Exotic tree seedlings are much more competitive than natives but show underyielding when growing together

Aims Invasive species continue to be a worldwide threat to ecosystems mainly as a cause for biodiversity loss. Forest ecosystems, for example, are subject to a change in species composition due to the invasion of exotic species. Specifying the attributes that cause the strong competitiveness of several exotic species may improve the ability to understand and effectively manage plant invasions in the future. In this study the following hypotheses were tested: (1) biomass production of below- and aboveground plant components of the exotic tree species is higher than that of the natives, resulting in a higher competitiveness of the exotics; (2) the exclusion of root competition has a positive effect on the biomass production of the inferior native species; and (3) mixtures of native and exotic species yield a higher biomass production than the respective monocultures.Methods A pot experiment, containing about 2000 tree seedlings, was established. We investigated the biomass productivity and growth reactions of two native (Quercus robur L., Carpinus betulus L.) and two exotic tree species (Prunus serotina Ehrh., Robinia pseudoacacia L.) in different intra- and interspecific, competitive situations with and without the influence of root competition.Important findings The biomass production of both exotic species was significantly higher and led to a strong competitive advantage, resulting in a biomass decrease of the less competitive native species. The high belowground biomass of both exotic species had a negative effect on the biomass production. The competitive pressure of exotic tree seedlings on the native ones was largely driven by root competition. Furthermore, mixtures of native and exotic tree species had a higher productivity than their growth in monocultures would have predicted. Competition was lower for exotic species in mixtures with the less productive native species compared to the competition in monocultures or in mixture with the other highly productive exotic species. Accordingly, both highly competitive exotic species produced less biomass in mixture with each other compared to monocultures. Despite the significantly higher biomass of P. serotina in all mixtures and in monoculture, R. pseudoacacia seemed to be the dominating species. Due to its strong root competition, R. pseudoacacia significantly reduced the biomass production of P. serotina .     

Selecting plants and nitrogen rates to vegetate crude-oil-contaminated soil

Phytoremediation can be effective for remediating contaminated soils in situ and generally requires the addition of nitrogen (N) to increase plant growth. Our research objectives were to evaluate seedling emergence and survival of plant species and to determine the effects of N additions on plant growth in crude-oil-contaminated soil. From a preliminary survival study, three warm-season grasses--pearlmillet (Pennisetum glaucum [L.] R. Br.), sudangrass (Sorghum sudanense [Piper] Stapf [Piper]), and browntop millet (Brachiaria ramosa L.)--and one warm-season legume--jointvetch (Aeschynomene americana L.)--were chosen to determine the influence of the N application rate on plant growth in soil contaminated with weathered crude oil. Nitrogen was added based on total petroleum hydrocarbon-C:added N ratios (TPH-C:TN) ranging from 44:1 to 11:1. Plant species were grown for 7 wk. Root and shoot biomass were determined and root length and surface area were analyzed. Pearlmillet and sudangrass had higher shoot and root biomass when grown at a TPH-C:TN (inorganic) ratio of 11:1 and pearlmillet had higher root length and surface area when grown at 11:1 compared with the other species. By selecting appropriate plant species and determining optimum N application rates, increased plant root growth and an extended rhizosphere influence should lead to enhanced phytoremediation of crude-oil-contaminated soil.     

Effects of ramet clipping and nutrient availability on growth and biomass allocation of yellow nutsedge

A glasshouse experiment was conducted to examine how the interactions of nutrient availability and partial ramet clipping affect growth, reproduction and biomass allocation of Cyperus esculentus, an invasive sedge. The plants sprouting from tubers were grown at low and high nutrient levels, and were subject either to no clipping, one, two or three clippings, with each clipping cutting half of the existing ramets at soil level. Our results show that nutrient availability and clipping frequency tended to independently affect most of growth, reproduction and biomass allocation parameters of Cyperus esculentus examined in the present study. Increased supply of nutrients led to an increase in plant productivity and its associated traits. All of the traits, except for the number of ramets, displayed a decreasing pattern with increasing clipping frequency, indicating that Cyperus esculentus had undercompensatory responses to ramet clipping. It is likely that the patterns of plants response to clipping are species specific, and depend on morphological characters of species. Its susceptibility to ramet clipping can offer opportunities for controlling this invasive species through mechanical methods such as mowing. Clipping had little effects on biomass allocation; however, root weight fraction increased with increasing clipping frequency. While nutrient availability and clipping frequency had no influence on leaf carbon concentration at harvest, both of them increased leaf nitrogen concentration, and hence reduced leaf C/N ratio.     

Nitrogen fertilization has minimal influence on rhizosphere effects of smooth crabgrass (Digitaria ischaemum) and bermudagrass (Cynodon dactylon)

Aims Plants generally respond to nitrogen (N) fertilization with increased growth, but N addition can also suppress rhizosphere effects, which consequently alters soil processes. We quantified the influence of N addition on rhizosphere effects of two C₄ grasses: smooth crabgrass (Digitaria ischaemum) and bermudagrass (Cynodon dactylon).Methods Plants were grown in nutrient-poor soil for 80 days with either 20 or 120 μg NH 4 NO 3 -N g dry soil^-1. N mineralization rates, microbial biomass, extracellular enzyme activities and bacterial community structure were measured on both rhizosphere and bulk (unplanted) soils after plant harvest.Important findings Fertilization showed nominal differences in net N mineralization, extracellular enzyme activity and microbial biomass between the rhizosphere and bulk soils, indicating minimal influence of N on rhizosphere effects. Instead, the presence of plant roots showed the strongest impact (up to 80%) on rates of net N mineralization and activities of three soil enzymes indicative of N release from organic matter. Principal component analysis of terminal restriction fragment length polymorphism (T-RFLP) also reflected these trends by highlighting the importance of plant roots in structuring the soil bacterial community, followed by plant species and N fertilization (to a minor extent). Overall, the results indicate minor contributions of short-term N fertilization to changes in the magnitude of rhizosphere effects for both grass species.     

Aboveground resilience to species loss but belowground resistance to nitrogen addition in a montane plant community

Aims Decades of empirical work have demonstrated how dominant plant species and nitrogen fertilization can influence the structure and function of plant communities. More recent studies have examined the interplay between these factors, but few such studies use an explicit trait-based framework. In this study, we use an explicit trait-based approach to identify potential mechanisms for community-level responses and to test ecological niche theory.Methods We experimentally manipulated plant communities (control, ?dominant species, ?random biomass) and nitrogen (N) inputs (control, +organic N, +inorganic N) in a fully factorial design. We predicted that traits related to plants' ability to take up different forms of soil N would differ between dominant and subordinate species, resulting in interactive effects of dominant species loss and N fertilization on plant community structure and function. The study took place in a montane meadow in the Rocky Mountains, Colorado, USA.Important findings After four years, the plant community in removal plots converged toward a species composition whose leaf and root functional traits resembled those of the previously removed dominant species. Ecosystem productivity generally increased with N addition: soil carbon efflux was ~50% greater when either form of N was added, while inorganic N addition increased aboveground biomass production by ~60% relative to controls. The increase in production was mediated by increased average height, leaf mass:area ratio and leaf dry matter content in plant communities to which we added inorganic N. Contrary to our predictions, there were no interactive effects of N fertilization and dominant species loss on plant community structure or ecosystem function. The plant community composition in this study exhibited resistance to soil N addition and, given the functional convergence we observed, was resilient to species loss. Together, our results indicate that the ability of species to compensate functionally for species loss confers resilience and maintains diversity in montane meadow communities.     

A quantitative review comparing the yield of switchgrass in monocultures and mixtures in relation to climate and management factors

Switchgrass (Panicum virgatum L.), a US Department of Energy model species, is widely considered for US biomass energy production. While previous studies have demonstrated the effect of climate and management factors on biomass yield and chemical characteristics of switchgrass monocultures, information is lacking on the yield of switchgrass grown in combination with other species for biomass energy. Therefore, the objective of this quantitative review is to compare the effect of climate and management factors on the yield of switchgrass monocultures, as well as on mixtures of switchgrass, and other species. We examined all peer‐reviewed articles describing productivity of switchgrass and extracted dry matter yields, stand age, nitrogen fertilization (N), temperature (growing degree days), and precipitation/irrigation. Switchgrass yield was greater when grown in monocultures (10.9 t ha^?1, n=324) than when grown in mixtures (4.4 t ha^?1, n=85); yield in monocultures was also greater than the total yield of all species in the mixtures (6.9 t ha^?1, n=90). The presence of legume species in mixtures increased switchgrass yield from 3.1 t ha^?1 (n=65) to 8.9 t ha^?1 (n=20). Total yield of switchgrass‐dominated mixtures with legumes reached 9.9 t ha^?1 (n=25), which was not significantly different from the monoculture yield. The results demonstrated the potential of switchgrass for use as a biomass energy crop in both monocultures and mixtures across a wide geographic range. Monocultures, but not mixtures, showed a significant positive response to N and precipitation. The response to N for monocultures was consistent for newly established (stand age <3 years) and mature stands (stand age ≥3 years) and for lowland and upland ecotypes. In conclusion, these results suggest that fertilization with N will increase yield in monocultures, but not mixtures. For monocultures, N treatment need not be changed based on ecotype and stand age; and for mixtures, legumes should be included as an alternative N source.     

Effects of plant species richness on stand structure and productivity

Aims Aboveground biomass production commonly increases with species richness in plant biodiversity experiments. Little is known about the direct mechanisms that cause this result. We tested if by occupying different heights and depths above and below ground, and by optimizing the vertical distribution of leaf nitrogen, species in mixtures can contribute to increased resource uptake and, thus, increased productivity of the community in comparison with monocultures.Methods We grew 24 grassland plant species, grouped into four nonoverlapping species pools, in monoculture and 3- and 6-species mixture in spatially heterogeneous and uniform soil nutrient conditions. Layered harvests of above- and belowground biomass, as well as leaf nitrogen and light measurements, were taken to assess vertical canopy and root space structure.Important findings The distribution of leaf mass was shifted toward greater heights and light absorption was correspondingly enhanced in mixtures. However, only some mixtures had leaf nitrogen concentration profiles predicted to optimize whole-community carbon gain, whereas in other mixtures species seemed to behave more 'selfish'. Nevertheless, even in these communities, biomass production increased with species richness. The distribution of root biomass below ground did not change from monocultures to three- and six-species mixtures and there was also no indication that mixtures were better than monocultures at extracting heterogeneously as compared to homogeneously distributed soil resources. We conclude that positive biodiversity effect on aboveground biomass production cannot easily be explained by a single or few common mechanisms of differential space use. Rather, it seems that mechanisms vary with the particular set of species combined in a community.     

Time course of plant diversity effects on Centaurea jacea establishment and the role of competition and herbivory

Aims Invasion resistance in experimental plant communities is known to increase with increasing diversity and further to depend on the presence of particular functional groups. To test whether these effects also hold true for the invader establishment phase beyond the seedling stage, we studied survival and performance of Centaurea jacea L. (brown knapweed) planted into experimental grassland communities of varying plant biodiversity over three consecutive years. Moreover, we analysed the role of insect herbivory and biomass of the recipient community for mediating diversity effects.Methods In 2005, seedlings of Centaurea were transplanted into experimental grassland communities (the Jena Experiment) covering a species richness (1–60) and functional group richness (1–4) gradient. Half of these transplants and the community surrounding them in each plot were sprayed with insecticide while the other half served as control. In 2006 and 2007 (during the second and third year after transplantation), we recorded survival, growth-related (e.g. transplant biomass, height) and reproduction-related traits (e.g. number of flower heads). Annual data on community aboveground biomass served as covariate to investigate mediating effects of aboveground competition with the recipient community.Important findings Species richness was the most important factor responsible for Centaurea limitation. Higher levels of diversity decreased survival and all performance traits in both years. These diversity effects were partly driven by community biomass, but not fully explained by that covariate, suggesting the importance also of further processes. The influence of functional group richness was strong in the second year after transplantation and weaker in the third year. Among the particular functional groups, only the presence of legumes showed strong negative effects on Centaurea survival and weak negative effects on growth and reproduction, the latter two being mediated by biomass. Insect herbivore reduction considerably benefited Centaurea in sprayed monocultures, where it grew significantly larger than in all other diversity levels and than in the control subplots. We conclude that effects of plant community properties on invading individuals change in the course of establishment, that plant species richness effects are also important during later stages of establishment, and that biomass (especially at high diversity) and herbivory (especially at low diversity) of the recipient community are important in mediating community effects on invaders.     

氮素和植物生长促进剂对羊草生长及竞争力的影响

选取羊草、斜茎黄芪、克氏针茅3种内蒙古典型草原常见植物,通过不同植物物种组合(单独种植或者羊草与其他两种混合种植)、施加不同剂量的氮素(0、0.1、0.2、0.4、0.8mg·g^-1)和植物生长促进剂——油菜素内酯(0.005 mg·g^-1)的温室盆栽试验,分析氮素和油菜素内酯对羊草生长及其与其他植物竞争力关系的影响.结果表明:添加氮素对羊草、斜茎黄芪的生物量影响显著,对克氏针茅的生物量影响较弱,并且氮素的影响因其伴生植物的不同而改变.氮素添加显著增加了羊草在单独种植和与斜茎黄芪混合种植时的地上生物量,但显著降低了与克氏针茅混合种植时的地下生物量.由于土壤可利用氮增加,羊草的地上与地下分配发生变化,表现为单独种植、与克氏针茅混合种植时,根冠比下降;与斜茎黄芪混合种植时,氮素添加对羊草根冠比的影响则不显著.油菜素内酯对植物生长的显著影响不普遍,仅显著降低了斜茎黄芪单独种植时的地下生物量;在羊草与斜茎黄芪混合种植时,显著增加了羊草地上生物量.油菜素内酯和氮适量联用可有效增加特定物种组合的生物量,从而在退化草地恢复中具有一定的应用前景.     

水分和养分添加对羊草功能性状和地上生物量的影响

研究水分和养分添加对植物功能性状的影响, 对于揭示植物对环境变化的响应和适应规律至关重要。该文采用盆栽试验的方法, 进行不同水平水分处理(增水50%, 减水50%, 以498 mm降水量作为对照)和养分添加(无养分添加, 单施氮肥, 单施磷肥, 氮磷共施), 研究羊草(Leymus chinensis)的10种功能性状和地上生物量对水分和养分添加的响应。得出以下结论: (1)双因素方差分析结果表明, 水分主效应对羊草株高、分蘖数、茎生物量、叶生物量、叶面积、叶质量、净光合速率、蒸腾速率、水分利用效率存在显著影响; 养分主效应对羊草分蘖数、茎生物量、净光合速率、蒸腾速率、水分利用效率存在显著影响; 水分和养分的交互作用对羊草分蘖数、茎生物量、蒸腾速率、水分利用效率存在显著影响。(2)各功能性状对降水量的响应在不同养分添加水平是不同的, 分蘖数和叶面积在单施氮肥和氮磷共施条件下随降水量增加而增加, 而在无养分添加和单施磷肥条件下无显著变化; 茎生物量在无养分添加、单施氮肥和单施磷肥条件下随降水量增加而增加, 而在氮磷共施条件下无增加趋势; 比叶面积在单施氮肥条件下增水处理显著低于对照组, 而在其他养分添加条件下无明显变化。(3)短期氮磷处理显著影响羊草叶片光合生理性状, 而对叶形态性状影响不显著。(4)羊草地上生物量随降水量的增加呈现上升趋势, 并且在单施氮肥条件下, 增水处理使地上生物量达到最高, 为522.55 g·m ^-2。总之, 羊草的功能性状对降水量增加表现出明显的响应, 响应格局在不同养分条件下不同, 反映了其对水肥环境变化的适应。     

Warming changes plant competitive hierarchy in a temperate steppe in northern China

Aims Quantifying changes in plant growth and interspecific interactions, both of which can alter dominance of plant species, will facilitate explanation and projection of the shifts in species composition and community structure in terrestrial biomes expected under global warming. We used an experimental warming treatment to examine the potential influence of global warming on plant growth and interspecific interactions in a temperate steppe in northern China.Materials and Methods Six dominant plant species were grown in monoculture and all 15 two-species mixtures for one growing season under ambient and elevated temperatures in the field. Temperature was manipulated with infrared radiators.Important findings Total biomass of all the six plant species was increased by 34–63% in monocultures and 20–76% in mixtures. The magnitude of the warming effect on biomass was modified by plant interactions. Experimental warming changed the hierarchies of both competitive response and competitive effect. The competitive ability (in terms of response and effect) of one C₄ grass (Pennisetum centrasiaticum) was suppressed, while the competitive abilities of one C₃ forb (Artemisia capillaris) and one C₃ grass (Stipa krylovii) were enhanced by experimental warming. The demonstrated alterations in growth and plant interactions may lead to changes in community structure and biodiversity in the temperate steppe in a warmer world in the future.     

Plant biomass production and soil nitrogen in mixtures and monocultures of old field Mediterranean annuals

We examine the relationship between plant diversity and ecosystem properties in a Mediterranean grassland. Five legumes, three grasses and two forb species are grown in monocultures and compared with mixtures that include these ten species. Trifolium angustifolium L. (a legume), Lolium rigidum Gaudin (a grass), and Centaurea solstitialis L. (a forb), are replicated in monocultures. Plant cover, root length and biomass, and concentrations of soil nitrate and ammonium are measured in all plots in March and May. Aboveground biomass is measured at a final harvest in late May to early June. Root biomass is significantly higher in the species mixtures than the average of the monocultures. Plant cover and root length are marginally significantly higher (0.05 < P ≤ 0.1) in the mixtures compared to the average of the monocultures. Soil inorganic nitrogen concentrations and aboveground biomass do not significantly differ between the average of the monocultures and the mixtures. Aboveground biomass in T. angustifolium monocultures is significantly higher than in the mixtures, and on average the legume monocultures do not differ significantly from the mixtures. Root length and biomass in L. rigidum monocultures are higher than in the mixtures in March. Nitrate concentrations (which are negatively correlated with root length and biomass) are the lowest in C. solstitialis in May. Thus, we have evidence that some of the measures of ecosystem performance decline in the average of the monocultures when compared with the mixtures, but mixtures never outperform or do more poorly than the best performing monocultures.     

氮肥和种植密度对达乌里胡枝子的生长与生物固氮的影响

氮供给和种植密度是影响植物生长的两个重要因素。豆科植物因其生物固氮能力而在受到氮限制的生态系统中具有重要作用, 氮含量增加促进植物生长的同时也会抑制豆科植物的生物固氮能力, 种植密度会通过种内竞争影响豆科植物的生长和生物固氮能力, 然而少有研究关注氮肥添加和种植密度对豆科植物生长和生物固氮能力的影响。该研究以达乌里胡枝子(Lespedeza davurica)为研究对象, 通过温室盆栽实验, 探究氮肥和种植密度对其生长和生物固氮的影响。实验设置4个氮添加水平(0、5、10、20 g·m^-2·a^-1)和3种种植密度(1、3、6 Ind.·pot^-1, 约32、96、192 Ind.·m^-2)。结果发现: 1)施肥和密度增加均影响了达乌里胡枝子的生长。叶片碳(C)、氮(N)含量、净光合速率随施氮量增加而增加, 氮添加也促进了植物的生长, 当施氮量为10 g·m^-2·a^-1时植物产量达到最大。叶片C、N含量、净光合速率随种植密度增加而下降, 密度增加可以促进每盆的总生物量, 但对单个植株的生长有负效应。2)氮肥对根瘤形成有抑制作用, 但种植密度增加会缓解氮肥对生物固氮能力带来的“氮阻遏”。该实验条件下, 当施氮量为10 g·m^-2·a^-1, 种植密度为3 Ind.·pot^-1, 或施氮量为5 g·m^-2·a^-1, 种植密度为6 Ind.·pot^-1时, 能最大程度发挥“施氮增产”和种植密度缓解“氮阻遏”的作用。氮添加降低了达乌里胡枝子的根瘤生物量和对根瘤形成的投资(根瘤生物量占总生物量的比例), 从而抑制达乌里胡枝子的生物固氮。种植密度增加导致达乌里胡枝子因种内竞争增加而使资源获取受限, 从而增加对根瘤的投资和根瘤生物量来获得更多来自大气中的氮。3)结构方程结果显示, 氮肥和种植密度通过直接或间接作用, 解释了64%的达乌里胡枝子生物量变化和42%的根瘤生物量变化。上述结果表明合理优化豆科植物的施肥量和种植密度可能对人工草地种植以及退化草地恢复管理具有重要意义。     

Traits of an invasive grass conferring an early growth advantage over native grasses

Aims Invasive species often have higher relative growth rates (RGR) than their native counterparts. Nutrient use efficiency, total leaf area and specific leaf area (SLA) are traits that may confer RGR differences between natives and invasives, but trait differences are less prominent when the invasive species belongs to the same plant functional type as the dominant native species. Here, we test if traits displayed soon after germination confer an early size advantage. Specifically, we predicted that invasive species seedlings grow faster than the natives because they lack trade-offs that more strongly constrain the growth of native species.Methods We quantified plant morphological and physiological traits and RGR during early seedling growth at high and low nutrient levels in three dominant perennial native C₄ grasses: Panicum virgatum L. (switchgrass), Schizachyrium scoparium (Michx.) Nash (little bluestem) and Andropogon gerardii Vitman (big bluestem); and a perennial C₄ exotic invasive grass, Sorghum halepense (L.) Pers. (Johnsongrass).Important findings After 2 weeks of growth, Johnsongrass seedlings had greater biomass, SLA and photosynthetic nitrogen use efficiency, but lower leaf N concentrations (% leaf N) and root:shoot ratio than natives. As growth continued, Johnsongrass more quickly produced larger and thicker leaves than the natives, which dampened the growth advantage past the first 2 to 3 weeks of growth. Investment in carbon gain appears to be the best explanation for the early growth advantage of Johnsongrass. In natives, growth was constrained by an apparent trade-off between allocation to root biomass, which reduced SLA, and production of leaves with high N content, which increased carbon gain. In Johnsongrass, root:shoot ratio did not interact with other traits, and % leaf N was decoupled from RGR as a result of a trade-off between the positive indirect association of % leaf N with RGR and the negative direct association of % leaf N with RGR.     

Nitrogen further promotes a dominant salt marsh plant in an increasingly saline environment

Aims Human alterations of the environment are combining in unprecedented ways, making predictions of alterations to natural communities a difficult and pressing challenge. Estuarine systems have been subject to a high degree of modification, including increased nitrogen (N) inputs and altered salinity, factors important in shaping estuarine plant communities. As human populations increase and the climate changes, both N and salinity levels are likely to increase in these coastal marshes. Our objective was to evaluate the interactive effects of N and salinity on US West Coast salt marsh species; in particular, the performance of the dominant species Sarcocornia pacifica (pickleweed) alone and in mixed species assemblages. We expected increased salinity to favor S. pacifica but that N enrichment could help maintain greater species richness through use of N in salinity tolerance mechanisms.Methods We crossed treatments of N (added or not) and salinity (salt added or not) in a field experiment at a salt marsh in the San Francisco Estuary, California, USA, in each of three habitats: (i) monotypic pickleweed on the marsh plain, (ii) monotypic pickleweed along channels and (iii) mixed assemblages along channels. In a greenhouse experiment, we crossed treatments of N (added or not) and salinity (at three levels to simulate brackish to saline conditions) in (i) pots of pickleweed only and (ii) the same species mix as in the field.Important findings N addition doubled S. pacifica biomass and branching in both channel and marsh plain habitats regardless of salinity and greatly increased its dominance over Distichlis spicata and Jaumea carnosa in mixed assemblages along channels. In the greenhouse, S. pacifica biomass increased 6- to 10-fold with N addition over the range of salinities, while D. spicata and J. carnosa biomass increased with N addition only at lower salinity levels. Thus, while localized management could influence outcomes, expected overall increases in both N and salinity with human population growth and climate change are likely to enhance the production of S. pacifica in US West Coast marshes while reducing the diversity of mixed species assemblages. This decline in diversity may have implications for the resilience of marshes already subject to multiple stressors as the climate changes.     

刈割留茬高度和不同播种组合对人工草地初级生产力和物种丰富度的影响

采用7种播种方式(3种单播和4种混播)和3种刈割留茬高度裂区试验,测定了青藏高原高寒地区人工建植3年的垂穗披碱草(Elymus nutans)、中华羊茅(Festuca sinensis)和羊茅(Festuca ovina)草地地上生物量、物种丰富度以及杂草生物量,以分析刈割对青藏高原人工草地初级生产力和物种丰富度的影响。结果显示:(1)在草地建植第2年,不刈割与刈割留茬60和20 mm的草地初级生产力均有显著差异;留茬60和20 mm刈割使单播草地的平均初级生产力分别降低20%和27%,使混播草地的平均初级生产力分别降低29%和37%。(2)草地建植第3年,不刈割、留茬60和20 mm 3个处理间的草地生产力均差异极显著;留茬60和20 mm刈割使单播草地的平均生产力分别降低19%和36%,使混播草地的平均生产力分别降低4%和18%。研究表明,刈割显著降低了人工草地的初级生产力,同时显著增加了垂穗披碱草单播草地的物种丰富度和杂草生物量以及3种牧草混播草地物种丰富度,其他播种草地的物种丰富度和杂草生物量与不刈割草地均无显著差异。     

Changes in species composition,diversity and biomass of herbaceous plant traits due to N amendment in a dry tropical environment of India

Aim European and North American studies have suggested that nitrogen (N) depositions reduce plant diversity and increase primary productivity due to changes in plant traits. To predict the vegetation response to future global change, experimental validations from other regions are widely needed. We assessed the effects of N treatment by urea fertilization on the diversity and biomass of the herbaceous plant traits (HPTs) in a dry tropical environment of India.Methods Diversity and biomass of different HPTs were determined on the basis of data collected in year 2010, from 135, 1 m × 1 m plots distributed over 15 locations. The plots were treated with urea fertilizer in different doses (Control, 60kgNha^-1 yr^-1 and 120kg N ha^-1 yr^-1) since 1st January 2007. The plots were ordinated and data were subjected to appropriate statistical analyses.Important findings Correspondence analysis (CA) suggested uniqueness of species composition due to N amendment. Species number and biomass of the trait categories varied due to N fertilization and traits. All studied trait categories (except N-fixers) yielded maximum mean species number at moderate level of N fertilization. Different levels of N fertilization exhibited different species diversity–primary productivity (D-P) relationships. Further, study showed reduction in plant diversity due to increase in biomass at high rates of N addition.Conclusions Tall, erect, non N-fixers, annuals, grasses HPTs were favoured by N enrichment. N dose above 60kg enhanced the biomass of fast growing, erect, annuals, non N-fixers, nitrophilic HPTs. The changes in traits with N addition, especially the increase in annuals and grasses and decrease in typically N-rich N-fixers, have implications for sustainable cattle production.     

养分添加对天山高寒草地植物多样性和地上生物量的影响

为揭示高寒草地物种多样性和地上生物量以及二者之间关系对养分添加的响应模式,该研究以天山高寒草地为对象,通过两年的多重养分添加实验,研究氮(N)、磷(P)、钾(K)3种养分单独和组合添加对天山高寒草地群落物种多样性和地上生物量的影响。结果表明:(1)养分添加使当地植物物种多样性不同程度地减少,其中以N+P、N+K、N+P+K添加的效应最为显著,多重养分添加导致的土壤生态位维度降低是当地物种丧失的重要原因。(2)养分添加能显著提高群落地上生物量,其中N为第一限制养分,解除N限制后P和K成为限制养分, N+P+K复合添加对地上生物量的提高最为显著。(3)养分添加两年后,地上生物量与物种丰富度之间无显著回归关系且地上生物量增加主要是由于禾草类生物量增加导致,说明地上生物量主要由少数优势种决定而非群落物种数。     

Switchgrass, Big Bluestem, and Indiangrass Monocultures and Their Two- and Three-Way Mixtures for Bioenergy in the Northern Great Plains

High yielding, native warm-season grasses could be used as renewable bioenergy feedstocks. The objectives of this study were to determine the effect of warm season grass monocultures and mixtures on yield and chemical characteristics of harvested biomass and to evaluate the effect of initial seeding mixture on botanical composition over time. Switchgrass (Panicum virgatum L.), indiangrass [Sorghastrum nutans (L.) Nash], and big bluestem (Andropogon gerardii Vitman) were planted as monocultures and in all possible two- and three-way mixtures at three USA locations (Brookings and Pierre, SD and Morris, MN) during May 2002. Biomass at each location was harvested after a killing frost once annually from 2003 to 2005. Total biomass yield significantly increased with year at all locations. Switchgrass monocultures or mixtures containing switchgrass generally out-yielded big bluestem or indiangrass in monocultures or the binary mixture. Cellulose and hemicellulose concentrations were higher in 2004 and 2005 compared with 2003. Switchgrass or mixtures containing switchgrass tended to have less cellulose than either big bluestem or indiangrass. Results were more variable for total N, lignin, and ash. Switchgrass was the dominant component of all mixtures in which it was present while big bluestem was dominant when mixed with indiangrass. Indiangrass was maintained only in monocultures and declined over years when grown in mixtures at all locations. Our results indicated if biomass yield in the northern Great Plains is a primary objective, switchgrass should be a component of binary or tertiary mixtures that also contain big bluestem and/or indiangrass.     

N capture by Plantago lanceolata and Brassica napus from organic material: the influence of spatial dispersion, plant competition and an arbuscular mycorrhizal fungus

This study investigated N capture by Plantago lanceolata L. and Brassica napus L. from complex organic material (dual-labelled with 15N/13C) added either as a thin concentrated layer (discrete patch treatment) or dispersed uniformly with the background sand:soil mix in a 10 cm band (dispersed treatment) when grown in monoculture or in interspecific competition and in the presence or absence of a mycorrhizal inoculum (Glomus mosseae). No 13C enrichments from the organic material were detected in the plant tissues, but 15N enrichments were present. Total plant uptake of N from the organic material on a microcosm basis was not affected by the spatial placement of the organic material, but Plantago monocultures captured less N than the species in interspecific competition (i.e. 23% versus 38% of the N originally added). N capture from Brassica monocultures was no different to either Plantago monocultures or both species in mixture. However, N capture from the organic material by both individual Plantago and Brassica plants was reduced when grown with Brassica plants (by 10-fold and by more than half, respectively). N capture from the organic material was directly related to the estimated root length produced in the sections containing the organic material: the individual that produced the greatest root length captured most N. Strikingly, when the organic material was added as a discrete patch the N captured by Brassica, a non-mycorrhizal species, actually increased when the G. mosseae inoculum was present compared to when G. mosseae was absent (i.e. 35% versus 19% of the N originally added).