Complex dynamics in a carbon-nitrogen model of a grass-legume pasture

A physiologically based model of a grass-legume pasture is used to study the dynamics of these competing species. In our model, we consider carbon and nitrogen pools and fluxes, incorporating competition for light and soil mineral nitrogen, and including the processes of nitrogen fixation, nitrogen losses and dry matter allocation. First, the steadystate responses of each species to nitrogen deposition, to leaching rate, and to other nitrogen losses are examined. We then consider the dynamic behaviour of these species when there is no time delay for nitrogen cycled through the soil organic matter pool. Next, the effects of various time delays associated with the soil organic matter nitrogen pool on the system dynamics are examined: the behaviour becomes complex, non-linear and exhibits lightly or heavily damped oscillations at two frequencies. The high sensitivity of the system both to the initial value of the soil organic matter nitrogen pool, and to any photosynthetic competitive advantage, is investigated. The implications of these results in relation to observations and experiments on grass-legume pastures are discussed.     

The role of legumes as a component of biodiversity in a cross-European study of grassland biomass nitrogen

To investigate how plant diversity loss affects nitrogen accumulation in above‐ground plant biomass and how consistent patterns are across sites of different climatic and soil conditions, we varied the number of plant species and functional groups (grasses, herbs and legumes) in experimental grassland communities across seven European experimental sites (Switzerland, Germany, Ireland, United Kingdom (Silwood Park), Portugal, Sweden and Greece). Nitrogen pools were significantly affected by both plant diversity and community composition. Two years after sowing, nitrogen pools in Germany and Switzerland strongly increased in the presence of legumes. Legume effects on nitrogen pools were less pronounced at the Swedish, Irish and Portuguese site. In Greece and UK there were no legume effects. Nitrogen concentration in total above‐ground biomass was quite invariable at 1.66±0.03% across all sites and diversity treatments. Thus, the presence of legumes had a positive effect on nitrogen pools by significantly increasing above‐ground biomass, i.e. by increases in vegetation quantity rather than quality. At the German site with the strongest legume effect on nitrogen pools and biomass, nitrogen that was fixed symbiotically by legumes was transferred to the other plant functional groups (grasses and herbs) but varied depending on the particular legume species fixing N and the non‐legume species taking it up. Nitrogen‐fixation by legumes therefore appeared to be one of the major functional traits of species that influenced nitrogen accumulation and biomass production, although effects varied among sites and legume species. This study demonstrates that the consequences of species loss on the nitrogen budget of plant communities may be more severe if legume species are lost. However, our data indicate that legume species differ in their N₂ fixation. Therefore, loss of an efficient N₂‐fixer (Trifolium in our study) may have a greater influence on the ecosystem function than loss of a less efficient species (Lotus in our study). Furthermore, there is indication that P availability in the soil facilitates the legume effect on biomass production and biomass nitrogen accumulation.     

Nitrogen fixation and nitrogen balance studies in Rhizobium-VA mycorrhiza inoculated legume-grass association by15N isotope dilution technique under a field condition

A mixed pasture comprising of buffel grass and a legume siratro was studied under field condition for a two-year period to know the fodder yield increase, nitrogen fixation and nitrogen balance with and without the inoculation of VA mycorrhiza to grass and Rhizobium to legume component.¹⁵N dilution technique was followed using labelled ammonium sulphate. The data showed that during the first year of the above study combined inoculation of VA mycorrhiza and Rhizobium to grass and legume respectively significantly increased the total dry matter (DM) (23,900 kg ha^–1 yr^–1) and total N content (308 kg ha^–1 yr^–1) of the mixed pasture over the uninoculated mixture. However, the above increase due to combined inoculation was maximum during second year with respect to DM yield (28,200 kg ha^–1 yr^–1), but the total N harvested through grass-legume mixture was comparatively lower than the first year (297 kg ha^–1 yr^–1). The amount of biologically fixed N was highest in the first year (79 kg ha^–1 yr^–1) and showed a very drastic reduction at the end of second year (39 kg ha^–1 yr^–1). A positive nitrogen balance was observed in the grass-legume mixture irrespective of inoculation of VA mycorrhiza and/or Rhizobium.     

Nitrogen Use Efficiency in Growth of Polygonum cuspidatum Sieb. et Zucc

The growth of Polygonum cuspidatum in sand culture was analysedunder varying nutrient conditions. Nitrogen availability influencednitrogen uptake of plants through the uptake rate per unit rootweight rather than the amount of root. In turn, the differentamounts of nitrogen taken up affected plant growth through theireffects on the rate of leaf expansion. Net assimilation rate (NAR) increased with nitrogen contentper unit leaf area (C), but further increase in leaf nitrogencaused diminishing returns of NAR Optimal nitrogen content perunit leaf area (C_opt) to maximize dry-matter production of aleaf could be determined by drawing a tangent from the onginto a curvilinear relation between NAR and C. This optimal contentdivides a nitrogen-limiting range (C < C_opt) from a carbon-limitingone (C> C_opt) along the axis of nitrogen content. Under nitrogenlimitation, efficiency of nitrogen use in dry-matter productioncould increase if the plant had a larger carbon sink. This givesa qualitative explanation to reduced shoot-to-root ratio underlimited availability of nitrogen. Polygonum cuspidatum Sieb. et Zucc, Japanese knotweed, carbon sink, growth analysis, leaf nitrogen, net assimilation rate, nitrogen use efficiency     

Relative Nitrogen Limitation at Steady-state Nutrition as a Determinant of Plasticity in Five Grassland Plant Species

Partitioning of biomass between roots and different shoot partshas often been used to explain the response of plants to variationsin resource availability. There are still many uncertaintiesin the importance of this trait for plant performance, and clearguidelines on how partitioning should be quantified in relationto growth rate and resource supply are of fundamental importancefor such an understanding. This paper reports an attempt toshow how plant nitrogen status relates to root:shoot partitioningand other plastic responses, in a manner that can be used forquantitative predictions. The reactions to nitrogen limitationof five grassland plant species, with different ecological demands,were compared. The species used were the forbs Polygala vulgarisand Crepis praemorsa, and the grasses Danthonia decumbens, Agrostiscapillaris and Dactylis glomerata. The experiment was conductedin a climate chamber where the plants were grown hydroponically(1) under non-limiting nutrient conditions and (2) at a steady-statenitrogen limitation, which enabled the plants to express halfof their growth potential. The relative growth rate (RGR) ofthe species was strongly related to plant nitrogen concentration(PNC) and leaf area ratio (LAR), whereas the effects on netassimilation rate (NAR) were very small. Despite large differencesin maximum relative growth rate, the species showed remarkablesimilarities in dry matter partitioning between root and shoot.It is concluded that root:shoot partitioning can be treatedas a direct function of the relative resource limitation ofthe plant. The difficulty of attaining well-defined levels ofresource limitation in soil, other solid substrates and manyhydroponic systems may be the most important reason for thedivergent results in earlier studies. Better knowledge of soil-rootinteractions, and plant responses to the whole span of resource-supplylevels, is required for a thorough understanding of how nutrientslimit growth. Copyright 1999 Annals of Botany Company Growth rate, plant strategies, plasticity, partitioning, biomass, nitrogen, nutrient limitation, grassland.     

Phosphorus and Defoliation Interact and Improve the Growth and Composition of the Plant Community and Soil Properties in an Alpine Pasture of Qinghai-Tibet Plateau

Pasture degradation caused by overgrazing and inappropriate fertiliser management is a major production and environmental threat in Qinghai-Tibet Plateau. Previous research has focused on the effects of mixed nitrogen (N) and phosphorus (P) fertiliser and reduced grazing pressure on the plant community of the grassland; however, the role of P and how it interacts with various defoliation (the process of the complete or partial removal of the above-ground parts of plants by grazing or cutting) intensities on the plant and soil of the grassland ecosystem have not been quantified. A field experiment was conducted to quantify how P application in combination of defoliation pressure could impact the dynamic change of the plant and soil in a native alpine grassland ecosystem of the Qinghai-Tibet Plateau, China, from May 2012 to September 2014. A split-plot design with 4 replicates and repeated measures was used to determine the growth and composition of plant community and soil physical and chemical properties under various levels of P fertiliser and defoliation intensity. The results showed that applying 20 kg P/ha increased the herbage yield of Melissitus ruthenica by 68% and total pasture yield by 25%. Close defoliation favoured the growth and plant frequency of the shorter species, whereas lax defoliation favoured that of the taller plant species. Medium P rate and cutting to 3 cm above ground gave an overall best outcome in pasture yield, quality and frequency and soil moisture and nutrient concentration. Application of P fertiliser with a moderate defoliation pressure to promote legume growth and N fixation has the potential to achieve multiple benefits in increasing pasture and livestock production and improving environmental sustainability in the alpine pasture of Qinghai-Tibet Plateau, a fragile and P-deficient ecosystem zone in China and its western neighbouring countries.     

Nitrogen fixation in annual and perennial legume-grass mixtures across a fertility gradient

Background and aims

The selection of legume species and species mixtures influences agroecosystem nitrogen (N) and carbon cycling. We utilized a fertility gradient to investigate the effects of plant species interactions on biological N fixation of an annual and perennial legume in response to shifting soil resource availability.

Methods

Legume N fixation of annual field pea (Pisum sativum) and perennial red clover (Trifolium pratense) grown in monoculture and mixtures with oats (Avena sativa) or orchardgrass (Dactylis glomerata) was estimated using the ¹⁵N natural abundance method across 15 farm fields and we measured six soil N pools ranging from labile to more recalcitrant.

Results

Evidence of complementary and facilitative species interactions was stronger for the perennial red clover-orchardgrass mixture than for the annual field pea-oat mixture (N Land Equivalency Ratios were 1.6 and 1.2, respectively). We estimated that the transfer of fixed N from red clover to orchardgrass increased aboveground N fixation estimates by 15% from 33 to 38?kg?N ha^?1. Despite a more than 2-fold range in soil organic matter levels and more than 3-fold range in labile soil N pools across field sites, the N fertility gradient was not a strong predictor of N fixation. While grass N assimilation was positively correlated with soil N pools, we found only weak, inverse correlations between legume N fixation and soil N availability. In grass-legume mixtures, soil N availability indirectly influenced N fixation through plant competition.

Conclusions

These results suggest that increasing diversity of cropping systems, particularly through the incorporation of perennial mixtures into rotations, could improve overall agroecosystem N cycling efficiency.     

高寒草甸土地退化及其恢复重建对植被碳、氮含量的影响

以青海省达日县高寒草甸原生高寒嵩草(Kobresia)草甸封育系统为对照,研究了土地退化对植被生产力的影响,检验了不同人工重建措施(两个人工种植处理:混播(HB)、翻耕单播(DBF)和1个退化草地封育自然恢复处理(NR)及1个退化草地自然状态(SDL))对植被生产力的相对影响程度。结果表明,原生植被封育处理(YF)地上总生物量为265.1 g·m^-2,混播(HB)和翻耕单播(DBF)处理中地上总生物量分别为原生植被封育处理的116%和68%。退化草地封育自然恢复处理(NR)和重度退化自然状态下地上总生物量分别为原生植被封育的76%和53%。YF处理根系生物量远大于其它处理。原生植被封育系统中植被地上部分碳储量为 110.14 g·m^-2,地下根系(0~30 cm)碳储量为2 957 g·m^-2,植被总碳储量为 3 067.14 g·m^-2;重度退化草地系统中植被地上部分碳储量为 57.07 g·m^-2,地下根系(0~30 cm)碳储量为 357 g·m^-2,植被总碳储量为 414.07 g·m^-2。由此可见,高寒草甸严重退化后,通过植物组织流失的碳达到2 653.35 g·m^-2,即86.5%的碳损失;原生植被封育系统植被总氮储量为 56.85 g·m^-2,而重度退化草地植被总氮储量为 18.02 g·m^-2,高寒草甸严重退化使植物组织68.30%氮损失。与重度退化地相比,由于恢复重建措施增加了植物的生物量输入和群落组成,除翻耕单播处理外,其它恢复重建措施均能恢复系统植被的碳氮储量。这些恢复重建措施将会逐步改善土壤的物理和化学特性,最终使这些生态系统逐步由碳源向碳汇方向的转变成为可能。     

Carbon-specific phytoplankton growth rates: a comparison of methods

Measurements of biomass and growth rate of two axenic algalcultures were carried out using three different methodologicalapproaches: the specific ¹⁴C-labelling of chlorophyll a, [³H]adenineincorporation into DNA and net organic carbon assimilation.Time-course experiments revealed that the specific activitiesof chlorophyll a were significantly higher than the specificactivity of total algal carbon in six of seven experiments.When the specific activity of chlorophyll a is used to calculatethe carbon biomass and growth rate, the carbon biomass of thealgae will thus be underestimated and the specific growth ratewill be too high. Determination of growth rates from incorporationof [³H]adenine gave lower values than those obtained from netorganic carbon assimilation and from ¹⁴C incorporation intochlorophyll a. Problems with adenine saturation are suggested.When [³H]adenine is used to measure growth rates in dense algalcultures, additions of >1 µM [³H]adenine are oftenrequired to maximally label the extracellular and intracellularadenine pools and hence DNA.     

The use of carbon isotope ratios to evaluate legume contribution to soil enhancement in tropical pastures

Soil carbon distribution with depth, stable carbon isotope ratios in soil organic matter and their changes as a consequence of the presence of legume were studied in three 12-year-old tropical pastures (grass alone —Brachiaria decumbens (C₄), legume alone —Pueraria phaseoloides (C₃) and grass + legume) on an Oxisol in Colombia. The objective of this study was to determine the changes that occurred in the¹³C isotope composition of soil from a grass + legume pasture that was established by cultivation of a native savanna dominated by C₄ vegetation. The¹³C natural abundance technique was used to estimate the amount of soil organic carbon originating from the legume. Up to 29% of the organic carbon in soil of the grass + legume pasture was estimated to be derived from legume residues in the top 0–2-cm soil depth, which decreased to 7% at 8–10 cm depth. Improvements in soil fertility resulting from the soil organic carbon originated from legume residues were measured as increased potential rates of nitrogen mineralization and increased yields of rice in a subsequent crop after the grass + legume pasture compared with the grass-only pasture. We conclude that the¹³C natural abundance technique may help to predict the improvements in soil quality in terms of fertility resulting from the presence of a forage legume (C₃) in a predominantly C₄ grass pasture.     

Nitrogen dynamics in grain crop and legume pasture systems under elevated atmospheric carbon dioxide concentration: A meta‐analysis

Understanding nitrogen (N) removal and replenishment is crucial to crop sustainability under rising atmospheric carbon dioxide concentration ([CO₂]). While a significant portion of N is removed in grains, the soil N taken from agroecosystems can be replenished by fertilizer application and N₂ fixation by legumes. The effects of elevated [CO₂] on N dynamics in grain crop and legume pasture systems were evaluated using meta‐analytic techniques (366 observations from 127 studies). The information analysed for non‐legume crops included grain N removal, residue C : N ratio, fertilizer N recovery and nitrous oxide (N₂O) emission. In addition to these parameters, nodule number and mass, nitrogenase activity, the percentage and amount of N fixed from the atmosphere were also assessed in legumes. Elevated [CO₂] increased grain N removal of C₃ non‐legumes (11%), legumes (36%) and C₄ crops (14%). The C : N ratio of residues from C₃ non‐legumes and legumes increased under elevated [CO₂] by 16% and 8%, respectively, but the increase for C₄ crops (9%) was not statistically significant. Under elevated [CO₂], there was a 38% increase in the amount of N fixed from the atmosphere by legumes, which was accompanied by greater whole plant nodule number (33%), nodule mass (39%), nitrogenase activity (37%) and %N derived from the atmosphere (10%; non‐significant). Elevated [CO₂] increased the plant uptake of fertilizer N by 17%, and N₂O emission by 27%. These results suggest that N demand and removal in grain cropping systems will increase under future CO₂‐enriched environments, and that current N management practices (fertilizer application and legume incorporation) will need to be revised.     

Inorganic soil nitrogen under grassland plant communities of different species composition and diversity

We measured aboveground plant biomass and soil inorganic nitrogen pools in a biodiversity experiment in northern Sweden, with plant species richness ranging from 1 to 12 species. In general, biomass increased and nitrate pools decreased with increasing species richness. Transgressive overyielding of mixed plant communities compared to the most productive of the corresponding monocultures occurred in communities with and without legumes. N₂-fixing legumes had a fertilizing function, while non-legumes had a N retaining function. Plant communities with only legumes had a positive correlation between biomass and soil nitrate content, whereas in plant communities without legumes they were negatively correlated. Both nitrate and ammonium soil pools in mixed non-legume communities were approximately equal to the lowest observed in the corresponding monocultures. In mixed legume/non-legume communities, no correlation was found for soil nitrate with either biomass or legume biomass as percentage of total biomass. The idea of complementarity among species in nitrogen acquisition was supported in both pure non-legume and mixed non-legume/legume communities. In the latter, however, facilitation through increased nitrogen availability and retention, was probably dominating. Our results suggest that diversity effects on biomass and soil N pools through resource use complementarity depend on the functional traits of species, especially N₂ fixation or high productivity.     

单户与联户经营对高寒草甸土壤理化性质与植物多样性的影响

在甘南高寒草甸单户经营草场与联户经营草场进行野外群落学调查,分析其物种多样性和功能多样性的变化,以揭示不同经营方式下土壤理化性质对植物多样性的影响机制。结果显示：（1）联户草场内植物群落的Margalef指数、Simpson指数、Shannon-Winener指数较大,而Pielou指数在不同放牧经营方式下的差异并不显著。（2）不同经营方式下,土壤含水量、土壤全氮、土壤全磷和土壤有机碳质量分数表现为联户大于单户的趋势且差异显著（P<0.05）;单户草场土壤电导率与土壤pH大于联户草场;随着土层的加深土壤含水量、土壤全氮质量分数、土壤全磷质量分数、土壤有机碳质量分数和土壤电导率增加,而土壤pH在土层之间的差异并不显著。（3）联户经营方式下的功能丰富度、功能均匀度与功能离散度显著大于单户草场（P<0.05）。（4）相关性分析表明,植物群落物种多样性和功能多样性与土壤含水量、土壤全氮质量分数和土壤全磷质量分数存在正相关关系,土壤电导率和土壤pH与植物群落物种多样性和功能多样性存在负相关关系。冗余分析表明,联户经营方式下土壤理化性质对植物多样性的影响更显著,且0＜h≤10 cm土层土壤理化性质与植物多样性的相关性更高。单户经营方式下,土壤全氮和土壤有机碳是植物多样性的显著影响因子,联户经营方式下,土壤全氮和土壤全磷是植物多样性的显著影响因子。     

三江源地区主要草地类型土壤碳氮沿海拔变化特征及其影响因素

以三江源地区主要草地类型为研究对象,分析了不同草地类型土壤有机碳和全氮的变化特征及其与环境因子、土壤特征等的相互关系。结果表明:沿着海拔的逐渐升高,土壤有机碳和全氮含量均呈现出 “V"字形变化规律,即土壤有机碳氮含量在海拔最高处(5 120 m)和最低处(4 176 m)比较高,而在中间海拔梯度较低,土壤有机碳与全氮含量极显著相关( r=0.905)且高寒草甸土壤碳、氮含量高于高山草原土壤碳、氮含量;土壤中有机碳含量和全氮含量均随着土壤含水量的增加而增加,偏相关分析结果表明:对0~30 cm土层中土壤有机碳和土壤全氮影响最大的是土壤含水量,偏相关系数为0.946 5、0.905 9(p<0.01);土壤有机碳含量和全氮含量与植被盖度和草地生产力存在正相关趋势;土壤有机碳含量和全氮含量与土壤pH值和全盐量存在负相关趋势。     

The Effect of Elevated [CO2] on Uptake and Allocation of 13C and 15N in Beech (Fagus sylvatica L.) during Leafing

Abstract: A continuous dual ¹³CO₂ and ¹⁵NH₄¹⁵NO₃ labelling experiment was undertaken to determine the effects of ambient (350μmol mol^-1) or elevated (700μmol mol^-1) atmospheric CO₂ concentrations on C and N uptake and allocation within 3-year-old beech ( Fagus sylvatica L.) during leafing. After six weeks of growth, total carbon uptake was increased by 63 % (calculated on total C content) under elevated CO₂ but the carbon partitioning was not altered. 56 % of the new carbon was found in the leaves. On a dry weight basis was the content of structural biomass in leaves 10 % lower and the lignin content remained unaffected under elevated as compared to ambient [CO₂]. Under ambient [CO₂] 37 %, and under elevated [CO₂] 51 %, of the lignin C of the leaves derived from new assimilates. For both treatments, internal N pools provided more than 90 % of the nitrogen used for leaf-growth and the partitioning of nitrogen was not altered under elevated [CO₂]. The C/N ratio was unaffected by elevated [CO₂] at the whole plant level, but the C/N ratio of the new C and N uptake was increased by 32 % under elevated [CO₂].     

Testing simulations of intra‐ and inter‐annual variation in the plant production response to elevated CO2 against measurements from an 11‐year FACE experiment on grazed pasture

Ecosystem models play a crucial role in understanding and evaluating the combined impacts of rising atmospheric CO₂ concentration and changing climate on terrestrial ecosystems. However, we are not aware of any studies where the capacity of models to simulate intra‐ and inter‐annual variation in responses to elevated CO₂ has been tested against long‐term experimental data. Here we tested how well the ecosystem model APSIM/AgPasture was able to simulate the results from a free air carbon dioxide enrichment (FACE) experiment on grazed pasture. At this FACE site, during 11 years of CO₂ enrichment, a wide range in annual plant production response to CO₂ (?6 to +28%) was observed. As well as running the full model, which includes three plant CO₂ response functions (plant photosynthesis, nitrogen (N) demand and stomatal conductance), we also tested the influence of these three functions on model predictions. Model/data comparisons showed that: (i) overall the model over‐predicted the mean annual plant production response to CO₂ (18.5% cf 13.1%) largely because years with small or negative responses to CO₂ were not well simulated; (ii) in general seasonal and inter‐annual variation in plant production responses to elevated CO₂ were well represented by the model; (iii) the observed CO₂ enhancement in overall mean legume content was well simulated but year‐to‐year variation in legume content was poorly captured by the model; (iv) the best fit of the model to the data required all three CO₂ response functions to be invoked; (v) using actual legume content and reduced N fixation rate under elevated CO₂ in the model provided the best fit to the experimental data. We conclude that in temperate grasslands the N dynamics (particularly the legume content and N fixation activity) play a critical role in pasture production responses to elevated CO₂, and are processes for model improvement.     

Control of nitrogen and carbon metabolism in root nodules

Because legume root nodules have high rates of carbon and nitrogen metabolism, they are ideal for the study of plant physiology, biochemistry and molecular biology. Many plant enzymes involved in carbon and nitrogen assimilation have enhanced activity and enzyme protein in nodules as compared to other plant organs. For all intents and purposes the interior of the root nodule is O₂ limited. Both plant and bacterial components of effective root nodules have unique adaptive features for maximizing carbon and nitrogen metabolism in an O₂-limited environment. Plant glycolysis appears to be shunted to malic acid synthesis with further reductive synthesis to fumarate and succinate. Nodule bacteroids utilize these organic acids for the energy to fuel nitrogenase activity. Activities of the plant enzymes phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), malate dehydrogenase (MDH, EC 1.1.1.37) and aspartate aminotransferase (AAT, EC 2.6.1.1), which are very high in nodules, may mediate the flux of carbon between organic and amino acid pools. Dark CO₂ fixation via nodule PEPC can provide up to 25% of the carbon needed for malate and aspartate synthesis. At least three of the plant proteins showing enhanced expression in root nodules are O₂ regulated. Isolation of alfalfa cDNAs encoding PEPC, AAT, NADH-glutamate synthase (NADH-GOGAT, EC 1.4.1.14) and aldolase (EC 4.1.2.13) will offer new tools to assess molecular events controlling nodule carbon and nitrogen metabolism.     

植株密度和氮素互作对垂穗披碱草生物量分配的影响

垂穗披碱草（Elymus nutans）是高寒地区建植和改良栽培草地的首选草种。虽然合理植株密度和氮素添加量是垂穗披碱草栽培草地稳产的关键因子,但两者之间是否存在最佳互作组合仍不清楚。采用盆栽试验,通过分析不同植株密度（58、102、146株/m²）和氮素添加量（0、200、400 mg/kg）组合状态下垂穗披碱草株高、单株分蘖数、地上生物量、地下生物量、根系体积和地上地下生物量比,以确定理论上是否存在植株密度和氮素添加量的最佳组合。结果表明：随植株密度增加,垂穗披碱草株高、地上生物量和地上地下生物量比值均先增加后降低,而单株分蘖数逐渐减小,根系体积和地下生物量先增加后保持相对稳定;随氮素添加量增加,垂穗披碱草单株分蘖数、地上生物量和地上地下生物量比值均表现为先增加后降低,地下生物量逐渐降低。植株密度与氮素添加量互作虽然对垂穗披碱草的根系体积和单株分蘖数没有显著影响,但两者互作显著影响了垂穗披碱草株高、地上生物量、地下生物量、地上地下生物量比（P<0.01）,这些指标与植株密度和氮素添加量的关系均表现为一个开口向下的抛物面。当植株密度为102株/m²和氮素添加量为200 mg/kg时,垂穗披碱草栽培草地产量最大,生物量分配最优。垂穗披碱草植株密度和氮素添加互作时理论上存在最佳组合,这为垂穗披碱草栽培草地的田间管理提供了理论依据。     

Plant Growth-promoting Rhizobacteria and Soybean [ Glycine max (L.) Merr.] Growth and Physiology at Suboptimal Root Zone Temperatures

Application of plant growth-promoting rhizobacteria (PGPR) hasbeen shown to increase legume growth and development under optimaltemperature conditions, and specifically to increase nodulationand nitrogen fixation of soybean [Glycine max (L.) Merr.] overa range of root zone temperatures (RZTs). Nine rhizobacteriaapplied into soybean rooting media were tested for their abilityto reduce the negative effects of low RZT on soybean growthand development by improving the physiological status of theplant. Three RZTs were tested: 25, 17.5, and 15 °C. At eachtemperature some PGPR strains increased plant growth and development,but the stimulatory strains varied with temperature. The strainsthat were most stimulatory at each temperatures were as follows:15 °C—Serratia proteamaculans 1–102; 17.5 °C—Aeromonashydrophila P73, and 25 °C—Serratia liquefaciens 2–68.Because enhancement of plant physiological activities were detectedbefore the onset of nitrogen fixation, these stimulatory effectscan be attributed to direct stimulation of the plant by thePGPR rather than stimulation of plant growth via improvementof the nitrogen fixation symbiosis. Legume; nitrogen fixation; nodulation; root zone temperature; PGPR     

放牧对呼伦贝尔草地植物和土壤生态化学计量学特征的影响

放牧通过畜体采食、践踏和排泄物归还影响草地群落组成、植物形态和土壤养分,植物通过改变养分利用策略适应环境变化。通过分析呼伦贝尔草原放牧和围封样地中的群落植物和土壤的碳氮磷养分及化学计量比,探讨放牧对生态系统化学计量学特征和养分循环速率的影响机制。结果如下:(1)群落尺度上,放牧和围封草地植物叶片C、N和P的含量没有显著差异;但是在种群尺度上,放牧草地植物叶片N含量显著高于围封草地;(2)放牧草地土壤全C、全N、有机C、速效P含量,低于围封草地,硝态N含量高于围封草地;土壤全P和铵态N指标没有显著差异;(3)放牧草地植物C∶N比显著低于围封草地,植物残体分解速率较快,提高了生态系统养分循环速率。