Researches on rhizosphere ecological processes and the underlying mechanisms have become one of the most active and sensitive hotspots in soil science. Root exudates have specialized roles in mediating the nutrient cycling and signal transduction within root-soil-microbe interactions. They are the key driving factors in regulating the functions of rhizosphere micro-ecosystem, and serve as a major premise for the concept and ecological processes in rhizosphere. However, due to the instinctive advantages of crops, such as short life cycles and convenient operation, most previous studies on root exudation mainly focused on agricultural ecosystems and were primarily targeted at providing practical guidelines. In contrast, there have been relatively few investigations on root exudates of trees, which highly limited the comprehensive knowledge of the potential mechanisms of root exudates in mediating soil biogeochemical processes in forest ecosystems. Hence, in this review, based on the main findings in our previous studies and the emerging frontiers in rhizosphere ecology, we specifically reviewed the ecological consequences and key remaining challenges in researches on root exudation in forests. Finally, we identify several topics and research outlooks for guiding future work to facilitate studies on root exudation and its ecological consequences in forest ecosystems.

Invasive alien plants not only influence plant community composition, biodiversity and ecosystem structure and function, but also have severe impacts on soil nitrogen transformation processes. The effects of invasive alien plants on nitrogen (N) cycling have been one of the hot topics in invasion ecology. Litter decomposition and its nutrient release play an important role in nutrient cycling. In addition, invasive alien plants have the potential to influence soil N transformation through allelopathy. All these processes are tightly related to soil microbes. Therefore, this review mainly focuses on litter decomposition and its nutrient release, and allelopathy to understand the effects of plant invasion on soil N transformation. Changes in soil N transformation and soil microbes (esp. Ammonia oxidizing bacteria and Ammonia oxidizing archaea) due to plant invasion, as well as the feedbacks of these changes to further invasion of alien plants were discussed. Finally, the interactions between arbuscular mycorrhizal fungi and plant invasion were reviewed.

Aims Temperature limit is the main cause of alpine treeline formation. Therefore, it is important to understand the response mechanisms of alpine treeline as well as their tree species under the global climate change. The present study focused on the spatio-temporal dynamics of treeline and ecological characteristics of the tree species in two treeline ecotones.
Methods Two vertical belt-transect plots were established in each treeline ecotone of the Zheduo Mountain and Jianziwan Mountain of the eastern Qinghai-Xizang Plateau. Top and bottom of each transects were lain between species line and forest line, respectively. Detailed information of each tree species treeline, including species name, latitude, longitude, height, age, base diameter, and coordinates, was recorded accordingly.
Important findings The temperatures of the two research areas have increased during the past 58 years. The precipitation has decreased slightly in both the Zheduo Mountain and Jianziwan Mountain. The age structure of Abies fabri from the Zheduo Mountain and A. squamata from the Jianziwan Mountain showed a reversed “J” shape curve and a bimodal shape, respectively. Within the two transects, due to the limitation of seed diffusion, the dominate species showed aggregated distributions at the small scale. At the large scale, A. fabri was aggregated at the Zheduo Mountain, while A. squanmata of the Jianziwan Mountain was randomly distributed due to the impact of surrounding environmental factors. Both tree height and base diameter decreased with the increase of elevation. The fir trees (Abies spp.) at the upper part of the treeline ecotone presented an allometric growth, whose height growth rate was higher than that of base growth, while the relationships between height growth and base growth were isometric at almost mid and lower part of the treeline ecotone. Compared with 10 years ago, there was no significant change at the position of treeline and tree species line of the Zheduo Mountain and the Jianziwan Mountain, neither of the tree density in the Jianziwan Mountain. However, the number of trees in the Zheduo Mountain increased by about 25%. Compared with 20 years ago, tree species lines of the Zheduo Mountain and Jianziwan Mountain were shifted upwards by 50 and 30 m, respectively. Besides, their treeline positions were increased by 75 and 40 m, respectively. Furthermore, the number of trees also increased significantly by 220% and 100%, respectively. Therefore, the treeline and its constructive species are mainly affected by temperature at the large spatio-temporal scale, while influenced by temperature and ambient environment at the small spatio-temporal scale.

Aims Root functional traits and their variations mediate coexistence and adaptive strategy of plant species. Yet, strong environmental constraints may induce convergence of root traits among different plant species. To study the variations of root traits and clarify the diverse adaptive strategies across plant species, we sampled three alpine grasslands along a precipitation gradient in the Xizang Plateau.
Methods In three grassland communities along a precipitation gradient: Nagqu, Baingoin and Nyima from east to west of Xizang Plateau, we collected 22 coexisting plant species and measured three key root traits: 1st-order root diameter, 1st-order lateral root length and root branch intensity.
Important findings The main results showed that: (1) the root of plants in the alpine grassland was generally thin, and the interspecific variation was also small (22.76%); (2) the root diameter of 86% plant species was in the range from 0.073 mm to 0.094 mm. Compared with the thick-root species, thin-root species had a higher root branching intensity, but shorter lateral root length. In addition, at community-level, plants mainly increased root diameter and lateral root length, but reduced root branching intensity to adapt to the decreasing precipitation; while at species-level, the plant species exhibited diverse adaptive strategies along the precipitation gradient.

Aim The objective of this study was to determine the amount and distribution of exogenous phosphorus (P) in different organs, as well as their changes in Chinese fir (Cunninghamia lanceolata) under different P supply levels. The results could be used as scientific base for selecting P-efficient genotypes.
Methods Seedlings of two Chinese fir genotypes (M1 and M4), both with high P use efficiency, were treated with different P supply levels and quantified by using 32P isotope tracer for P distributions in different organs. The seedlings used in this study were selected by our team through previous research as the experimental materials.
Important findings We found that the distribution of exogenous P in M1 and M4 was the highest in the roots and the lowest in the stems, and at an intermedia level in the needles. The ³²P content of each organ under the same treatment was ranked as root > stem > needle on the horizontal projection plane. The exogenous P content by the roots, stems and needles of M1 and M4 under low-P treatment appeared lower than that under the high-P treatment. The blackening degree of low-P image of roots, stems and needles under the same treatment was also lower than that under high-P treatment. The content of exogenous P in these organs under the low-P treatment increased slowly, indicating that the low-P stress significantly affected the absorption and accumulation of P in the seedlings. P allocation rates in the roots of M1 and M4 showed an initial decreasing and increasing later under low-P stress, while under the high-P treatment, the root P level increased first and stabilizing later. These findings indicate that M1 and M4 could adapt to external low-P stress through redistribution of P within the plants by transferring P from roots to above-ground parts at the early stage under low P stress. With the extension of stressing time, P from above-ground parts was shifted to roots. However, the distribution of exogenous P in M1 and M4 was significantly different under the low P treatment. The distribution of exogenous P from the beginning to the end of M1 was greater in the roots than that in above-ground parts, while M4 showed a similar pattern in early stages but a higher rate toward the above-ground parts later. This indicates that M1 has a higher degree of strengthening P circulation in vivo than M4 with low P stress, i.e. the tendency of P transfer from above-ground parts to roots is stronger in M1 than in M4.

Aims The vulnerability of xylem embolism is one of the key physiological factors that are related to plant mortality. Vulnerability curves are typically used for determining the vulnerability of xylem embolism. However, the shapes of vulnerability curves vary with the methods of assessment, especially in plant species with long xylem vessels. This study aims to investigate the feasibility of using different methods for establishment of vulnerability curves.
Methods Robinia pseudoacacia branches, with long xylem vessels, were used as plant materials for comparison of three different methods in establishing vulnerability curves, including bench top dehydration, Cochard Cavitron centrifugation and Sperry centrifugation. In the Sperry centrifugation method, rotors of two different sizes were used to test the ‘open vessel artifact’ hypothesis.
Important findings The vulnerability curve established by the bench top dehydration method displayed an “s” shape, while both the Cochard Cavitron centrifugation and Sperry centrifugation methods produced “r” shape curves. Vulnerability curves derived from the bench top dehydration method and the centrifugation methods were significantly different. Using the Sperry centrifugation method, the R. pseudoacacia branch samples in the 14.4 cm rotor had a higher proportion of open vessels, while the embolic vulnerability curves established on the 27.4 cm and 14.4 cm long stem segments were similar, indicating that the Sperry centrifugation method does not produce “open vessel artifact”.

Aims Exotic plant invasion has been a global eco-environmental issue, which declines biodiversity and influences ecosystem structure and function. Over the past decades, more and more studies showed that influences of exotic plant invasion on soil nitrogen (N) cycles, and soil N availabilities can facilitate the success and aggravation of invading plants.
Methods Based on differences in soil N contents between invaded and uninvaded areas in natural ecosystems at the same study sites, this study explored magnitudes and ecophysiological mechanisms of soil N variations under exotic plant invasion.
Important findings Based on the data integrated from 107 papers, we found that contents of soil total N, ammonium-N, nitrate-N, inorganic N, microbial biomass N under exotic plant invasion were increased by (50 ± 14)%, (60 ± 24)%, (470 ± 115)%, (69 ± 25)%, (54 ± 20)% respectively relative to those under no invasion. The increment in the soil nitrate-N pool was highest, suggesting higher nitrification rate, which potentially promoted plant nitrate-N utilization and the coexistence of nitrate-preferring species. The increment of soil nitrate-N pool under invasion was higher in the temperate zone than the subtropical zones significantly. Invasion of N₂-fixing plants caused obviously larger increments of soil total N and nitrate-N contents compared to invasion of non-N₂-fixing plants. The invasion of woody and evergreen invasive plants caused larger increments of soil total N than herbaceous and deciduous plants, respectively. The increases in soil ammonium-N under invasion did not differ substantially among different life forms and showed no clear relationship with the percentage of N₂-fixing plants. Differently, soil nitrate-N showed much larger increments under invasion and showed positive linear relationship with the percentage of N₂-fixing invasive plants. The N₂-fixing function, litter quality and quantity of invasive plants are important factors regulating soil N mineralization and nitrification under invasion. This study provides novel insights into the mechanisms underlying the success and aggravation of plant invasion and into the relationships between soil N dynamics and plant functional traits in ecosystems under exotic plant invasion.