Nitrogen competition between three dominant plant species and microbes in a temperate grassland

Background and aims

To test the hypothesis that dominant plant species could acquire different nitrogen (N) forms over a spatial scale and they also have the ability to compete for available N with microbes.

Methods

A short-term ¹⁵N labeling experiment was conducted in the temperate grassland ecosystem of North China in July of 2013. Three N forms (NO₃ ^? _, NH₄ ⁺ and glycine) labeled with ¹⁵N were injected into the two soil depths (0–5 and 5–15 cm) surrounding each plant to explore N acquisition by plants and microbes. Three dominant plant species (Artemisia frigida, Cleistogenes squarrosa and Artemisia capillaris) were investigated.

Results

Two hours after ¹⁵N labeling, all three dominant plant species absorbed both organic and inorganic N, but different patterns were observed at two soil depths. Uptake of NO₃ ^? was significantly higher at 0–5 cm than at 5–15 cm soil depth among all the dominant plant species. ¹⁵N recovery by microbes was significantly higher than plants. However, ¹⁵N recovery by plants showed different patterns over soil depths.

Conclusions

Dominant plant species in the temperate grassland have different patterns in acquisition of N added to soil in organic form and absorption of inorganic N, and microbes were more effectively than plants at competing for N in a short-term period.

     

Asymmetric competition between plant species

Despite extensive interest in the role of plant size in competition, few formal attempts have been made to quantify the magnitude of asymmetric competition, particularly for interactions between members of different species. This paper introduces the concept of asymmetric interspecific competition at the population livel (i.e. mean plant performance) in mixtures of species. It proposes an index of interspecific competitive asymmetry which allows for a progressively greater asymmetric effect as the average size differences between competing species increase, and allows for such an effect whether individuals of focal species are larger or smaller, on average, than competitors. This index of competitive asymmetry is evaluated in the study of interactions between two widely coexisting annuals of disturbed habitats, Stellaria media and Poa annua. An experiment was conducted in which the density, relative frequency and relative seedling sizes (emergence times) of Poa and Stellaria individuals were varied. The relative growth rate (RGR) for both species was measured over a 22-day period. An inverse linear model was fitted for each species, relating the RGR of the focal species to the initial biomass of each species. Each response model included an asymmetry coefficient () to assess whether the impact of a unit of initial biomass of the associate species changed with the relative sizes of seedlings of the two species. A zero value of implies symmetric competition between the two populations; i.e. the competitive effect of a unit of associate species biomass does not change with its initial seedling size. If is positive the smaller the initial relative size of seedlings of the associate species, the smaller their per unit biomass effect on the response of the focal species. The model fitted our data for Stellaria and Poa well and was validated by an alternative modelling approach. Asymmetry coefficients were estimated as 0.508 (P<0.05) for the effect of Poa in the Stellaria model, and 0.0001 (NS) for the effect of Stellaria in the Poa model; i.e. the effect of Poa on Stellaria was asymmetric while the effect of Stellaria on Poa was symmetric. Differences in interspecific species asymmetric competitive effects are discussed within the context of shoot architecture, and the relative importance of competition for light versus soil resources. Finally, we discuss the relationship of this model to earlier models of competitive asymmetry, and consider the implications of interspecific competitive asymmetry for a number of current theories of plant competition and community organisation.     

松嫩盐碱草地几种植物的土壤营养位分析

针对松嫩盐碱草地上的几种主要植物,以植物地上生物量和总和优势度（SDR）为土壤营养位效能指标,分别从多项土壤因子综合、可溶盐含量、总碱度、Na^ 含量、有机质含量、全氮含量角度进行了土壤营养位分析。讨论了松嫩盐碱草地植物种群分布格局与土壤营养的关系,指出松嫩盐碱草地植物种群分布格局的形成是可溶盐含量、总碱度、Na^ 含量、有机质含量、全氮含量5项主要土壤因子综合作用的结果,对于各植物种群土壤营养位的分化结果而言,没有一项土壤因子是起决定性作用的。     

Interspecific pollen transfer and competition between co-occurring plant species

Summary Computer simulations of a pollinator foraging in a mixture of two species were used to explore how plant reproduction can be influenced by interspecific pollination movements. Interspecific pollen transfer led to strong competitive effects when availabilities of pollen, receptive stigma surfaces, or pollinator movements were limited relative to the total number of fertilizations possible in the mixed population. Results from simulations suggest that competition for pollination through interspecific pollen transfer can result in rapid exclusion of one of two species, and that such competition represents a selective force promoting stable divergence of potential competitors in habitat affinity, flowering time, or other characteristics related to pollinator sharing.     

生物间非传递性竞争研究进展

生物间的竞争关系是决定群落中物种共存和生物多样性的关键因素。传统研究主要关注物种两两之间的竞争作用, 而对多物种相互竞争形成的网络研究相对较少。近年来, 类似于“石头-剪刀-布”游戏的非传递性竞争被认为是一种重要的物种共存和生物多样性的维持机制, 越来越受到生态学家们的关注。本文首先回顾了非传递性竞争定义的发展过程, 并介绍了非传递环的不同结构。其次介绍了基于竞争结局矩阵以及入侵增长率的非传递性竞争度量指标, 并对比不同指标的特点与适用情形。随后通过多个研究实例介绍了非传递性竞争在自然群落中的普遍性, 并指明物种之间的权衡是非传递性竞争产生的生物学机制。最后介绍了非传递性竞争对生物多样性与生态系统功能的影响。非传递性竞争本质上是物种两两之间相互作用的组合, 是只包含单一作用类型的特殊网络结构。因此, 非传递性竞争如何影响生物多样性-生态系统功能关系和群落稳定性, 如何受到环境与高阶相互作用的影响, 以及如何将竞争网络拓展到包含不同相互作用类型的生态网络, 将是未来非传递性竞争研究的重要方向。对非传递性竞争的研究有助于整合生物间的各种相互作用, 构建更加现实合理的生态网络, 并加深对物种共存和生物多样性维持机制的认识, 进而有助于指导生物多样性的保护和恢复工作。     

Convergence in plant traits between species within grassland communities simplifies their monitoring

Plant trait measurement is a very powerful and promising method for assessing the effects of land use change on ecosystem behavior in grasslands, but it is very time-consuming. Hence we pose the following questions for simplifying diagnosis and monitoring: (i) are plant traits (PTs) similar between plant life forms (PLF: grasses, rosettes, upright forbs, legumes) within a plant community? (ii) is it possible to define the main plant community characteristics by measuring traits on one PLF or a limited number of dominant species?Six PTs known for their ability to characterize the capacity of species to exploit resource-rich or -poor environments and for their competitive dominance in response to disturbance (specific leaf area (SLA), leaf dry matter content (LDMC), plant height (H), C and N contents, flowering time) were measured on the species of 18 plant communities located in Central Pyrenees. The experiment combined 2 fertility levels and 3 defoliation regimes (cutting, grazing). Comparisons were made between the weighted values at community, PLF and two dominant species levels. Regression analysis shows that there were significant correlations between grasses and rosettes for 4 PTs. For H, N and C:N ratio, data for both grass and rosette PLFs were close to the bisecting line. The largest difference in the intercept was observed for LDMC. On the basis of plant traits weighted for all the species, plant communities were ranked in similar ways for SLA and H (Spearman r > 0.93; p < 0.001) and to a lesser extent for LDMC (r = 0.72; p < 0.001). Convergence in weighted plant traits for different PLFs within a plant community mean that in the studied grasslands, defoliation regime and nutrient availability act as strong filters that impose, at least at PLF level, very similar PFTs. This determines a specific local community structure and composition. An application of this result in managed grasslands is the possibility of focusing on one PLF or a limited number of species for vegetation diagnosis and monitoring.     

Interspecific competition alters leaf stoichiometry in 20 grassland species

The extensive use of traits in ecological studies over the last few decades to predict community functions has revealed that plant traits are plastic and respond to various environmental factors. These plant traits are assumed to predict how plants compete and capture resources. Variation in stoichiometric ratios both within and across species reflects resource capture dynamics under competition. However, the impact of local plant diversity on species‐specific stoichiometry remains poorly studied. Here, we analyze how spatial and temporal diversity in resource‐acquisition traits affects leaf elemental stoichiometry of plants (i.e. the result of resource capture) and how flexible this stoichiometry is depending on the functional composition of the surrounding community. Therefore, we assessed inter‐ and intraspecific variations of leaf carbon (C), nitrogen (N), and phosphorus (P) (and their ratios) of 20 grassland species in a large trait‐based plant diversity experiment located in Jena (Germany) by measuring leaf elemental concentrations at the species‐level along a gradient in plant trait dissimilarity. Our results show that plants showed large intra‐ and interspecific variation in leaf stoichiometry, which was only partly explained by the functional group identity (grass or herb) of the species. Elemental concentrations (N, P, but not C) decreased with plant species richness, and species tended to become more deviant from their monoculture stoichiometry with increasing trait dissimilarity in the community. These responses differed among species, some consistently increased or decreased in P and N concentrations; for other species, the negative or positive change in P and N concentrations increased with increasing trait difference between the target species and the remaining community. The strength of this relationship was significantly associated to the relative position of the species along trait gradients related to resource acquisition. Trait‐difference and trait‐diversity thus were important predictors of how species’ resource capture changed in competitive neighbourhoods.     

土壤微生物增强了外来植物紫茎泽兰对本地植物种的竞争力

【Background】The soil microbial community plays an important role in plant establishment, growth and nutrition. Invasion success may be linked to plant microbe interactions. 【Method】Under glasshouse conditions, we compared the effect of soil microbial communities to the growth and interactions between the exotic weed Ageratina adenophora and native plants. The microbial communities were from soil invaded by A.adenophora (IS) vs. that dominated by native weeds (NS). 【Result】A.adenophora which received inoculum from IS had higher arbuscular mycorrhizal colonization rate than that from NS, especially when Medicago falcata or Setaria viridis grew near A.adenophora. Microbial inoculum from IS accelerated the growth of A.adenophora, when planted in polyculture with the native plant S.viridis, but the native species growth was not affected. A.adenophora, receiving an inoculum from IS, inhibited the growth of its two neighboring native species, while no such effect was observed when using inoculum from NS. A.adenophora responded positively to the inoculum taken from IS in all planting combinations, but responded negatively to inoculum from NS both in monoculture and in polyculture with M.falcata. 【Conclusion and significance】Soil microbes, including arbuscular mycorrhizal fungi present in soil in the rhizosphere of A.adenophora enhanced the competiveness of this invasive weed against native species, which may be an important invasion mechanism of exotic plants.     

Soil microbiome mediates positive plant diversity‐productivity relationships in late successional grassland species

Which processes drive the productivity benefits of biodiversity remain a critical, but unanswered question in ecology. We tested whether the soil microbiome mediates the diversity‐productivity relationships among late successional plant species. We found that productivity increased with plant richness in diverse soil communities, but not with low‐diversity mixtures of arbuscular mycorrhizal fungi or in pasteurised soils. Diversity‐interaction modelling revealed that pairwise interactions among species best explained the positive diversity‐productivity relationships, and that transgressive overyielding resulting from positive complementarity was only observed with the late successional soil microbiome, which was both the most diverse and exhibited the strongest community differentiation among plant species. We found evidence that both dilution/suppression from host‐specific pathogens and microbiome‐mediated resource partitioning contributed to positive diversity‐productivity relationships and overyielding. Our results suggest that re‐establishment of a diverse, late successional soil microbiome may be critical to the restoration of the functional benefits of plant diversity following anthropogenic disturbance.     

Rareness starts early for disturbance-dependent grassland plant species

Ecological communities always contain a few common species and an abundance of uncommon species. Given that most plant mortality occurs in seeds and seedlings, recruitment success often predicts plant community assemblage and patterning, but observational patterns do not reveal whether plant populations are seed or habitat limited. Grassland plant species make up a sizable portion of the overall native flora in northeastern North America (N.A.), but approximately 30 % of the area’s threatened and endangered flora are grassland species, possibly leftovers from the post-glacial landscape. Yet, close relatives of many rare grassland species thrive in the same range. We investigated whether seed or habitat limitation explained rarity and commonness in remnant grassland species. We used seed addition experiments coupled with microhabitat manipulations (burning and herbivore exclusion) in three different habitat types to evaluate recruitment (germination and seedling survival) limitation for three rare and three common grassland species. Rare grassland species successfully recruited where burning reduced initial competitor density, but seedling survival suggested they were severely limited by interspecific competition. Both the rare and common plant species survived equally well in forest habitats where herbaceous density was low whereas neither survived in the edge habitats. Only the common plants thrived in the high-competition meadow habitat, further suggesting that the rare grassland species are poor competitors. Commonness and rarity are temporal designations that can change as disturbance alters the landscape. Glacial retreat and low precipitation in northeastern N.A. created a landscape suitable for poor competitors that tolerated poor conditions. Our results suggest that rare remnant grassland plants, unlike their close relatives, display more biotic than abiotic limitation as they do not compete well with other plants. These results suggest that suitable habitat is not a spatial location but a temporally transient assemblage of species requirements.     

[CO2]- and density-dependent competition between grassland species

The predicted ongoing increase of atmospheric carbon dioxide levels is considered to be one of the main threats to biodiversity due to potential changes in biotic interactions. We tested whether effects of intra‐ and interspecific planting density of the calcareous grassland perennials Bromus erectus and Carex flacca change in response to elevated [CO₂] (600 ppm) by using factorial combinations of seven densities (0, 1, 2, 4, 8, 16, 24 tillers per 8 × 8 cm² cell) of both species in plots with and without CO₂ enrichment. Although aboveground biomass of C. flacca was increased by 54% under elevated [CO₂], the combined aboveground biomass of the whole stand was not significantly increased. C. flacca tended to produce more tillers under elevated [CO₂] while B. erectus produced less tillers. The positive effect of [CO₂] on the number of tillers of C. flacca was strongest at high intraspecific densities. On the other hand, the negative effect of [CO₂] on the number of tillers of B. erectus was not present at intermediate intraspecific planting densities. Seed production of C. flacca was more than doubled under elevated [CO₂], while seed production of B. erectus was not affected. Moreover, the mass per seed of C. flacca was increased by elevated [CO₂] at intermediate interspecific planting densities while the mass per seed of B. erectus was decreased by elevated [CO₂] at high interspecific planting densities. Our results show that the responses of C. flacca and B. erectus to elevated [CO₂] depend in a complex way on initial planting densities of both species. In other words, competition between these two model species is both [CO₂]‐ and density dependent. On average, however, the effects of [CO₂] on the individual species indicate that the composition of calcareous grasslands is likely to change under elevated [CO₂] in favor of C. flacca.     

接种AM真菌对玉米和油菜种间竞争及土壤无机磷组分的影响

在温室盆栽条件下,分别模拟单作、间作和尼龙网分隔种植,比较接种丛枝菌根(arbuscular mycorrhizal, AM)真菌Glomus intraradices和Glomus mosseae对菌根植物玉米和非菌根植物油菜生长和磷吸收状况的影响,并分析土壤中各无机磷组分的变化。结果发现,接种AM真菌可以促进土壤中难溶性磷(Ca₁₀-P和O-P)向有效态磷转化,并显著降低总无机磷含量 (P<0.05),显著提高菌根植物玉米的生物量和磷吸收量(P<0.05),特别是在间作体系中使玉米的磷营养竞争比率显著提高了45.0%-104.1% (P<0.05),显著降低了油菜的生物量和磷吸收量(P<0.05),从而增强了了菌根植物的竞争优势,降低了非菌根植物与菌根植物的共存能力。揭示了石灰性土壤中AM真菌对植物物种多样性的影响,有助于更加全面地理解AM真菌在农业生态系统中的作用。     

Effects of nitrogen and sulphur gradients on plant competition, N and S use efficiencies and species abundance in a grassland plant mixture

Sulphur (S) depletion of grassland soils has occurred in Europe for many decades. This is known to promote a decrease in ecosystem productivity and is suspected to alter plant community structure. Considering the strong links between nitrogen (N) and S metabolism in plants, these effects should depend on N availability. We tested this hypothesis in a pot experiment, considering a four grassland species plant mixture (three Poaceæs: Lolium perenne, Agrostis capillaris and Poa pratensis and one Fabaceæ: Trifolium repens), and submitted it to a double N and S gradient. We used labelled ¹⁵N-fertilizer and ³⁴S-fertilizer in order to determine both nutrient use efficiencies by each species and to analyze the influence of competition for these nutrients on plant mixture dynamics. We compared species relative physiological performance (RPP) in the monoculture and their relative ecological performance (REP) in the mixture of the four species. We analysed gradient effects at establishment and at regrowth after cutting. At establishment, grass production and S use efficiency increased along the N gradient. The S gradient slightly favoured the dominance of L. perenne, increased A. capillaris production and enhanced N use efficiency of both species. At regrowth, increased S promoted more significant effects, enhancing T. repens performance in increasing its N₂ fixation ability and maintaining this at high N. It also induced a change in grass species relative performance (dry matter production and N use efficiency) at high N, enhancing that of L. perenne and decreasing that of A. capillaris. At both establishment and regrowth, RPP did not reflect REP, meaning that species behave differently along the gradient when grown in mixture. Finally, the S gradient and the N gradient modulated relative plant species abundance. It appears that modulation of S availability could be used as a tool to drive grassland community structure.     

Variation in nectar quality across 34 grassland plant species

• Floral nectar is considered the most important floral reward for attracting pollinators. It contains large amounts of carbohydrates besides variable concentrations of amino acids and thus represents an important food source for many pollinators. Its nutrient content and composition can, however, strongly vary within and between plant species. The factors driving this variation in nectar quality are still largely unclear.
• We investigated factors underlying interspecific variation in macronutrient composition of floral nectar in 34 different grassland plant species. Specifically, we tested for correlations between the phylogenetic relatedness and morphology of plants and the carbohydrate (C) and total amino acid (AA) composition and C:AA ratios of nectar.
• We found that compositions of carbohydrates and (essential) amino acids as well as C:AA ratios in nectar varied significantly within and between plant species. They showed no clear phylogenetic signal. Moreover, variation in carbohydrate composition was related to family-specific structural characteristics and combinations of morphological traits. Plants with nectar-exposing flowers, bowl- or parabolic-shaped flowers, as often found in the Apiaceae and Asteraceae, had nectar with higher proportions of hexoses, indicating a selective pressure to decelerate evaporation by increasing nectar osmolality.
• Our study suggests that variation in nectar nutrient composition is, among others, affected by family-specific combinations of morphological traits. However, even within species, variation in nectar quality is high. As nectar quality can strongly affect visitation patterns of pollinators and thus pollination success, this intra- and interspecific variation requires more studies to fully elucidate the underlying causes and the consequences for pollinator behaviour.

     

Interspecific interactions and biomass allocation among grassland plant species

Although a handful of studies have shown how interspecific interactions may influence plant shoot to root ratios, the issue of how these interactions influence biomass partitioning among coexisting plant species remains largely unexplored. In this study, we determined whether a given plant species could induce other plant species to allocate relative biomass to each of four zones (aboveground, and three soil depth layers) in a different manner to what they would otherwise, and whether this may influence the nature of competitive or facilitative interactions amongst coexisting plant species. We used a glasshouse study in which mixtures and monocultures of ten grassland plant species were grown in cylindrical pots to determine the effects of plant species mixtures versus monocultures on the production of shoots and of roots of other species for each of three soil depths. Across all experiments, stimulation of production in mixtures was far less common than suppression of production. Different plant species shifted their allocation to shoots or roots at different depths, suggesting that interspecific interactions can either: (1) increase the ratio of deep to shallow roots, perhaps because competition reduces root growth in the uppermost part of the soil profile; or (2) decrease this ratio by reducing plant vigour to such an extent that the plant cannot produce roots that can reach deep enough to exploit resources at lower depths. Further, these results suggest that there are instances in which competition may have the potential to enforce resource partitioning between coexisting plant species by inducing different species to root at different depths to each other.     

Modification of competition between two grass species by a hemiparasitic plant and simulated grazing

Plant parasitism and herbivory are common phenomena in natural grasslands, where they may significantly affect competition between plant species. However, only few studies have simultaneously examined these two processes. We investigated whether the root hemiparasite Odontites litoralis ssp. litoralis affects the outcome of competition between two clonal graminoids, the endangered Puccinellia phryganodes and the common species Agrostis stolonifera, and whether simulated grazing affects the interaction among these three species. This study system simulates the community of early successional stages of the Bothnian Bay salt marsh meadows, which are intensively grazed by greylag geese (Anser anser). We conducted a factorial greenhouse experiment to study the effects of interspecific competition (one or two host species present), hemiparasitic infection (hemiparasite present or not), and simulated grazing (host clipped or not) on Puccinellia and Agrostis. Puccinellia was clearly an inferior competitor to Agrostis, whereas the two species did not differ as hosts for the hemiparasite. Infection by the hemiparasite reduced the aboveground biomass of Puccinellia and Agrostis by 59% and 45%, respectively. Competition with Agrostis decreased the biomass of parasitised Puccinellia by 36% and that of non-parasitised Puccinellia by 56%. Parasitism thus seemed to benefit Puccinellia indirectly by decreasing the relative competitive advantage of Agrostis. Moreover, parasitism increased the relative contribution of Puccinellia to the total aboveground host plant biomass. Simulated grazing decreased the aboveground biomass of Agrostis significantly more than that of Puccinellia and thus increased the competitive ability of Puccinellia. Simulated grazing of the two host species did not affect the performance of Odontites. These results suggest that both hemiparasitic plants and herbivory may play a significant role in the maintenance of plant species diversity by promoting competitively inferior species.