Plants have evolved diverse defense strategies to herbivory over time. Chemical defenses based on plant secondary metabolites play a key role in plants-herbivores interspecific interactions. There is a trade-off between the biosynthesis of secondary metabolites for plants and resource allocation to growth and reproduction. Recently, numerous studies have examined how plant secondary metabolites affect behavior of herbivores as well as plant growth and fitness. However, a comprehensive and in-depth elaboration of the chemical defense strategies of plants is still lacking. We systematically review the factors influencing the synthesis and release of plant secondary metabolites, the chemical defense strategies and their formation mechanisms in response to herbivory. Plant tissues and organs, population and species composition in the community, species identity and feeding intensity of herbivores, soil resource availability, seasons, environmental stresses all can affect the synthesis and release of plant secondary metabolites. Plants respond to herbivory by enhancing the plasticity of chemical defense, regulating the partitioning pattern of photosynthetic products and the trade-offs relationship between growth, reproduction and defense. Many hypotheses have been proposed to explain plant chemical defense strategies, including growth-differentiation balance hypothesis, plant apparency hypothesis, optimal defense theory, carbon/nutrient balance hypothesis, growth rate hypothesis, plant defense syndromes hypothesis, and error management theory. As the intensification of human activities (e.g., livestock grazing) and climate change, researches on plant defense strategies in response to large herbivore feeding, environmental stress and global change should be strengthened in the future with a multidisciplinary perspective, which will be helpful for deeper understanding of the defense processes and mechanisms of plants in response to herbivores.

Biogenic volatile organic compounds (BVOCs) are secondary metabolites of plants that play various ecological roles, such as defense and signaling. They have become a popular topic in environmental and ecological studies. Nitrogen deposition is a dual factor of nutrient and environmental stress for plants that alters the emission of plant defense-related BVOCs (dBVOCs). It affects inter-species communication, their survival and development, and structure and function of the entire ecosystem. However, relevant research is limited. In this study, we focused on three types of dBVOCs, namely, terpenes, green leaf volatiles, and methyl salicylate, and elucidated their main components, synthesis pathways, ecological functions, and responses to various biotic and abiotic stressors. We further examined the impact of nitrogen deposition on these dBVOCs, exploring their response trends and related mechanisms under nitrogen-phosphorus limitation. Future research is necessary to deepen and expand the understanding of dBVOCs by uncovering their underlying mechanisms. Additionally, efforts should be made to overcome the challenges in research methods and detection devices for dBVOCs and strengthen interdisciplinary collaboration. Such advancements can provide theoretical support for tree planting management, green pest prevention and control, and promotion of sustainable development in the context of global changes.

Aims In the context of global climate change, frequent extreme precipitation events will affect the soil nitrogen transformation processes in the semi-arid steppe. However, it remains unclear how the key processes of soil nitrogen transformation respond to precipitation change and how the total nitrogen mineralization rate is sensitive to different precipitation levels.

Methods This study was conducted on the meadow steppe, typical steppe, and desert steppe of the Global Change Network test platform of the northern China Steppe. In this study, the gross nitrogen mineralization rate (GNM) and gross nitrification rate (GN) of soil were measured by the ¹⁵N isotopic pool dilution method, as well as related biological (microbial biomass carbon content, microbial biomass nitrogen content, background biomass (BGB)), and abiotic (soil temperature (ST), soil water content (SWC)), soil substrate (soil ammonium nitrogen (NH₄⁺-N) content, nitrate nitrogen (NO^-₃-N) content, soluble organic carbon content, soluble organic nitrogen content) indexes.

Important findings The results showed significant differences in GNM among different steppe types. The highest GNM was in meadow steppe ((3.284 ± 0.613) mg·kg^-1·d^-1), followed by typical steppe ((1.370 ± 0.167) mg·kg^-1·d^-1) and desert steppe ((0.724 ± 0.216) mg·kg^-1·d^-1). However, the 50% decrease in precipitation had no significant effects on the GNM and GN of the three grasslands. The sensitivity of GNM and GN to precipitation reduction in typical steppe and desert steppe soil was significantly higher than that of precipitation increase. In contrast, the sensitivity of GNM and GN in meadow steppe soil was not significantly different between precipitation increase and decrease. Structural equation model (SEM) analysis revealed that SWC was the main factor affecting the soil GNM. These results indicate that short-term extreme precipitation has no significant effects on the GNM and GN of three important grasslands in northern China, but changes their sensitivity to precipitation change. How the increase of extreme precipitation events will affect the soil nitrogen conversion process in terrestrial ecosystems in the future needs to be systematically studied over long time scales and large spatial patterns.

Aims In the salinized grassland of the agro-pastoral ecotone with limited nutrients, the increase of nitrogen input induced by agricultural fertilization and the change of land use patterns usually causes changes in soil available nutrients, which could further affect the net primary productivity of plants. However, due to the impact of annual precipitation, it is still uncertainty about whether the response of net primary productivity of plants to nitrogen addition and mowing varies with changes in rainfall.

Methods This study investigated plant net primary productivity under nitrogen addition and mowing at salinized grassland in the agro-pastoral ecotone of northern China, based on the experimental platform for nitrogen forms and mowing at the National Research Station of Grassland Ecosystems on the Loess Plateau in Youyu, Shanxi Province. There were six treatments including control and simulated nitrogen deposition experiments with two common nitrogen compounds (ammonium nitrate and urea), combined with and without mowing.

Important findings The results showed that: (1) Regardless of mowing or un-mowing treatments, short-term addition of ammonium nitrate and urea significantly increased the content of inorganic nitrogen in the soil, thereby increasing the aboveground (ANPP), belowground (BNPP), and total net primary productivity (NPP); (2) ANPP, BNPP, NPP, inorganic nitrogen content, and soil water content showed significant interannual differences, with higher values observed in the wet year (2018) than in the dry year (2017); (3) The interaction between short-term nitrogen addition and year had a significant impact on NPP. In the wet year, the positive effect of nitrogen addition on NPP was significantly higher than that in the dry year, which was mainly related to the synergistic effect of soil nitrogen and water; (4) Mowing decreased NPP and had a significant interactive effect with the year on the BNPP:ANPP. In the dry year, mowing generally decreased BNPP:ANPP. However, in the wet year, this negative effect gradually weakened and even turned into a positive effect. These results highlighted the crucial role of natural precipitation in regulating the response of net primary productivity of salinized grassland in the agro-pastoral ecotone to anthropogenic disturbances, and further indicated that the salinized grassland ecosystem in the agro-pastoral ecotone is jointly limited by nitrogen and water.

Aims The objective of this study was to explore the effects of long-term nitrogen (N) addition on leaf litter production and nutrient return of the dominant tree species in broadleaf evergreen forests of subtropical regions.

Methods A long-term N addition manipulation experiment with three levels (0 kg·hm^-2·a^-1, CK; 20 kg·hm^-2·a^-1, LN; and 40 kg·hm^-2·a^-1, HN) was conducted to examine the effects of N addition on leaf litter production, contents of leaf litter carbon (C), N, and phosphorus (P), as well as their return in the Michelia wilsonii forests located on the western margin of the Sichuan Basin.

Important findings (1) Significant inter-annual variations were observed in the production of M. wilsonii leaf litter, as well as in the mean contents of litter C, N, and P and their annual return, but N addition did not affect these inter-annual dynamics. (2) The HN treatment significantly increased the annual leaf litter production of M. wilsonii by 24.9%. Additionally, the effect of N addition on the average annual litter production decreased as the duration of N addition increased. (3) The HN treatment resulted in a significant increase in the mean content of C of M. wilsonii leaf litter by 3.4%. The effects of HN treatment on leaf litter N and P contents exhibited a turning point during 2019-2020. Specifically, the annual mean N content decreased from 2016 to 2019, but then experienced a significant increase after 2019. In contrast, the annual mean P content decreased from 2018 to 2019, but then experienced a significant increase in 2020. (4) N addition significantly increased the annual return of C, N, and P in leaf litter, with higher effect as the level of N addition increased. However, the effect of N addition on the annual P return showed a significant linear decline over the study period. These findings highlight that high N additions significantly promoted the productivity of M. wilsonii forests and increased carbon and nutrient inputs within the ecosystem. Distinct effects of short-term versus long-term N inputs on the N cycling processes of M. wilsonii forest ecosystems were observed. Additionally, the future exogenous N addition may potentially inhibit the leaf litter yield and P return of M. wilsonii.

Aims Plants and soil microorganisms play crucial roles in regulating the function and stability of terrestrial ecosystems. Exploring the variations in community composition and diversity of plants and soil microorganisms and their driving mechanisms along altitude gradients provides a better understanding of their responses to changes in terrestrial ecosystems and the mechanisms that maintain biodiversity under global change.

Methods In this study, a vertical gradient experiment was conducted across three grassland types on the northern slope of Kunlun Mountains. High-throughput sequencing was employed to assess soil bacterial and fungal communities. The study examined the patterns of variation in community structure, composition, and species diversity of plants and soil microorganisms along an altitudinal gradient (2 200-3 800 m) and explored their potential interrelationships.

Important findings The species diversity and productivity of plants, as well as the diversity of soil microorganisms (including bacteria and fungi) and soil nutrients content, all increased monotonically along the altitudinal gradient, with peak values observed in the alpine steppe. Soil fungi were more sensitive to environmental gradient than bacteria. The aboveground plant community and soil fungal community showed significant variation along the altitude gradient, while the structure and composition of the soil bacterial community remained largely unchanged. The species diversity and aboveground biomass of the plant community were significantly correlated with the diversity of the soil microbial community, with a stronger relationship observed with bacterial diversity than with fungal diversity. The species diversity of soil microorganisms and plants was positively correlated with mean annual precipitation (MAP) and soil carbon and nutrient (phosphorus and potassium) contents, but negatively correlated with mean annual air temperature (MAT). The pattern analysis showed that altitude (geographical distance) and climate factors had greater contributions to microbial and plant diversity than soil factors. The variation in climate factors (MAT and MAP) due to the altitude gradient was the dominant drivers of community dynamics in both plant and soil microorganisms, though their roles differed. Soil nutrients also played an important role in the diversity of both soil microorganisms and plants. In summary, the regional distribution pattern of aboveground plant and soil microbial communities in mountain steppe ecosystems is co-regulated by climate and soil factors associated with the altitude gradient in arid regions. This study highlights that climatic factors in arid areas are the dominant drivers of variations in the distribution pattern and structural composition of plant and soil microbial communities. Moreover, soil microbial diversity, especially bacterial diversity, plays a crucial role in enhancing grassland community productivity. The findings of this research provide valuable insights for grassland ecosystem management and biodiversity conservation in arid regions.

Aims In order to understand the interspecific competition strategy of the new fruit-soybean agroforestry system and find a planting pattern that balances ecological and economic benefits, the Malus pumila‘Saiwaihong’ - Glycine max agroforestry system (MS) was set up.

Methods Through the study on root length density (RLD), specific root length (SRL), root vertical center, competitiveness index and interspecific relative competitiveness of the MS, their interspecific competition strategy was revealed.

Important findings The results showed that the RLD of intercropping fruit trees and soybean was lower than that of monoculture, and the trend of SRL was opposite. Soybean roots were mainly distributed at 0-20 cm soil depth and 150-200 cm away from the fruit tree, and decreased with the increase of soil depth and the decrease of distance from the fruit tree. The root system of intercropping ‘Saiwaihong’ mainly distributed in the soil depth of 20-40 cm. Within the growth period, the vertical center of fine roots moved downward. In the MS, each plant adopted different interspecific competition strategies. Fine roots of soybean became a strong competitor in the surface soil of the MS by increasing SRL and reducing root mass. Intercropping ‘Saiwaihong’ could reduce niche overlap and resource competition with intercropping soybean by moving down the root distribution center, lengthening root length and increasing root mass. The overall interspecific relative competitiveness of ‘Saiwaihong’ was stronger than that of soybean, but the competitiveness index of soybean was still stronger than that of ‘Saiwaihong’ in the topsoil. The yield of intercropping soybean was 34.12% lower than that of monoculture. There was no significant difference in the yield of ‘Saiwaihong’. The land equivalent ratio (LER) and income equivalent ratio (IER) of the agroforestry system were greater than one. The results showed that the underground competition of MS was weak, and it had good intercropping advantages which were suitable for planting in Xinjiang. The results of this study can provide a reference for the selection of planting patterns of agroforestry systems in arid and semi-arid areas.

Aims To understand the current state of the population of Fraxinus sogdiana, a second-grade endangered species in China, and predict its development trend, a large sample survey was conducted in the Xinjiang Ili Fraxinus sogdiana National Nature Reserve.

Methods By replacing the age structure with the diameter at breast height structure, a population static life table was compiled and related population structure was analyzed. The survival curve, mortality curve and survival analysis function curve were drawn, and population dynamic index was calculated, and the population fluctuation cycle was explored via using spectral analysis.

Important findings 1) The populations of F. sogdiana on the east and west of the Kax River and total population exhibited growing age structure, with the survival curves towards Deevey-II type. 2) The mortality peaks for the population on the east of the Kax River and total population were in the age classes of IV, VII, and X, while those on the west of Kax River population class had a mortality peak in age class V. 3) The dynamic change index ignoring the external interference (V_pi) and dynamic change index under external interference ${{{V}'}_{pi}}$ of the populations on the east and west of the Kax River and total population were greater than 0, indicating that they were growing-type populations. But the ${{{V}'}_{pi}}$ of population on the east of the Kax River was close to 0, suggesting that it was more resistant to external random environmental interference. 4) Survival analysis revealed that the survival rate and cumulative mortality rate of populations in different habitats reached equilibrium in age classes II-III. The hazard rate curve of the population on the east of the Kax River exhibited relatively smaller fluctuations, indicating that this area is more suitable for the growth of F. sogdiana. 5) Spectral analysis showed that the fluctuations of F. sogdianapopulations are primarily controlled by their own biological characteristics. The population on the east of the Kax River and total population exhibited small cycle fluctuation characteristics with multiple harmonics superimposed during the large cycle. It is recommended protecting and improving habitat of F. sogdiana, accurately nurturing individuals in age classes I and II, and promoting the natural regeneration and development of the population.

Aims Alfalfa (Medicago sativa) is an important forage in the intercropping system. However, the extreme weak light environment in the intercropping system often leads to a significant decrease in alfalfa yield, and breeding shade-tolerant cultivars is the main way to solve this bottleneck problem.

Methods In this study, 20 alfalfa germplasm resources were used as test materials, and the shading rates were 0 (full light), 52.9% (moderate shading) and 71.8% (heavy shading) were used to determine the changes in six morphological indexes (stem diameter, height, number of primary lateral root, number of root nodule, number of root crown tiller, angle of stem to leaf), seven growth indexes (leaf mass, plant biomass, leaf area, specific leaf area, the percentage of leaf mass, stem mass and root mass in total biomass), and five physiological indexes (chlorophyll a content, chlorophyll b content, total chlorophyll content, carotenoid content, chlorophyll a/b ratio) during the tillering stage of alfalfa under different light intensities. Based on the shading rates of 0 and 52.9%, principal component analysis, membership function analysis, cluster analysis, and stepwise regression analysis were used to comprehensively evaluate the shade tolerance of alfalfa germplasm resources and screen their identification indicators.

Important findings (1) The stem diameter, the number of root nodule, and the number of root neck tillering of alfalfa decreased with the decrease of light intensity. The total chlorophyll content showed an increasing trend. The number of primary lateral root, leaf dry mass, biomass per plant, and leaf area showed a trend of first increase and then decrease, while the trends of other indexes were inconsistent, and there were significant differences among different cultivars. (2) Combining principal component analysis, membership function analysis and cluster analysis, the 20 test materials could be divided into three types according to their shade tolerance: shade tolerant (4 test materials), semi-shade tolerant (8 test materials), and shade-sensitive (8 test materials). (3) By stepwise regression analysis, the optimal regression equation was established: D = -0.108 + 0.071X₁₀ + 0.049X₆ + 0.208X₁₄ + 0.027X₄ + 0.096X₇ + 0.052X₃ + 0.048X₅, the precision of estimation was above 93.72%. Seven indexes, including leaf area, leaf dry mass, angle between stem and leaf, number of root nodules, number of primary lateral roots, number of root neck tillering, and chlorophyll a content were selected as shade tolerance identification indexes for alfalfa. The results would provide good materials and scientific basis for the breeding of alfalfa cultivars with shade tolerance, and contribute to the construction and optimization of alfalfa intercropping systems.

Aims The aim of this study was to better understand the regulatory mechanism of growth and development of Reaumuria songorica seedlings, and to provide some references for the cultivation and restoration of desert plants.

Methods This study uses one-year-old seedlings of R. songorica as materials to investigate the effects of photoperiods and light quality ratios on their growth. LED lights were used as the light source, and three photoperiods and six light quality ratios were set up for cross-cultivation. The plant height, branch number, biomass and photosynthetic pigment content of the R. songorica seedlings were measured to evaluate their growth, and the effects of photoperiod and light quality ratio on their growth were investigated.

Important findings The appropriate photoperiod and light quality ratio played an important role in the growth and development of R. songorica seedlings. Increasing the light duration can promote the growth of R. songorica. When the ratio of red- to blue-light was 3:1, the growth rate of plant height increased by 20.20% and 88.47% for 14 and 16 h daily light duration, respectively, in comparison with 12 h. When the ratio of red- to blue-light was 1:3 and the daily light duration was 14 and 16 h, the number of primary branches increased by 6.67% and 66.67%, and the underground biomass increased by 259.85% and 551.82%, respectively, in comparison with 12 h. High proportion of red-light and high proportion of blue-light treatment can promote the growth of R. songoricaseedlings, but the growth rate of plant height and the change trend of primary branch number is opposite. Under the light quality ratio of red- to blue-light was 4:1, the growth rate of plant height increased with photoperiod, while the number of primary branches decreased. However, under the light quality ratio of red- to blue-light was 1:4, the growth rate of plant height and the number of primary branches were opposite. Through the comprehensive application of principal component analysis and membership function analysis, the results show that the growth condition of R. songorica seedlings is better under treatments T2 (12-hour light, red-blue light ratio of 4:1) and T5 (12-hour light, red-blue light ratio of 1:4).

The Xiangquan River valley of Zanda in the western Ngari Prefecture is one of the main distribution areas of the mountainous desert on the Qingzang Plateau, but there is a lack of data on the background of desert vegetation in this area. Based on the field survey of 42 desert plots in Zanda County, Nagri, Xizang from 2018-2023, this study analyzed the classification and community characteristics of desert vegetation in the Xiangquan River basin. The results showed that there were 8 desert alliances in Zanda County, Xizang, including two temperate shrub deserts: Caragana versicolor desert and Ephedra intermedia desert, and six temperate semi-shrub and herbaceous deserts, Ajania fruticulosa desert, Ephedra gerardiana desert, Krascheninnikovia ceratoides desert, Krascheninnikovia eversmanniana desert, Lactuca orientalis desert and Seriphidium thomsonianum desert. Notably, five desert alliances (C. versicolor, E. gerardiana, K. eversmanniana, L. orientalis, and S. thomsonianum deserts) are newly recorded desert types. A total of 82 species of vascular plants belonging to 51 genera and 20 families were recorded. Among them, the life forms were mainly hemicryptophytes and perennial forbs. The water ecological type were mostly hyperxerophytic and typical xerophytes, including most of the edificators and companions of desert vegetation, such as Ajania fruticulosa, Ephedra intermedia, Krascheninnikovia ceratoides, Lactuca orientalis, and Krascheninnikovia eversmanniana. The Central Asia-Middle Asia component is the most important component of the geographical floristic type, followed by the Qingzang Plateau and the Eastern Paleoarctic. This study provides basic data and information for understanding the status and distribution of desert vegetation resources in this region, and for classifying the desert vegetation types in the Qingzang Plateau for better regional biodiversity conservation.