Understanding the response patterns and potential mechanisms of structure and function in grassland ecosystems to nitrogen (N) enrichment is essential to evaluate ecological impacts of external N input. The muti-level N manipulative experiment offers the possibility to explore the nonlinear response patterns and associated mechanisms of structure and function in grassland ecosystems to additional N input. In this review, we summarized the impacts of additional N inputs on community diversity, carbon (C) and N cycling in grassland ecosystems around the world. Numerous studies illustrated that N enrichment induced the decline of plant species diversity, plant functional diversity and soil bacteria richness in grassland ecosystems, yet the change of fungal diversity was not significant. Above- and below-ground plant productivity showed different responses to N input: aboveground plant productivity exhibited initial increasing and subsequent saturation trends, but root productivity and root:shoot ratio showed downward patterns, and root turnover rate appeared a single-peak pattern of first increasing and then decreasing with the continuous increase of N addition rate. Meanwhile, different C decomposition processes responded variously to N enrichment. Specifically, litter decomposition rates exhibited multiple response of “exponential decrease, liner increase or insignificant change with N addition level”. However, the relationship of soil respiration and CH₄ consumption with N addition was dominated by a single peak trend of increasing at low to medium N levels but declining at high N levels. Likewise, different soil C fractions showed multiple response patterns to N input. N addition generally stimulated soil C storage and particulate organic C accumulation, while the mineral-associated organic C exhibited divergent responses of “increase, unaltered, and decrease” along the N addition gradient. In addition, plant N uptake exhibited initial increasing and subsequent situation trends along N addition gradients, while different soil N transformation processes showed differentiated responses along N addition gradients and the relationship between N₂O emission and N addition rate varied among various grassland ecosystems. An exponential increase of N₂O fluxes with N addition rate was observed in temperate grasslands, while the patterns of first increase and then saturation or linear increase of the N-induced changes in N₂O emissions had been discovered in alpine grasslands. Future studies should focus on the nonlinear responses of rhizosphere processes and phosphorus (P) cycle to external N input, and also explore potential mechanisms from the aspect of multi-dimensional biodiversity changes.

Stable hydrogen and oxygen isotope analysis provides an important tool for calculating plant root water uptake amount, determining the contribution to plant water source, and evaluating plant water use strategy, and is thus of great relevance to ecohydrological studies with respect to exploration of the water transmission mechanism of the atmosphere-vegetation-soil system. However, the stable hydrogen and oxygen isotope ratios (δ²H and δ¹⁸O) offset between soil and xylem water can cause inconsistency in the calculated contribution rate of plant water source, but the reasons for differences in hydrogen and oxygen isotope results are unclear. In this review, we first briefly introduced the phenomenon of hydrogen-oxygen stable isotope ratio offset; secondly, the framework was constructed along the water transport path of the soil-plant-atmosphere continuum. We systematically expounded the natural effects of δ²H and δ¹⁸O offset in three interfaces (plant-atmosphere interface, soil-atmosphere interface, and root-soil interface) and two spaces (plant and soil layer). At the same time, we summarized the methodological artifacts that are associated with soil and xylem sample extraction and δ²H and δ¹⁸O determination technologies. Finally, we identify main knowledge uncertainties according to the existing research progress; and highlight three areas that deserve future research attention: the acquisition of isotope spatiotemporal data, the cause of micro-scale isotope offset, and the optimization of extraction and determination technology.

Aims Vegetation traits are one of the research hotspots in plant ecology and they reflect the strategies of plant survival. According to the CSR survival strategy model, plant species may be classified into three categories: C strategy that can maximize biomass in resource-rich environments (competitive species); R strategy that can rapidly intake resources and reproduce in environments with a high frequency of disturbance (opportunistic species); and S strategy that can maintain individual survival in resource-poor environments (tolerant species). Leaf traits have adaptive changes in response to environmental gradients, which have impacts on plant adaptation. The mechanisms of how leaf traits affect CSR survival strategies in the alpine grassland of Qingzang Plateau remain unclear. The objective of this paper is to investigate the spatial patterns of CSR survival strategies of alpine plants and the mechanisms by which environmental factors influence plant survival.
Methods We surveyed a field transect which consists of 53 sample sites in an alpine grassland on the Qingzang Plateau from July to August 2020. Vegetation traits of leaf area, leaf fresh mass, and leaf dry mass were measured and C, S, R values were calculated. Finally, we analyzed the critical drivers and mechanism of plant CSR strategies in response to geographical elements.
Important findings Our results showed that: (1) In the alpine grassland of the Qingzang Plateau, 41.6%-96.7% of plants are identified as S strategy. (2) With the increase of the longitude, the proportion of C strategy plants increased gradually from the west to the east, whereas along the altitude gradient, the proportion of C strategy plants decreased with the increasing altitude. (3) Random forest analysis showed that the contribution of precipitation to C strategy is the highest (25.74%), and the contribution of altitude to S strategy is the highest (27.34%). Additionally, both precipitation and temperature had significant effects on leaf area, and leaf water content significantly affects plant CSR strategies. In summary, results of our study highlighted that precipitation is the most critical factor that governs plant CSR survival strategies. This finding has important implications for studying the ecological adaptation along environmental gradients in alpine grasslands.

Aims The Qingzang Plateau is highly sensitive to global climate change. The unique natural conditions lead to extremely vulnerable vegetation and its ecosystem, making this region ideal for analyzing responses of vegetation to climate change. However, different types of vegetation may have different responses to seasonal variability. This study explores and analyzes vegetation changes on the Qingzang Plateau and the response characteristics of different vegetation types to moisture variations (i.e., dry and wet conditions) during the growing season.
Methods The standardized precipitation evapotranspiration index (SPEI) and the normalized difference vegetation index (NDVI) were used here as indicators of dry humidity and vegetation greenness, respectively. Sen’s slope estimation, BFAST model and correlation analyses were used to quantify the spatiotemporal variability of vegetation greenness and its response to drought-wet variations on the Qingzang Plateau from 2000 to 2018.
Important findings Results show that vegetation greenness on the Qingzang Plateau generally increased over the time period analyzed. Additionally, the rate of spatial variation reveals striking regional differences. The breaks of vegetation greenness occurred in most regions during 2012-2015, after which there was general upward trend after the breaks, the various trend is most apparent in northern Qingzang Plateau. Positive correlations between NDVI and multi-time scale SPEI were observed in most regions during the growing season, and gradually increased in the middle and latter part of the growing season. The responses of each vegetation type to SPEI also showed a distinct periodicity during the year. Meadow and steppe areas were more sensitive to multi-time scale SPEI than forest and shrub areas, and this response differed significantly during different stages of the growing season and for different time scales of SPEI.

Aims As an anomalous feature of the species-area relationship (SAR), the small-island effect (SIE) is an important pattern in the fields of island biogeography and biodiversity science. However, the existing work has rarely explored the SIE in fragmented mountaintops. Moreover, the underlying factors determining the SIE remain largely untested. In this paper, we aimed to evaluate the prevalence and underlying factors determining the occurrence of SIEs in mosses on mountaintops of the Middle Taihang Mountains.
Methods We investigated and compiled data on the species richness of all mosses and six most common moss families on 19 mountaintops of the Middle Taihang Mountains. For each of the seven taxonomic groups, we applied six SAR models, including four piecewise regressions with two segments, a linear regression, and a zero-slope regression as a null model, to detect the SIE and then used the corrected Akaike’s information criterion (AICc) as a criterion to select the best model. We obtained three environmental variables, including island height, annual temperature range, and net primary productivity per unit area that are linked to the habitat diversity hypothesis, the extinction hypothesis, and the subsidized island biogeography hypothesis, respectively, in order to explore the underlying drivers of the SIE. We applied iterative multiple linear regression and variation partitioning to determine the effects of island area and each of environmental variables on species richness for islands of varying sizes. Finally, we analyzed the trends of environmental variables’ contributions to species richness variation over the iterative process by using generalized linear regressions with polynomials of area up to the second degree.
Important findings A total of 131 moss species, belonging to 23 genera under 68 families, were identified. SIEs were detected for all mosses and six most common moss families, with the area thresholds varing between 0.36 km² and 106.91 km². The driving factors of the SIE differentiate among groups. Among them, Grimmiaceae did not support the habitat diversity hypothesis; Pottiaceae, Bryaceae, Grimmiaceae, and Hypnaceae did not support the extinction hypothesis; whereas, the subsidized island biogeography hypothesis was generally supported by each group to varying degrees. On the whole, mountaintops with an area of ​​over 10 km² maintain a large diversity of moss species and thus be identified as site conservation targets. For mosses with relatively unique habitat requirement, such as Grimmiaceae, protecting the stony habitat is the key to maintain its species diversity; as for other mosses, conservation of habitat diversity is the key to maintain their species diversity. The general applicability of the subsidized island biogeography hypothesis suggests that the resource replenishment of the forest ecosystem below the mountaintops counteracts the removal of mosses’ species richness due to area reduction. Therefore, conservation of species diversity and community stability in the forest below the mountaintops is essential for sustaining moss species diversity in fragmented mountaintops.

Aims Interspecific interactions are a key determinant of wetland plant community assembly. As plant traits vary in different life history stages, the form, intensity and mechanisms of interspecific interactions may change with the ontogenetic process of plants. Seed germination is vital for plant life cycle and determines, to some extent, population distribution and community structure. In field sites, various seeds often appear in the same place, which makes the germination process affected by neighbouring conspecific or heterospecific seeds. Although some previous studies have discussed plant-to-seed and seed-to-seed interactions, the intensity of intra- and interspecific interactions during seed germination and the factors influencing these interactions are not well understood.
Methods We investigated the effects of density (80 seeds per dish, 160 seeds per dish) and seed proportion (monoculture, 3:1 mixture, 2:2 mixture, 1:3 mixture) on the germination rate and mean germination time of Spartina alterniflora, Scirpus × mariqueter and Phragmites australis seeds using petri dish experiments. In addition, the intensity of intra- and interspecific interactions under different species combinations was assessed by the relative neighbour effect (RNE) index.
Important findings The germination rates of Spartina alterniflora, Scirpus × mariqueter and Phragmites australis seeds increased at higher densities. When mixed with Spartina alterniflora seeds, the mean germination time of Phragmites australis seeds under the lowest proportion (25%) treatment was significantly longer compared to the other treatments. No competitive hierarchy was found among the seeds of these three plants, and there appeared to be intransitive competition. During seed germination, Scirpus × mariqueter had competitive dominance over Spartina alterniflora, and the latter was more competitive than Phragmites australis, which in turn, dominated Scirpus × mariqueter. Considering the complexity of intra- and interspecific seed-to-seed interactions in natural ecosystems, directly extrapolating the results from laboratory experiments to field environment may have constraints. Hence, further studies are needed to investigate the contribution of vegetative and sexual reproduction during regeneration stages via field surveys and manipulative experiments to improve our understanding of the assembly mechanisms of salt marsh plant communities.

Aims Root hairs and mycorrhizal fungi, by increasing absorbing area of plant roots, promote ability of plants in acquiring soil resources such as phosphorus (P). The combination of absorptive roots, root hairs, and mycorrhizal fungi differs among distinct plant species, resulting in variations in acquisition of soil resources. At present, in situ quantifications of microstructures on the surface of roots is difficult, and relationships among absorptive roots, root hairs, and mycorrhizal fungi underlying the adaptation of soil P gradient in different woody plants remain largely unknown.
Methods We investigated 21 tree species in a subtropical forest and examined the existence of root hairs. We quantified the variation of root hair characteristics and analyzed the relationships between root hairs, mycorrhizal colonization rate, root traits along with soil P availability.
Important findings 1) Root hairs did not commonly exist in 21 subtropical tree species. We only observed root hairs in seven species, including four arbuscular mycorrhizal (AM) trees and three ectomycorrhizal (ECM) trees. Root hair occurrence rate was highest in Pinus massoniana (86%). 2) Mycorrhizal type is key to understanding the relationships between root hairs and mycorrhizal fungi. Root hair density was significantly positively correlated with mycorrhizal fungi colonization rate in AM trees. In contrast, in ECM trees, root hair diameter was significantly negatively correlated with mycorrhizal fungi colonization rate. 3) Root hair length and diameter were negatively correlated with soil available P content in AM trees, while root hair occurrence rate decreased with the soil available P content in ECM trees. Our results improve the understanding of root hairs and their interaction with other plant traits in natural forest stands, which lay the foundation for better predictions of nutrient acquisition strategies.

Aims The aims of this study were to investigate the effects of different stubble height on non-structural carbohydrate metabolism of Cyperus esculentus, to further clarify the relationship between stubble height and aboveground biomass of C. esculentus, and to seek the best clipping height.
Methods The growth physiology parameters, non-structural carbohydrates and aboveground biomass of C. esculentus leaves at six stubble heights (10, 20, 30, 40, 50 cm and uncut) were determined.
Important findings The results showed that clipping can stimulate the photosynthesis of C. esculentus. The peak of regeneration and growth was reached from 1 to 14 days after clipping. The contents of soluble sugars in leaves of 30 cm stubbles (7th, 21st and 28th days) were higher than those in leaves of other treatments, which were 9.22%, 10.83% and 9.07%, respectively, and the starch contents (14th and 21st days) were 4.88% and 4.11%, respectively. The sucrose contents in leaves of 40 cm stubbles (21st and 28th days) were higher than those in leaves of other treatments, which were 7.88% and 11.38%, respectively. The fructose contents (14th and 21st days) were also higher than those in leaves of other treatments, which were 5.29% and 6.40%, respectively. The activities of sucrose phosphate synthase and sucrose synthase in 30 cm stubbles and 40 cm stubbles were higher than those in other treatments in different periods after clipping. Stubble 10 cm inhibited the increase of sucrose content and related enzyme activities of C. esculentus. The total amount of forage at clipping and harvest, the hay mass of 30 cm stubbles was up to 10 605.11 kg·hm^-2, 19.93% higher than that of uncut. The hay mass of 40 cm stubbles was up to 8 976.93 kg·hm^-2, 1.52% higher than that of uncut. In addition, through redundancy analysis, soluble sugar content, the activities of sucrose synthase and sucrose phosphate synthase were important factors affecting the forage yield and regeneration rate. In the long period of cutting (day 7-28), 30-40 cm stubbles were more conducive to regeneration and growth, accumulation and synthesis of non-structural carbohydrate, as well as related enzyme activities and forage yield. Therefore, the suitable stubble height range was 30-40 cm.

Aims Nitrogen oxides (NO_x = NO + NO₂) are the world’s main atmospheric pollutants. Especially in China, NO_x deposition has been increasing in recent years. When NO_x enter the forest ecosystems accompanied by dry deposition, they reach the forest canopy first. Studies have shown that trees can assimilate NO₂ deposited on leaf surfaces, but this process has been ignored in the forest N cycle models. Currently, the quantity of NO₂ that can be assimilated by trees through foliage and the mechanisms of how the assimilated NO₂ is distributed remain unclear.
Methods In this study, we conducted a ¹⁵NO₂ fumigation experiment with static chambers under dark and light conditions. We chose Schima superba and Pinus massoniana seedlings, the common tree species in southern China, as our experimental materials, and analyzed the whole ¹⁵N recovery and the distribution of assimilated ¹⁵NO₂ in different tissues.
Important findings The results show that trees assimilated NO₂ mainly through stomata. Schima superba and P. massoniana could take up 10.3% ± 5.9% and 20.4% ± 7.0% ¹⁵NO₂ in the whole plant under the dark condition, respectively; while 35.9% ± 5.4% and 68.2% ± 7.6% under light condition. The sequence of ¹⁵N recovery per dry mass in different tissues was leaves > branches > stem > roots. Most recovered NO₂ remained in the leaves in a short period after fumigation. The ¹⁵N recovery in leaves of S. superba and P. massoniana accounted for 72% and 49% of the total recovery under the dark condition, and 91% and 96% under the light condition, respectively. This study indicates that the foliar assimilation of NO₂ in forest ecosystems cannot be ignored. The process of foliar NO₂ assimilation plays a key role in the N budget of forest ecosystems.

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.

Quantitative estimation of the contribution of predictary variables to community composition is a hotspot in community ecology. However, multicollinearity and joint contributions among predictors make it difficult to estimate the importance of predictor in specific analysis scenarios. To address this issue, the “rdacca.hp” package provides a new quantitative indicator by introducing the concept of hierarchical partitioning (HP) to assign individual effects for individual predictors (or groups of predictors) across all possible model subsets. The package solves the problem of estimating the relative importance of predictors with multicollinearity in canonical analysis. The “rdacca.hp” package has become an important tool for community ecological analysis. To further promote users’ understanding and use of the “rdacca.hp” package, we demonstrate the general steps for using this package in canonical analysis with an example analyzing the important environmental and spatial drivers that shape the oribatid mites (Oribatida) community. Subsequently, we conduct a bibliometric analysis of recent studies using “rdacca.hp” package. The results show that, since its launch, the package has been widely used as a fundamental quantitative framework in ecology, environmental science and related disciplines. Finally, we discuss the further application and extension of the “rdacca.hp” package. In conclusion, this paper aims to advocate the understanding and application of the “rdacca.hp” package for domestic researchers.