Plant nutrient dynamics and stoichiometric homeostasis of invasive species Spartina alterniflora and native Cyperus malaccensis var. brevifolius in the Minjiang River estuarine wetlands

Aims Stoichiometric homeostasis is an important mechanism in maintaining ecosystem structure, function, and stability. The invasion of exotic species, Spartina alterniflora, has largely threatened the structure and function of native ecosystems in the Minjiang River estuarine wetland. However, how S. alterniflora invasion affect plant stoichiometric homeostasis is largely unknown. This could enhance our understanding on wetland ecosystem stability and expand the applications of ecological stoichiometry theory.
Methods Nitrogen (N) and phosphorus (P) contents of plant organs and soils in the S. alterniflora, Cyperus malaccensis var. brevifolius, and S. alterniflora-C. malaccensis var. brevifolius mixture were measured, and the homeostatic index (H) was calculated according to the stoichiometric homeostasis theory.
Important findings Our results showed that the invasion of S. alterniflora significantly increased soil N:P ratio (p < 0.05), but did not affect soil N or P contents. The N and P contents of leaf and stem were the highest for S. alterniflora, and those of the stem were the highest for C. malaccensis var. brevifolius. At the ecosystem level, the average of homeostatic index (H) of N (H_N, 25.31) was larger than those of P (H_P, 10.33) and N:P (H_N:P, 2.50). At the organ level, root H_N was significantly larger than stem H_N (p < 0.05) and sheath H_N:P was greater than root H_N:P (p < 0.05), while there was no significant difference for H_P among root, stem, leaf, and sheath (p > 0.05). As for species, root H_N of S. alterniflora was significantly larger than that of C. malaccensis var. brevifolius in the mixture community (p < 0.05). In the monoculture, stem H_N:P of S. alterniflora was significantly higher than that of C. malaccensis var. brevifolius (p < 0.05). Furthermore, root H_N, leaf H_N and sheath H_N of S. alterniflora in the mixed community was significantly larger than that of S. alterniflora in the monoculture (p < 0.05), suggesting that S. alterniflora invasions increased their stoichiometric homeostasis. Meanwhile, the stoichiometric homeostasis of invasive and native plants were influenced by multiple factors, such as nutrients, organs, vegetation, and invasion. However, larger homeostasis was found in S. alterniflora than in C. malaccensis var. brevifolius in some particular organs either in mixture or monoculture communities. Therefore, the successful invasion of S. alterniflora may result from higher homeostatic index than the native species, C. malaccensis var. brevifolius.     

Trade-off between leaf size and vein density of Achnatherum splendens in Zhangye wetland

Aims Trade-offs between leaf size and vein density are the basis of the theory of leaf economics spectrum, and are to understand the relationship between the physical build and physiological metabolism of plant leaves under different degrees of competition for resources. Our objective was to study the changes in the relationship between leaf size and vein density (leaf dry biomass and leaf area) in Achnatherum splendens populations with four plant bundle densities located in the flood plain wetland of Zhangye. Methods The study site was located at floodplain wetlands of Zhangye, Gansu Province, China. Survey and sampling were carried out in the communities that A. splendens dominated. According to the plant bundle density, the A. splendens communities were divided into four density gradients with “bundle” for the sampling units, high density (I, > 12 bundle·m^-2), medium density (II, 8-12 bundle·m^-2), medium density (III, 4-8 bundle·m^-2) and Low density (IV, <4 bundle·m^-2). According to the density of each combination, we chose seven (5 m × 5 m) A. splendens samples, resulting in a total of 28 samples (4 × 7). The soil physical and chemical properties of four density gradients were investigated and six samples of A. splendens were used to measure the leaf area, leaf dry biomass and vein density in laboratory, and biomass of different organs was measured after being dried at 85 °C in an oven. 28 plots were categorized into three groups: high, medium and low density, and the standardized major axis (SMA) estimation method was used to examine the allometric relationships between leaf area, leaf dry biomass and vein density. Important findings The results showed that with the population density changed from high, medium, to low, the soil moisture decreased, and soil electric conductivityincreased. The leaf area, leaf biomass and height of A. splendens decreased, and the vein density, specific leaf area and photosynthetically active radiation (PAR) increased gradually. In addition, leaf net photosynthetic rate (P_n), transpiration rate (T_r) and twig number firstly increased then decreased. There was a highly significantly negative correlation (p < 0.01) between the leaf size and vein density on the high- and low-level densities (I, IV), whereas less significant (p < 0.05) on the level of medium density (II, III). The SMA slope of regression equation in the scaling relationships between leaf size and vein density was significantly smaller than -1 (p < 0.05).     

氮磷添加对巨桉幼苗生物量分配和C:N:P化学计量特征的影响

巨桉(Eucalyptus grandis)是一种优良的速生用材树种, 了解氮(N)和磷(P)对巨桉生长、养分限制、化学计量特征的影响对于科学合理施肥具有重要意义。该实验以巨桉无性系组培苗为研究对象, 通过在酸性紫色土中设置不同施N或施P梯度, 研究巨桉幼苗各器官(根、茎、叶)生物量及碳(C)、N、P的分配和化学计量特征以及巨桉生长的养分限制状况。结果表明: 施N处理对巨桉根茎叶及总生物量的影响极显著, 增加了地上部分的生物量比例而显著降低了根系的生物量比例; 施P对巨桉幼苗总生物量影响不显著, 但显著提高了根的生物量分配比例, 对茎和叶的生物量分配没有显著影响。施N或施P显著改变了巨桉幼苗的N、P含量和化学计量比, 同时也显著影响了土壤与植物N:P的关系。施N可以促使酸性紫色土条件下巨桉对N的吸收而抑制对P的吸收, 施P则促进巨桉幼苗对P的吸收。施N对巨桉幼苗根茎叶的C、N、P分配特征有极显著影响, 而施P对巨桉幼苗根茎叶的C、N、P分配没有显著影响。施N极显著降低了巨桉幼苗N的利用率, 显著提高了P的利用率, 而施P处理极显著降低了巨桉幼苗P的利用率。从巨桉生物量沿施肥梯度和N:P的变化规律可以判断, 当叶片N:P < 15时, 巨桉的生长主要受到N的限制作用。施N可以显著地提高根茎叶的N:P比值, 缓解巨桉缺N的现象, 施P则进一步加剧了N元素的缺乏。     

Duration of mulching caused variable pools of labile organic carbon in a Phyllostachys edulis plantation

Aims Soil total organic carbon and labile organic carbon are important indicators in evaluating soil quality. Mulching is widely applied to promote the emergence of bamboo shoot in winter time through stand management. Yet the consequences of mulching on soil quality in Phyllostachys edulis have not been well studied. We aim at the quantitative effect of mulching duration on soil quality in P. edulis stands.
Methods Several P. edulis stands located in Huangyan District of Taizhou, Zhejiang Province of China, had been applied with mulching for 1-2 years and were used in this study to assess the mulching effects. We also selected stands without mulching treatment as the reference sites (or control, CK) for comparisons.||||Important findings Total soil organic carbon (TOC), light fraction organic matter (LFOM), and easily-oxidized carbon (EOC) contents at stands with 1-year and 2-year mulching treatments were significantly increased compared with those at the CK sites. The 1-year mulching increased TOC, LFOM and EOC by 11.2%-74.2%, 31.7%-196.9% and 5.0%-79.6%, respectively, than those of CK sites, while by 22.2%-90.8%, 36.7%-238.5%, and 21.9%-97.5% with 2-year treatment. However, the contents of water-soluble organic carbon (WSOC) changed insignificantly. Among the indicators, we found that WSOC:TOC in CK was higher than that with the mulching treatments, while EOC:TOC with 1-year treatment was higher than that with 2-year treatment, and EOC:TOC with 2-year treatment was higher than that of CK. Additionally, WSOC, EOC, and LFOM at all three treatments showed high correlations with TOC, with a higher correlation coefficient of WSOC with TOC of 0- 30 cm soil layers in CK than those with mulching treatments. The correlation coefficient of EOC and LFOM with TOC was the highest at the 2-year mulching sites. More importantly, TOC, WSOC, EOC, and LFOM were significantly (p < 0.05), or extremely significantly (p < 0.01), correlated with soil nutrient content, including total N, hydrolysis N, available P, available K, exchangeable Ca, and exchangeable Mg in all treatments. In sum, it appeared that mulching in short term can increase the contents of TOC, soil labile organic carbons and soil nutrients in bamboo soils, yielding an improved soil quality and thus can be promoted as a plausible practice for the sustainable management of P. edulis stands.     

Stoichiometric characteristics of carbon,nitrogen and phosphorus of Sibiraea angustata shrub on the eastern Qinghai-Xizang Plateau

Aims Little is known about the stoichiometric characteristics of carbon (C), nitrogen (N) and phosphorus (P) in plateau shrubs across China. Sibiraea angustata is a typical and representative shrub species on the eastern Qinghai- Xizang Plateau, and exploring its C, N and P distribution patterns and stoichiometric properties in different organs (including root, shoot, leaf, twig and fruit) would help us better understand the mechanisms of C, N and P cycling and balance in the S. angustata dominated shrub ecosystem.
Methods Sixteen sampling sites were selected on the eastern Qinghai-Xizang Plateau by the stratified sampling method. The height and coverage of the dominant shrubs, latitude, longitude and altitude of the sites were recorded. Three 5 m × 5 m plots were selected at each site. At least 128 biological samples of plant organs of S. angustata were collected and measured, respectively. The C and N concentrations of plant samples were analyzed using an elemental analyzer (2400 II CHNS). The P concentration was analyzed using the molydate/ascorbic acid method after H₂SO₄-H₂O₂ digestion.
Important findings The C, N and P concentrations of different organs followed the order of: shoot (495.07 g·kg^-1) > twig (483.37 g·kg^-1) > fruit (480.35 g·kg^-1) > root (468.47 g·kg^-1) > leaf (466.33 g·kg^-1); leaf (22.27 g·kg^-1) > fruit (19.74 g·kg^-1) > twig (7.98 g·kg^-1) > shoot (4.54 g·kg^-1) > root (4.00 g·kg^-1) and fruit (2.85 g·kg^-1) > leaf (1.92 g·kg^-1) > twig (0.96 g·kg^-1) > root (0.52 g·kg^-1) > shoot (0.45 g·kg^-1), respectively. The ranges of the coefficient of variation (CV) for C, N and P concentrations were 1.71%-4.44%, 14.49%-25.50% and 11.46%-46.15%, respectively. Specifically, the C concentration was relatively high and stable, and the maximum CV values for N and P were found in roots. The N:P value of different organs varied from 7.12-12.41 and the minimum CV for N:P was found in twig, which indicated that N:P in twig had higher internal stability. In addition, correlation analysis indicated that the C concentration was significantly negatively correlated with N and P concentrations and correlation coefficients were -0.407 and -0.342, respectively. However, N concentration had dramatically positive correlation with P concentration and the correlation coefficient was 0.814. These results also could indicate that the C, N and P stoichiometric characteristics in the S. angustata shrub accorded with the homeostatic mechanism and growth rate hypothesis to some extent, the distributions of C, N and P concentrations were closely related to the function of the organs and it should be prudent to use ecological stoichiometric ratios to judge the condition of nutrient limitation at the species level.     

Correlation between vein density and water use efficiency of Salix matsudana in Zhangye Wetland,China

Aims The correlation between vein density and water use efficiency (WUE) affects the balance between water supply and demand of plant leaves, which is significant for comprehending the ecological adaptation strategies of plants. The objective of this study was to study how Salix matsudana modulated vein density and WUE along a soil moisture gradient in Zhangye Wetland, China. Methods The study was conducted in floodplain wetland near Heihe River in Zhangye City, Gansu Province, China. Three sample plots, at a spatial interval of 70 m, were set up along a soil moisture gradient ordinally from the area near the water body to the wetland edge, plot I (69.23%), spot II (48.38%) and spot III (35.27%). Community traits were investigated by using diagonal method, and all individuals of S. matsudana were used for measurements of height and canopy. At each plot, 5 individuals of S. matsudana at 4 vertices and diagonal intersection were selected for measurements of vein density, WUE, net photosynthetic rate (P_n), transpiration rate (T_r), photosynthetically active radiation (PAR), saturated vapor pressure differences (VPD), specific leaf area, stomatal conductance (G_s) and intercellular CO₂ concentration (C_i). We used mathematical methods of correlation analysis and standardized major axis to investigate relationships between vein density and WUE. Important findings With decreasing soil moisture, the height, canopy, specific leaf area, G_s and C_i of S. matsudana decreased gradually, while the vein density, WUE, P_n, T_r, PAR and VPD increased gradually. The correlation between vein density and WUE was positive in all the three plots, but the relationship varied along the soil moisture plots gradient. There was a highly significant positive correlation (p < 0.01) between the vein density and WUE at plot I and III, whereas the correlation only reached a significant level (p < 0.05) at plot II; The correlation coefficient between vein density and WUE is significantly smaller than 1 at plot I (p < 0.05), while the correlation coefficient is significant greater than 1 at plot II and III (p < 0.05). We can conclude that varied relationships between vein density and WUE of S. matsudana along a soil moisture gradient could reflect plant acclimation.     

Characteristics of nitrogen and phosphorus stoichiometry across components of forest ecosystem in Shaanxi Province

Aims Nitrogen (N) and phosphorus (P) stoichiometry between vegetation, litter, and soil were important for understanding biogeochemical cycles in terrestrial ecosystems, but remain poorly understood. Here, our aims were to study characteristics of N and P stoichiometry for the plant, litter, and soil and the interactions between its components across forest ecosystems in Shaanxi Province.Methods A total of 121 sampling sites, covering the most main forest types in Shaanxi, were established across the whole province in 2012. And N and P concentration of vegetation (tree and understory), litter and soil layers were measured for each site.Important findings 1) There were significant differences in the N and P stoichiometry among the forest ecosystem components (p < 0.05). N and P contents were higher in understory and litter layer, and lower in tree and soil. Whereas the N:P was slightly different, highest in litter and lowest in the soil layer, with little differences among remaining components. The contents of N, P and N:P ranged from 0.72 to11.99 mg·g^-1, 0.47 to 1.07 mg·g^-1, and 1.86 to 14.84, respectively. Within top 1 m soil layer N content and N:P decreased with soil depth (p < 0.05), however the P content did not exhibit significant changes. 2) N and P contents, and N:P of each component were higher in broadleaf forest than in coniferous forest, although the difference is not significant. 3) N was positively correlated with P content except for soil and N:P was negatively correlated with P content, but positively correlated with N content within each component. In addition, there was a significant positive correlation for N and P stoichiometric characteristics between litter layer and tree, herb, soil layer. 4) Although spatial pattern of N and P contents, and N:P differed in relation to longitude, latitude, and elevation for forest ecosystem components, a stable distribution was exhibited in general.     

Relationships between leaf thickness and vein traits of Achnatherum splendens under different soil moisture conditions in a flood plain wetland,Heihe River,China

Aims The correlations between leaf thickness and vein traits influenced the leaf hydraulic dynamic balance, and there were important meanings to reveal ecophysiological mechanisms of plant leaves water transport and growth rate. Our objective was to study the changes in the relationship between leaf thickness and vein traits (vein diameter, vein density) of Achnatherum splendenspopulations by using standardized major axis estimation (SMA) method under different soil moisture conditions located in flood plain wetland of Zhangye.Methods The study site was located at flood plains wetland of Zhangye, Gansu Province, China. Selecting a starting point along the vertical direction of the river, in turn, along the soil moisture gradient, four plots were set up at intervals of 40 m, plot I (50.07%), plot II (38.77%), plot III (31.5%) and plot IV (20.4%). From each of the four sample plots, seven samples were collected, resulting in (5 m × 5 m) a total of 28 samples. Community traits (height, density) and soil physical and chemical properties were investigated. Six individual samplings of A. splendens from each plot were used to measure the leaf thickness, vein density and vein diameter in laboratory. In addition, the photosynthetically active radiation (PAR), leaf net photosynthetic rate (P_n) and transpiration rate (T_r) of A. splendens were measured in natural environment. The 28 plots were categorized into groups I, II, III and IV, and SMA estimation method was then used to examine the allometric relationship among leaf thickness, vein density and vein diameter. Important findings With a decreased soil moisture, the plant density and height displayed a pattern of steadily declining, while the soil electrical conductivity increased, In addition, the vein density, leaf thickness, water use efficiency (WUE), PAR and twig number of A. splendens displayed a pattern of initial decrease, whereas the vein diameter and T_r increase gradually, P_n and plant high displayed changing trends of increasing-decreasing. The leaf thickness was negatively associated with the vein density, vein diameter, and the relationship varied with the soil moisture conditions (p< 0.05). There was a significant positive relationship (p < 0.05) between the leaf thickness and vein density. The SMA slope of the regression equation gradually decreased and was significantly different from 1.0 (p < 0.05) on plot I and IV. In addition, along decreased soil moisture, the standardized major axis slope of regression equation in the scaling relationships between the leaf thickness and vein diameter gradually increased and was significantly different from -1.0 (p < 0.05) on plot I and IV.     

Responses of plant community biomass to nitrogen and phosphorus additions in natural and restored grasslands around Qinghai Lake Basin

Aims Plant biomass reflects the primary productivity of community vegetation, and is the main resource of carbon input in the terrestrial ecosystem. It is usually limited by nitrogen (N) and phosphorus (P) availability in the soil. Alpine grassland around Qinghai Lake Basin has experienced extensive land-use changes due to the cultivation of native grassland and vegetation recovery on cropped land. In this experiment, two grassland types were chosen, natural alpine grassland (NG) and its adjacent restored grassland (RG), to determine the responses of plant community biomass to N and P additions with different land-use. Methods NH₄NO₃ and Ca(H₂PO₄)₂·H₂O were added in a completely randomized block design, with medium levels of 10 g N·m^-2 and 5 g P·m^-2. Soil NO₃^--N and available P contents, and the plant community biomass were measured in the two grasslands. Two-way ANOVA was used to determine the effects of nutrient additions on all measured indicators, and regression analysis was used to analyze the correlations between plant biomass and soil NO₃^--N and available P contents.Important findings Results showed: (1) N and P additions both increased grass biomass in the NG, and significantly elevated the total aboveground biomass, with the promoting effect of N addition higher than that of P addition; N addition significantly increased both grass and forb biomass in the RG, and markedly promoted the total aboveground biomass, while P addition had no effects on the functional groups and total aboveground biomass (p > 0.05). (2) N and P additions both had no effects on the belowground and total biomass in the NG, whereas N addition significantly increased the total biomass by 34% in the RG, which suggested that the effect of N limitation on the vegetation primary productivity was stronger in the RG at present stage. (3) The aboveground biomass in the NG increased with soil NO₃^--N content (p < 0.05), and the above- and below-ground as well as the total biomass were all positively correlated with soil NO₃^--N content in the RG (p < 0.01). These results indicated that the plant growth in alpine grassland around Qinghai Lake Basin was prone to N limitation, and the effect of P limitation changed with land-use. Soil available N might be the key limiting factor for vegetation restoration and reconstruction in the RG. The “Grain for Green” project (the land-use policy) and atmospheric N deposition are benefiting both plant growth and C accumulation in the alpine grassland ecosystem around Qinghai Lake Basin.     

Responses of phenolic acid and defensive enzyme activities to mechanical damage in Artemisia frigida

Aims The study aims at understanding the effects of feed intake and trample damage on the phenolic acid formation and antioxidant enzyme activities in Artemisia frigida, and elucidating the adaptive mechanisms in A. frigida to grazing in secondary metabolites and their related enzyme activities.
Methods We analyzed the phenolic acid content and the activities of polyphenol oxidase (PPO), phenylalanine ammonia-lyase (PAL) and protective enzymes in leaves and roots in A. frigida under three levels (light, moderate, and heavy) of manipulative grazing condition. The measurements of the 9 phenolic acid contents started after 6 h of the mechanical damage of the plants by using the high performance liquid chromatography (HPLC), and the enzyme activities in leaves and roots were measured by a spectrophotometry method.
Important findings The light damage treatment induced productions of PPO, PAL and significantly (p < 0.05) increased antioxidant enzyme activities in the leaves and roots of A. frigida. The contents of PPO, PAL and antioxidant enzymes increased with increasing intensity of mechanical damage. Compared to the control, the content of free caffeic, syringic, ferulic and cinnamic acid in the leaves A. frigida were significantly elevated (p < 0.05) by 150.4%, 93.5%, 154.4% and 121.7%, respectively. They were significantly (p < 0.05) positively correlated with PAL activity in the moderate damage treatment. The content of free chlorogenic acid and catechol decreased by 91.1%, and 69.3%, respectively, compared with the control they had a negative correlation with PPO activity in the heavy damage treatment. The contents of gallic and protocatechuic acids increased (p < 0.05) by 280.6% and 215.7%, respectively, in the heavy damage treatment. With increasing intensity of mechanical damage, the content of 9 free phenolic acids significantly increased in roots but the increasing range was less than the one in leaves. Mechanical damage induced an increasing trend in the total amount of free and bounded phenolic acids in the leaves but a decreasing trend in the total amount of bounded phenolic acids in the roots of A. frigida. The results indicated that mechanical damage could firstly induce an increase of antioxidant enzymes and key enzymes in phenolic metabolism in A. frigida, leading to the accumulation of antioxidant substances of phenolic acids, further regulate the biosynthesis of lignins, quinones and tannins, and then enhance the resistance to mechanical damage and improved the tolerance of A. frigida to grazing.     

Correlation analysis between vein density and stomatal traits of Robinia pseudoacacia in different aspects of Beishan Mountain in Lanzhou

AimsThe relationship between leaf venation and stomatal traits reflects leaf water potential of plants, and is fundamental to probe the relationship between physiological functions and water use. Our objective was to study the changes in the relationship between venation density and stomatal traits (stomatal density and stomatal length) in Robinia pseudoacacia with the changes in slope aspects.MethodsIn Beishan Mountain of Lanzhou in Gansu Province, China, 20 transects were laid out horizontally along the contour at intervals of 50 m from an elevation of 1550-1750 m in four different slope aspects, and 12 plots were set up along each transect at intervals of 5 m. A handed GPS (global positioning system) was used to measure latitude, longitude and altitude at each plot. Community characteristics, including crown density, tree height and soil moisture, were investigated. Robinia pseudoacacia within all plots were sampled and used for measurements of individual leaf area, venation density, stomatal density and stomatal length in laboratory in each plot. The 240 plots were categorized into groups of southern, eastern, western and northern aspects, and the standardized major axis (SMA) estimation method was then used to examine the relationship between the stomatal density, stomatal length and vein density.Important findings The results showed that with a change of the aspect from south to east, west, and north, the crown density, average tree height and soil moisture of the plant community increased, while leaf net photosynthetic rate (P_n), transpiration rate (T_r), photosynthetically active radiation (PAR), vein density and stomatal density decreased. In addition, stomatal length and individual leaf area increased. Venation density of each aspect of R. pseudoacacia was positively correlated with stomatal density (p < 0.05) and negatively with stomatal length (p < 0.05). These correlations were strongest in south slope (p < 0.01).     

Effects of nitrogen addition on soil respiration of Rhododendron simsii shrubland in the subtropical mountainous areas of China

Aims As the second largest C flux between the atmosphere and terrestrial ecosystems, soil respiration plays a vital role in regulating atmosphere CO₂ concentration. Therefore, understanding the response of soil respiration to the increasing nitrogen deposition is urgently needed for prediction of future climate change. However, it is still unclear how nitrogen deposition influences soil respiration of shrubland in subtropical China. Our objectives were to explore the effects of different levels of nitrogen fertilization on soil respiration, root biomass increment, and litter biomass, and to analyze the relationships between soil respiration and soil temperature and moisture.
Methods From January 2013 to September 2014, we conducted a short-term simulated nitrogen deposition experiment in the Rhododendron simsii shrubland of Dawei Mountain, located in Hunan Province, southern China. Four levels of nitrogen addition treatments (each level with three replicates) were established: control (CK, no nitrogen addition), low nitrogen addition (LN, 2 g·m^-2·a^-1), medium nitrogen addition (MN, 5 g·m^-2·a^-1) and high nitrogen addition (HN, 10 g·m^-2·a^-1). Soil respiration was measured by LI-8100 soil CO₂ efflux system. At the same time, we measured root biomass increment and litter biomass in each plot.
Important findings Soil respiration exhibited a strong seasonal pattern, with the highest rates found in summer and the lowest rates in winter. Annual accumulative soil respiration rate in the CK, LN, MN and HN was (2.37 ± 0.39), (2.79 ± 0.42), (2.26 ± 0.38) and (2.30 ± 0.36) kg CO₂·m^-2, respectively. Annual mean soil respiration rate in the CK, LN, MN and HN was (1.71 ± 0.28), (2.01 ± 0.30), (1.63 ± 0.27) and (1.66 ± 0.26) μmol CO₂·m^-2·s^-1, respectively, and it was 17.25% higher in the LN treatment compared with CK (p = 0.06). The root biomass increment was increased by LN, MN, and HN treatments by 18.36%, 36.49% and 61.63%, respectively, compared to CK. The litter biomass was increased by LN, MN, and HN treatments by 35.87%, 22.17% and 15.35%, respectively, compared with CK. Soil respiration exhibited a significant exponential relationship with soil temperature (p < 0.01, R² is 0.77 to 0.82) and a significant linear relationship with soil moisture at the depth of 5 cm (p < 0.05, R² is 0.10 to 0.15). The temperature sensitivity (Q₁₀) value of CK, LN, MN and HN plots was 3.96, 3.60, 3.71 and 3.51, respectively. These results suggested that nitrogen addition promoted plant growth and decreased the temperature sensitivity of soil respiration. The increase of root biomass under N addition may be an important reason for the change of soil respiration in the study area.     

Geostatistical analysis of spatial variations in leaf traits of woody plants in Tiantong,Zhejiang Province

AimsExploring spatial variations in leaf traits and their relationships with environmental properties is crucial for understanding plant adaptation strategies and community assembly. This study aimed to reveal how leaf traits varied spatially and the role of environmental factors.MethodsThe study was conducted in a 5-hm² forest plot in Tiantong, Zhejiang Province. Three leaf traits, including individual leaf area (ILA), specific leaf area (SLA), and leaf dry matter content (LDMC) were measured for 20253 individual trees with diameter at breast height (DBH) ≥1 cm. Soil properties measured included contents of soil total nitrogen, soil total phosphorus, soil total carbon, soil pH value, soil volumetric water content, bulk density, and humus depth. Topographic variables measured included elevation, slope and convexity. We used geostatistical analysis to reveal spatial variations of the three leaf traits. Relationships between leaf variability and environmental factors were analyzed using principal component analysis (PCA) and Pearson’s correlation.Important findings Spatial variability followed the order of ILA > SLA > LDMC. Spatial autocorrelation of three leaf traits was weak within a distance of 5.16 m. The optimal model of the semi-variogram function was Gaussian model for ILA, and exponential model for SLA and LDMC. ILA showed the largest variability at the direction of northeast-southwest, and smallest variability at the direction of northwest-southeast. In contrast, SLA and LDMC had the highest variability at the direction of northwest-southeast and least variability at the direction of northeast-southwest. There were significantly negative relationships between ILA and topographic factors (r = -0.12, p < 0.0001), and between SLA and soil nutrients (r = -0.16, p < 0.0001). In contrast, LDMC was positively correlated with soil nutrients (r = 0.13, p < 0.0001). Relative to soil nutrients, topographic factors affected much more variations in ILA, SLA and LDMC at the direction of northeast-southwest. Distinctly, at the direction of northwest-southeast, variability of ILA was affected mainly by topographic factors, while soil nutrients resulted in the most variability of SLA and LDMC. In conclusion, leaf traits varied considerably with spatial direction in the studied forest plot. Associations between leaf traits and topographic factors and soil nutrients indirectly indicated effects of environmental filtering on community assembly.     

Foliar nitrogen and phosphorus stoichiometry of alien invasive plants and co-occurring natives in Xishuangbanna

Aims How alien invasive plants and co-occurring native plants utilize nutrients is one of major issues in invasion ecology. Foliar nitrogen (N) and phosphorus (P) contents and stoichiometry can elucidate the uptake ability and limitation status of nutrients in plants, which provides basic knowledge for understanding the invading ability and co-occurrence or disappearance of plants.
Methods Based on typical alien invasive plants (Chromolaena odorata, Ageratina adenophora) and native plants in southwestern China, this study focused on strategies of N and P utilization among invasive plants and native plants under different invasion conditions. The species compositions, aboveground biomass, leaf N and P contents and leaf N:P were investigated for plants in plots with no invasion and with different invasion extents (estimated by the plot-based percentage of invaders’ biomass in total community) at Mt. Kongming in Xishuangbanna region, Yunnan Province, China.
Important findings The species number decreased significantly with the invasion extent of both C. odorata and A. adenophora, although the aboveground biomass was greatly enhanced. Leaf N and P contents did not differ between the two studied invaders, but they showed significantly higher N and P levels than both co-occurring and only native species (p < 0.05). Besides, leaf N and P contents of invaders increased with the invasion extent, and leaf N of native plants also showed an increasing trend with the invasion extent. When the influence of invasion was checked for the same species, leaf P contents decreased, whereas leaf N and N:P increased for most native plants under invasion. Based on the absolute foliar N and P contents, N:P values, we inferred that native plants were still limited by N, although N availability might be enhanced by invasion. Both invasive plants had leaf N:P values lower than 10, suggesting a higher P uptake relative to N uptake. All above results highlighted a higher N and P uptake of typical alien invasive plants in southwestern China.     

Effects of tree species on soil organic carbon density: A common garden experiment of five temperate tree species

Aims Forest trees alter litter inputs, turnover and rhizospheric activities, modify soil physical, chemical and biological properties, and consequently affect soil organic carbon (SOC) storage and carbon sink strength. That how to select appropriate tree species in afforestation, reforestation and management practices is critical to enhancing forest carbon sequestration. The objective of this study was to determine the effects of tree species on SOC density and vertical distributions.Methods A common garden experiment with the same climate, soil, and management history was established in Maoershan Forest Ecosystem Station, Northeast China, in 2004. The experimental design was a completely randomized arrangement with twenty 25 m × 25 m plots, consisting of monocultures of five tree species, including white birch (Betula platyphylla), Manchurian walnut (Juglans mandshurica), Manchurian ash (Fraxinus mandshurica), Dahurian larch (Larix gmelinii), and Mongolian pine (Pinus sylvestris var. mongolica), each with four replicated plots. A decade after the establishment (2013-2014), we measured carbon density and related factors (i.e., bulk density, total nitrogen concentration, microbial biomass carbon, microbial biomass nitrogen, pH value) in soils of the 0-40 cm depth for these monocultures. Important findings Results showed that tree species significantly influenced the SOC density in the 0-40 cm depth (p < 0.05). SOC density in the 0-10 cm depth varied from 2.79 to 3.08 kg·m^-2, in the order of walnut > ash> birch > larch > pine, in the 10-20 cm depth from 1.56 to 2.19 kg·m^-2, in the order of pine > walnut > ash > birch > larch, in the 20-30 cm depth from 1.17 to 2.10 kg·m^-2, and in the 20-40 cm depth from 0.84 to 1.43 kg·m^-2. The greatest SOC density occurred in the birch stands in the 20-40 cm depth. The vertical distributions of SOC density varied with tree species. The percentage of SOC in the 0-10 cm depth over the total SOC in the soil profile was significantly higher in the walnut and larch stands than in others, while the percentage of SOC in the 20-40 cm depth over the total SOC was highest in the birch stands. SOC concentration and soil bulk density differed significantly among the stands of different tree species, and were negatively correlated. SOC density was positively correlated with soil microbial biomass and soil pH in the walnut, ash, and larch stands, and with total nitrogen density in all the stands. We conclude that tree species modifies soil properties and microbial activity, thereby influencing SOC density, and that different patterns of vertical distributions of SOC density among monocultures of different tree species may be attributed to varying SOC controls at each soil depth.     

Effects of different nitrogen:phosphorus levels on the growth and ecological stoichiometry of Glycyrrhiza uralensis

Aims The increase in atmospheric nitrogen (N) deposition has accelerated N cycling of ecosystems, probably resulting in increases in phosphorus (P) demand of ecosystems. Studies on the effects of artificial N:P treatment on the growth and carbon (C), N, P ecological stoichiometry of desert steppe species could provide not only a new insight into the forecasting of how the interaction between soils and plants responses to long-term atmospheric N deposition increase, but also a scientific guidance for sustainable management of grassland in northern China under global climate change. Methods Based on a pot-cultured experiment conducted for Glycyrrhiza uralensis (an N-fixing species) during 2013 to 2014, we studied the effects of different N:P supply ratios (all pots were treated with the same amount of N but with different amounts of P) on aboveground biomass, root biomass, root/shoot ratio, and C:N:P ecological stoichiometry both in G. uralensis (leaves and roots) and in soils. Additionally, through the correlation analyses between biomass and C:N:P ecological stoichiometry in leaves, roots, and soils, we compared the differences among the C:N:P ecological stoichiometry of the three pools, and discussed the indication of C:N:P ecological stoichiometry in soils for the growth and nutrient uptake of G. uralensis. Important findings The results showed that, reducing N:P decreased C:P and N:P ratios both in G. uralensis (leaves and roots) and in soils but increased aboveground biomass and root biomass of G. uralensis, indicating that low to moderate P addition increased P availability of soils and P uptake of G. uralensis. However, excessive low N:P (high P addition) led to great decreases in soil C:P and N:P ratios, thus hindering N uptake and the growth of G. uralensis. C:N:P ratios in the two pools of G. uralensis (especially in leaves) had close correlations with soil C:N:P ratio, indicating that the change in soil C:N:P ratio would have a direct influence on plants. Our results suggest that, through regulating C:N:P ratio in leaves and soils, appropriate amounts of P addition could balance soil P supply and plant P demand and compensate the opposite influences of long-term atmospheric N deposition increase on the structure of desert steppe.     

纳帕海高原湿地不同干扰强度下土壤真菌的分布格局

将无干扰的原生沼泽作为对照, 运用比较法研究了纳帕海高原湿地不同干扰强度下形成的湿地利用类型, 即沼泽(无干扰)、沼泽化草甸(轻度干扰)、草甸(中度干扰)和垦后湿地(重度干扰) 4个湿地利用类型的碳氮含量及其分布格局, 揭示干扰对纳帕海不同湿地利用类型碳氮及土壤真菌分布的影响。研究表明: (1) 4个湿地利用类型上下层土壤有机质(SOM)、全氮(TN)、碳氮比(C:N)和pH值均有显著的差异性(p < 0.01), 并且随着干扰强度的增大, SOM和TN含量逐渐减少。(2)土壤真菌经PDA培养基培养后计数, 在同一湿地类型上层的真菌数量大于下层, 随着干扰强度的增加, 真菌的数量逐渐增加。相关性分析表明: 真菌的数量与pH值、SOM和TN呈极显著负相关, 与C:N呈显著正相关。(3)系统发育研究表明: 纳帕海湿地分布有土壤真菌Ascomycota、Basidiomycota和Zygomycota, 其中Ascomycota是优势类群, 在高原湿地土壤碳氮分解等物质循环过程中Ascomycota处于主导地位。     

Canopy structure and radiation interception of Salix matsudana: Stand density dependent relationships

Aims Plants can enhance their photosynthetic efficiency and competitiveness by adjusting canopy structure and radiation interception. The objective of this paper was to quantify the relationship between canopy structure (crown depth and crown area) and light interception (LI) in a Salix matsudana stand under three different stand densities in a flood plain of Zhangye.Methods Our study site is located at the Heihe flood plain of Xichengyi in Ganzhou district, Zhangye City, Gansu Province in the middle Heihe River, where S. matsudana is the dominant species. Based on stand density (10 m × 10 m), the S. matsudana community is divided into three types: low density (I, 25-36 Ind.·plot^-1), medium density (II, 37-48 Ind.·plot^-1), and high density (III, 49-60 Ind.·plot^-1). Community characteristics, soil physical and chemical properties of each type were measured. At each plot, we measured photosynthetically active radiation (PAR), LI, net photosynthetic rate (P_n), transpiration rate (T_r), crown depth, crown area, leaf area index (LAI), twig numbers, twig length, and bifurcation angle. The standardized major axis (SMA) estimation method was used to determine the relationships between LI and canopy structure.Important findings With increasing in stand density, we found that soil moisture increased, and soil electric conductivity decreased, while twig length and crown depth increased, and PAR, twig numbers, bifurcation angle and crown area decreased. LAI and LI, P_n and T_r reached their maximum at the stand of medium density. There was a significant, positive correlation and negative correlation (p < 0.01), respectively, between the LI, crown depth and crown area at low density (I), whereas low significant (p < 0.05) at high density (III), and high significantly positive correlation (p < 0.01) at the medium density (II). S. matsudana has more horizontal branches that reduce LI. Canopy thickness and increased crown area at low density. More vertical distribution of branches at high density, and a more balanced spacial distribution were found at medium density.     

Effects of grazing intensity and grazing exclusion on litter decomposition in the temperate steppe of Nei Mongol,China

Aims Grazing intensity and grazing exclusion affect ecosystem carbon cycling by changing the plant community and soil micro-environment in grassland ecosystems. The aims of this study were: 1) to determine the effects of grazing intensity and grazing exclusion on litter decomposition in the temperate grasslands of Nei Mongol; 2) to compare the difference between above-ground and below-ground litter decomposition; 3) to identify the effects of precipitation on litter production and decomposition. Methods We measured litter production, quality, decomposition rates and soil nutrient contents during the growing season in 2011 and 2012 in four plots, i.e. light grazing, heavy grazing, light grazing exclusion and heavy grazing exclusion. Quadrate surveys and litter bags were used to measure litter production and decomposition rates. All data were analyzed with ANOVA and Pearson’s correlation procedures in SPSS. Important findings Litter production and decomposition rates differed greatly among four plots. During the two years of our study, above-ground litter production and decomposition in heavy-grazing plots were faster than those in light-grazing plots. In the dry year, below-ground litter production and decomposition in light-grazing plots were faster than those in heavy-grazing plots, which is opposite to the findings in the wet year. Short-term grazing exclusion could promote litter production, and the exclusion of light-grazing could increase litter decomposition and nutrient cycling. In contrast, heavy-grazing exclusion decreased litter decomposition. Thus, grazing exclusion is beneficial to the restoration of the light-grazing grasslands, and more human management measures are needed during the restoration of heavy-grazing grasslands. Precipitation increased litter production and decomposition, and below-ground litter was more vulnerable to the inter-annual change of precipitation than above-ground litter. Compared to the light-grazing grasslands, heavy-grazing grasslands had higher sensitivity to precipitation. The above-ground litter decomposition was strongly positively correlated with the litter N content (R² = 0.489, p < 0.01) and strongly negatively correlated with the soil total N content (R² = 0.450, p < 0.01), but it was not significantly correlated with C:N and lignin:N. Below-ground litter decomposition was negatively correlated with the litter C (R² = 0.263, p < 0.01), C:N (R² = 0.349, p < 0.01) and cellulose content (R² = 0.460, p < 0.01). Our results will provide a theoretical basis for ecosystem restoration and the research of carbon cycling.     

Relationships between soil nutrients and plant functional traits in different degradation stages of Leymus chinensis steppe in Nei Mongol,China

Aims Understanding ecological implications of plant functional traits is helpful in exploring community assembly under different environments of nature and human disturbances, and then to reveal the maintenance mechanism of the ecosystem services. By analyzing vegetation and soil data derived from field observations in Leymus chinensis steppe of Xilin River Basin in Nei Mongol, we aimed to explore the responses of plant functional traits to changing soil nutrients at different degradation stages. Methods We observed 69 plots for both plant community structure and soil attributes using quadrat and soil-drilling methods. Five plant functional traits, namely the specific leaf area (SLA), leaf dry matter content (LDMC), leaf carbon to nitrogen ratio (C:N), leaf lignin content (LLC), and maximum height (MH), were measured for each plot. We also tested soil attributes, such as total nitrogen (TN), total phosphorus (TP), available nitrogen (AN), available phosphorus (AP), and organic carbon (OC). The sixty-nine communities were classified into four groups (undegraded L. chinensis + forbs, slightly degraded L. chinensis + Stipa sp., moderately degraded L. chinensis + Cleistogenes squarrosa, and heavily degraded L. chinensis + Artemisia frigida) using TWINSPAN software. The relationships between plant functional traits and soil nutrient variables were analyzed for the four community groups using the Pearson’s correlation test with SPSS 21.0 software. Important findings (1) The soil nutrients decreased with the grassland degradation process and there were significant differences in TN and TP between the undegraded L. chinensis + forbs and heavily degraded L. chinensis + A. frigida communities; (2) plant functional traits also showed strong differences between the degradation stages. MH and C:N decreased with degradation. A significant difference was observed in MH between the undegraded L. chinensis + forbs and slightly degraded L. chinensis + Stipa sp. communities. The difference in C:N was also significant between the undegraded L. chinensis + forbs and heavily degraded L. chinensis + A. frigida communities; (3) the effects of soil nutrients on plant functional traits changed with grassland degradation. AN was negatively correlated with MH, LLC, and C:N in the slightly degraded L. chinensis + Stipa sp. community. In the moderately degraded L. chinensis + C. squarrosa community, those three traits mentioned above showed significantly positive correlations with TP; (4) while analyzing the degraded grassland, different relationships between plant functional traits were found. In the slightly degraded L. chinensis + Stipa sp. community, LLC was positively correlated with all other traits. Moreover, positive correlations also occurred between C:N and MH, C:N and LDMC, and C:N and LLC. In the heavily degraded L. chinensis + A. frigida community, all traits demonstrated the most significantly positive correlations.