots promotes the conversion of moderately labile organic and primary mineral phosphorus to labile phosphorus, thus maintaining the growth of C. lanceolata seedlings.

Aims The plant absorptive roots function to absorb water and nutrients. Investigations of anatomical traits of the roots are to understanding environmental adaptations of plant species. Ferns in tropical and subtropical regions are abundant and of important ecological and economic values. However, the anatomical traits of the absorptive roots of ferns are not fully comprehended.

Methods We investigated anatomical traits of absorptive roots for 26 fern species from four typical tropical/ subtropical forests by analyzing inter-specific differences in the traits across the species to explain the influence of phylogeny and climate. In addition, we compiled relevant root traits of subtropical angiosperm tree species and temperate fern species from literature to explore the trait differences among the groups.

Important findings (1) We found significant differences in eight root anatomical traits among the 26 fern species, with coefficient of variation ranging from 20.61% to 41.75%. (2) Root traits showed no significant phylogeny signal except in cortex thickness (CT), indicating little affection from phylogeny. However, climate might exert significant impacts on root traits, i.e., root diameter (RD) and CT significantly increased with decreasing precipitation of the driest month (quarter). (3) As RD decreases, the subtropical angiosperm woody plants showed a significant decrease in the ratio of CT to stele diameter (SD), but fern had an opposite pattern. Compared to temperate ferns, the tropical and subtropical ferns had higher RD, CT, and tracheid diameter (TD).

Aims To determine whether there is hydraulic redistribution in the root system of Populus tomentosa, and to explore its characteristics and influencing factors.
Methods The heat ratio method was used to monitor the long-term sap flow of 7 lateral roots (R1-R7) of four-year P. tomentosa trees, and the soil moisture and meteorological factors were measured simultaneously.
Important findings This study showed two patterns of hydraulic redistribution of P. tomentosa, namely, drought-induced hydraulic lift and rainfall-induced hydraulic descent. The occurrence and characteristic of hydraulic redistribution were affected by the distribution depth and diameter size of the lateral roots. In general, the magnitude of hydraulic redistribution was relatively low. In the growing season, the amount of water redistributed by P. tomentosa roots was low; however, under extreme drought conditions, the amount of water redistributed by the lateral roots could reach 64.6% of its total daily sap flow, indicating that hydraulic redistribution would provide plenty of water for dry lateral roots. This study showed that the root water uptake was significantly related to the meteorology-soil coupling factors (solar radiation (R_s) × soil water content (SWC), vapor pressure deficit (VPD) × SWC, reference evapotranspiration (ET_o) × SWC), but not to the hydraulic redistribution. In addition, this study found a unique daytime reverse sap flow occurred in shallow lateral roots. The reverse sap flow could account for up to 79.2% (R1) and 90.7% (R2) of the total daily sap flow, which could play an essential role in the drought resistance of shallow roots.

Aims Accurately quantifying the contribution of shallow, middle and deep soil water sources to the root water uptake is the prerequisite for understanding water uptake strategy of plants. This paper evaluates the effects of different water isotopes input methods on plant water sources analysis results in Bayesian mixing model MixSIAR.

Methods Soil and plant xylem samples were taken five times from May to September in 2019 in two apple (Malus pumila) orchards at 7- and 18-year age in Changwu Tableland of Shaanxi Province. Soil water contents and isotope ratios (δ²H and δ¹⁸O) were measured, and the different input methods of single isotope (²H and ¹⁸O), dual isotopes (²H & ¹⁸O) and xylem hydrogen corrected dual isotopes (²H(+8.1) & ¹⁸O) coupled with MixSIAR model were used to estimate the contribution ratio of different soil layers (0-0.4, 0.4-2, >2 m root depth) to orchards root water uptake.

Important findings The results showed that compared to the ²H isotope method, the contribution from soil layer below 2 m was lower and that from the surface 0-0.4 m was higher using the ¹⁸O isotope method, which was close to the ²H(+8.1) & ¹⁸O isotope method. Compared with ²H & ¹⁸O dual isotopes method, the contribution ratio from surface 0-0.4 m soil layer was higher using the ²H(+8.1) & ¹⁸O method when the surface soil water isotope was enriched, and that was lower when the surface soil water isotope was depleted. The corrected apple xylem hydrogen isotopes were closer to the evaporation line of soil water isotopes, thus the analysis methods of ¹⁸O and ²H(+8.1) & ¹⁸O accorded more with the isotope mass balance during root water uptake than ²H and ²H & ¹⁸O methods. Soil water contents in 0-2 m in 18 years old apple orchard showed greater seasonal variation than that in 7 years old apple orchard, and are more dependent on 0-0.4 m surface soil water. For the root water uptakes in 7- and 18-year apple orchard, the yearly-averaged contributions of deep soil water were 19% and 23%, respectively, showing no significant difference. We suggest that more attention should be drawn on the influence of different isotope input methods when using water stable isotopes to estimate plant water sources contribution in future studies.

Aims The research on dynamic niche partitioning of soil resource uptake is crucial for the understanding of plant coexistence mechanisms. However, there are still knowledge gaps in the interpretation of this field.

Methods In the growing season of 2019, a mature mixed forest of Populus tomentosa and Robinia pseudoacacia on the North China Plain was repeatedly sampled for water isotopes and soil water content, and fine root sampling was performed at the end of the growing season. Seasonal water uptake patterns of trees were determined by the hydrogen-oxygen stable isotope method and Bayesian mixture model (MixSIAR). The degree of niche overlap between P. tomentosaand R. pseudoacaciawas assessed by Pianka’s normalized overlap value.

Important findings Both tree species have deep root systems. Nevertheless, P. tomentosa tended to develop horizontal lateral roots and a higher proportion of fine roots were distributed in the shallow soil layers (0-30 cm). In contrast, R. pseudoacacia inclined to develop vertical taproots and a high proportion of fine roots were distributed in the deep soil layers (100-600 cm). In terms of the mean values of the whole growing season, the primary water sources for P. tomentosa and R. pseudoacacia were water from the middle (30-100 cm) and deep soil horizons. However, the contribution of water from shallow and middle soil horizons to the water uptake of P. tomentosa was higher than that of R. pseudoacacia, while the opposite was true for deep soil water and groundwater for the two species. Populus tomentosa and R. pseudoacacia showed completely opposite water uptake strategies in response to drought and heavy summer rainfall. During the dry season, P. tomentosa enhanced the water uptake contribution of the middle soil layer, while R. pseudoacacia promoted the relative water uptake from the groundwater. When heavy rainfall events occurred, P. tomentosa increased the water uptake contribution of the shallow soil layer, while R. pseudoacacia increased the water uptake contribution of the deep soil layer. In conclusion, there was niche complementarity in the fine root and water uptake between P. tomentosa and R. pseudoacacia. Furthermore, the degree of niche complementation in water uptake varied with seasons, and the water-uptake niche complementation degree in the dry season was relatively higher than that in other seasons. In addition, the study also showed that the root niche partitioning was not representative of the water uptake partitioning of trees. This study provides support for further understanding of plant coexistence mechanisms, and will provide an important reference for the formulation of future mixed forest management strategies to cope with climate change.

Selenium is an essential micronutrient element for humans. The range between “beneficial” and “harmful” levels of selenium is very narrow, so biofortification through plants is a safe and effective way to supplement selenium. This article reviewed the processes of selenium uptake, transport, and metabolism in plants. Plants mainly absorb selenate, selenite, and organic selenium from soil. The root system has different absorption mechanisms for different forms of selenium, and the absorption process is participated by different transporters. The absorbed selenium is mainly transported in plants in the form of selenate ion, transported to the aboveground through the xylem and the phloem, and metabolized under the action of various enzymes. Finally, part of the selenium absorbed by the roots is stored in the plant as organic selenium, and the other part is released into the atmosphere in the form of selenide. This article also focuses on the effects of different types of rhizosphere microorganisms on plant selenium biofortification. Arbuscular mycorrhizal fungi, ectomycorrhizal fungi, and rhizosphere promoting bacteria can promote the absorption of selenium in plants to a certain extent, but their internal mechanisms are still unclear. Based on the current research status, the future research focuses are put forward: 1) the process of selenium absorption by plants and its gene regulation; 2) the underlying mechanism and application potential of microorganisms on selenium biofortification in plants.

Fine roots are the most active and sensitive part of the root system, and play an important role in the biogeochemical cycles of forest ecosystems. Fine root growth and turnover have a strong impact on the root carbon flux into soil. The effect of global warming on below-ground ecological processes has become a hot issue in global change research, and the response of fine root dynamics to warming will directly affect the carbon balance of forest ecosystems. In this paper, the effects of warming on fine root production, mortality, biomass and turnover are reviewed to reveal the effects of warming on fine root dynamics. Generally, warming affects the fine root production and mortality by changing soil moisture, nutrient availability and distribution of recent photosynthetic products, accelerates fine root turnover process, and then reduces fine root biomass. However, fine root growth is affected by many factors, making the research results of the impact of warming on fine roots inconsistent among different studies, due to the difference in tree species, regions, warming methods and other factors. Therefore, comprehensively analyzing the response of forest fine roots under warming is very important for studies on below-ground ecological processes. In the future, we call for more research in the following aspects: (1) according to the advantages and disadvantages of each warming method, compare the effects of different warming methods and warming durations on the growth dynamics of fine roots and above-ground parts; (2) combined with various fine root observation and experimental methods, comprehensively analyze the effect of warming on fine root growth dynamics, and strengthen the research on the effect of warming on fine root order structure; (3) strengthen the research on the interaction of warming and nutrient, water and CO₂ on fine root growth dynamics; (4) focus on the effects of warming on fine root dynamics in different soil layers, especially in deep soils; and (5) deeply study the potential response of the relationship among fine roots, soils and microorganisms to warming.

Aims The aim of this research was to clarify the effects of intercropping and inoculation with arbuscular mycorrhizal fungi (AMF) on carbon transport and phosphorus uptake in black locust (Robinia pseudoacacia) and konjac (Amorphophallus konjac). The results could provide empirical evidence to reveal the mechanisms of black locust intercropping for disease control and plant growth promotion of konjac, and popularize the green and high-yielding cultivation technique of konjac under black locust.

Methods The experiment was carried out in two-compartment rhizoboxes separated by a 25-μm nylon net, each of which comprised compartment A (non-inoculated or AMF-inoculated black locust) and compartment B (monocropped black locust or intercropped konjac). A ¹³C stable isotope labeling technique was used to label the leaves of black locust in compartment A with ¹³CO₂. Carbon transport from black locust to konjac and the effects of AMF colonization on agronomic traits, ¹³C abundance, and phosphorus content in both crops were investigated.

Important findings The result showed that: (1) After inoculation, the AMF infection rate of black locust and konjac plants by hyphal links in compartment B reached 47.1% and 60.4%, respectively. For black locust, this AMF infection rate was 14.1% lower than that of directly inoculated plants under monocropping. In the case of intercropping, the biomass (dry mass) of AMF-inoculated konjac plants was 9.7% (aboveground parts) and 36.2% (belowground roots) higher than that of non-inoculated plants. (2) Compared with the non-inoculated plants under monocropping, the carbon fixed by photosynthesis of black locust plants in other treatments (non-inoculated + intercropping, inoculated + monocropping, and inoculated + intercropping) was mainly allocated to the plant roots and rhizosphere soil in compartment A, and more carbon passed through the nylon net in the form of root exudates to reach the rhizosphere of neighboring crop plants. (3) Compared with the respective non-inoculated controls, AMF inoculation in the monocropping and intercropping systems prominently improved phosphorus contents in the leaves, stems/petioles, roots, and total plants of black locust and konjac in compartment B. The findings suggest that AMF colonization could facilitate carbon transport from black locust to the rhizosphere soil and plant tissues of konjac. Intercropping konjac with black locust is an effective practice to promote AMF colonization and phosphorus uptake by both host plants.

Aims Due to complex root-soil interactions, the responses of carbon (C) dynamics in the rhizosphere soil to nitrogen (N) deposition may be different from those in bulk soil. However, the potentially different responses of C dynamics between the rhizosphere and bulk soil and their contributions to soil C sequestration under N deposition are still not elucidated.
Methods In this study, a typical subalpine coniferous plantation (Picea asperata) with chronic N addition treatments in southwestern China was selected as the research object. Based on the experimental plots of simulated N deposition (control: 0 kg·hm^-2·a^-1; N addition: 25 kg·hm^-2·a^-1), we measured the contents of soil organic carbon and its different physical and chemical fractions. Afterwards, by combining the rhizosphere spatial numerical model, we explored the differences in the C pool size of SOC and its fractions and their relative contribution to SOC pools between the rhizosphere and bulk soil, and further quantified the effects of N addition on soil C sequestration in rhizosphere soil.
Important findings The results showed that: 1) Although the addition of N increased the content of SOC and its physical and chemical components in the rhizosphere and non-rhizosphere at the same time, it only reached a significant level in the rhizosphere. Specifically, the rhizosphere SOC content increased by 23.64% under N addition, in which particulate organic carbon (POC), mineral-associated organic carbon (MAOC), labile carbon (LP-C) and recalcitrant carbon (RP-C) content increased by 19.63%, 18.01%, 30.48% and 15.01%, respectively. 2) The total SOC pool increment of spruce forest (0.88 kg·m^-2) was verified with the results of the rhizosphere space numerical model, and the effective rhizosphere extent of the southwest mountain coniferous forest was estimated to be 1.6 mm. Within this extent, N addition increased the SOC stocks of the rhizosphere and bulk soil by 33.37% and 7.38%, contributing to 45.45% and 54.55% of the total SOC pool increment, respectively. Among them, labile C components (POC and LP-C) are the major contributors to rhizosphere SOC accumulation under N addition. These results suggested that the rhizosphere and bulk soil of coniferous forest in southwestern mountainous area had great C sequestration potential under N deposition, and the C sink was more obvious in the rhizosphere soil. Our results highlight the importance of integrating rhizosphere processes into land surface models to accurately predict ecosystem functions in the context of increasing N deposition.

Trade-offs among plant functional traits reflect the trade-off relationships between resource acquisition and conservation of different plants, which are of pivotal importance for understanding the mechanisms by which plants adapt to the environment. However, due to the heterogeneity of the soil environment and the limitations of technical means, the study of below-ground root functional traits and their interrelationships is currently lagging behind that of above-ground functional traits. Traditionally, fine roots have been defined as all roots ≤2 mm in diameter. The acquisition and utilization of soil resources by plants depends on architectural traits, morphological traits, chemical traits and biotic traits of fine roots and so on, including fine roots associations with mycorrhizal fungi. Recently, the root economics space has been proposed, which demonstrates the existence of trade-offs between the do-it-yourself strategy of plants increasing their own root surface area and the outsourcing strategy of investments into fungal symbionts for enhanced nutrient mobilization from hyphal expansion, in addition to the traditional trade-offs between fast (high nitrogen content and metabolic rate) and slow (high tissue density) investment return. It was found that thin-root species obtained soil resources mainly by increasing specific root length, whereas thick-root species relied more on mycorrhizal fungi. However, the carbon economy of resource acquisition through the root and mycorrhizal hyphal pathways remains unclear. In future research, the key issues of root functional traits were suggested as follows: 1) for research methods, it is urgent to establish a unified set of definitions and research methods for root classification, sampling, storage, functional traits and their research methods; 2) for research traits, the research of “hard” traits of fine roots should be strengthened; 3) for the trade-offs between functional traits of fine roots, it is of great significance to strengthen the study of the trade-offs between construction costs and resource benefits between plant roots and mycorrhizal fungi.

The fine root phenology of forests is an important indicator to observe the impact of global warming. It can reflect not only the growth status of forests under the background of global change but also the dynamics of the carbon cycle and below-ground carbon distribution of terrestrial ecosystems. Meanwhile, the response of forest fine root phenology to climate change is considered a hotspot and challenge in the field of global change effects, and thus has been studied extensively. Recent studies claim that soil warming will prolong the growing season of fine roots in forests as the spring phenology and growth peak will begin earlier in some areas of the northern hemisphere, however, atmospheric warming may inhibit the growth of fine roots and delay the phenological events. In addition, some studies found that the root phenology in the surface soil may be more affected by warming than that in the deep layer. Concurrently, some researchers associated fine root phenology with rhizosphere soil environment, microorganisms and above-ground phenology to reveal the response mechanism. However, the response of fine root phenology to climate warming and its underlying mechanisms have not been fully explained. This paper systematically reviewed changes in fine root phenology in forests under global warming, and aimed to provide references for the research on below-ground phenology, and the response and adaptation mechanism of forests to global changes. Future studies should enhance the research on the following aspects: 1) improving and exploring more accurate simulation warming devices and carrying out long-term quantitative research; 2) exploring the relationship between different functional modules of roots (such as absorbing root/transporting root, fibrous root/pioneering root) and their phenology under changing environments, i.e. “environment-traits- phenology”; 3) considering variations of the control factors of root phenology under different below-ground phenological phases (the beginning, peak and end of root growth), species and soil layers; 4) focusing on the relationship between below- and above-ground phenology, and its impacts on plant productivity; 5) focus on the changes of forest below-ground phenology and ecosystem functions (such as carbon sinks, nutrient cycling, etc.) under the combined effect of warming and other environmental factors (CO₂ concentration, nitrogen deposition, etc.).