四种荒漠草原植物的生长对不同氮添加水平的响应

大气氮(N)沉降增加加速了生态系统N循环, 从而会对生态系统的结构和功能产生巨大的影响, 尤其是一些受N限制的生态系统.研究N添加对荒漠草原植物生长的影响, 可为深入理解N沉降增加对我国北方草原群落结构的影响提供基础数据.该文基于2011年在宁夏荒漠草原设置的N沉降增加的野外模拟试验, 研究了两年N添加下4个常见物种(牛枝子(Lespedeza potaninii),老瓜头(Cynanchum komarovii),针茅(Stipa capillata)和冰草(Agropyron cristatum))不同时期种群生物量和6-8月份相对生长速率的变化特征.并通过分析物种生长与植物(群落和叶片水平)和土壤碳(C),N,磷(P)生态化学计量学特征的关系, 探讨C:N:P化学计量比对植物生长养分限制的指示作用.结果显示N添加促进了4个物种的生长, 但具有明显的种间差异性, 且这种差异也存在于相同生活型的不同物种间.总体而言, 4个物种种群生物量与叶片N浓度,叶片N:P,群落N库,土壤全N含量和土壤N:P存在明显的线性关系, 与植物和土壤C:N和C:P的相关关系相对较弱.几个物种相对生长速率与植物和土壤N:P也呈现一定程度的正相关关系, 但与其他指标相关性较弱.以上结果表明, 短期N沉降增加提高了植物的相对生长速率, 促进了植物生长, 且更有利于针茅和老瓜头的生物量积累, 从而可能会逐渐改变荒漠草原群落结构.植物N:P和土壤N:P对荒漠草原物种生长具有较强的指示作用: 随着土壤N受限性逐渐缓解, 土壤N含量和N:P相继升高, 可供植物摄取的N增多, 因而有利于植物生长和群落N库积累.     

Responses of growth of four desert species to different N addition levels

Aims The increase in atmospheric N deposition has accelerated N cycling of ecosystems, thus altering the structure and function of ecosystems, especially in those limited by N availability. Studies on the response of plant growth to artificial N addition could provide basic data for a better understanding of how the structure of grasslands in northern China responds to increasing N deposition. Methods We investigated the seasonal dynamics of plant growth of four species after 2-year multi-level N addition in a field experiment conducted in a desert steppe of Ningxia in 2011. Plant biomass and the relative growth rate (RGR) of the studied species were measured and their relationships with C:N:P ratios of plants (community and leaf levels) and soils were analyzed. Important findings Results in 2012 showed that 2-year N addition promoted the growth of the four species and the effects were different among growth forms and were species-specific. In general, the plant biomass of the studied species was significantly correlated with leaf N concentration, leaf N:P ratio, community N pool, soil total N content and soil N:P ratio, while only weak relationships were observed between plant biomass and C:N and C:P ratios of plants and soils. In contrast, there was a significant linear relationship between RGR and N:P ratios both of plants and soils.Our results suggest that short-term N addition promoted the accumulation of plant biomass, and the species-specific responses to stimulated N addition can directly affect the structure of the desert steppe ecosystem. Plant N:P ratio and soil N:P ratio could indicate nutrient limitation of plant growth to a certain extent: N addition increased soil N content and N:P ratio, and thus relieved N limitation gradually. Once more N is available to plants, the growth of plants and the accumulation of community N was stimulated in turn.     

刨花楠不同相对生长速率下林木叶片碳氮磷的适应特征

分别对9年生与13年生刨花楠林木叶片氮磷养分之间关系及林木生物量相对生长速率与叶片碳氮磷化学计量比关系进行分析,探讨不同相对生长速率下的林木叶片N、P养分适应特征,并检验相对生长速率假说理论对刨花楠树种的适应性。结果表明:两种年龄刨花楠林木生物量相对生长速率、叶片C、N、P含量及其计量比值均存在显著差异;同一年龄的林木叶片N、P之间存在显著相关性,二者具有协同相关性;9年生林木叶片P含量及C∶P、N∶P与生物量相对生长速率呈二次曲线相关,而13年生林木叶片N、P含量及C∶N、C∶P、N∶P则与生物量相对生长速率均呈线性相关。研究表明,在能满足植物生长所需养分供给的土壤环境中,叶片N、P含量与林木相对生长速率间呈线性正相关,但当土壤中养分供应满足不了植物高速生长时,植物则会对有限的养分资源进行适应性调整。     

The relationship between relative growth rate and whole-plant C:N:P stoichiometry in plant seedlings grown under nutrient-enriched conditions

Aims Recent theories indicate that N is more in demand for plant growth than P; therefore, N concentration and N : C and N : P ratios are predicted to be positively correlated with relative growth rate (RGR) in plants under nutrient-enriched conditions. This prediction was tested in this study.Methods We examined the whole-plant concentrations of C, N and P and RGR, as well as the relationship between RGR and the concentrations and the ratios of N : C, P : C and N : P, for different harvest stages (the days after seed germination) of the seedlings of seven shrub species and four herbaceous species grown in N and P non-limiting conditions. The relationships among plant size, nutrient concentrations and ratios were subsequently determined.Important findings RGR was positively correlated with N concentration and the ratios of N : P and N : C when the data were pooled for all species and for each shrub species, but not for individual herbaceous species. However, the relationship between RGR and P concentration and P : C was not significantly correlated for either shrubs or herbs. The variation of N among harvest stages and species was much greater than that of P, and the variation in N : P ratio was determined primarily by changes in N concentration. The shrub species differed from the herbaceous species in their N and P concentrations, nutrient ratios and in intraspecific relationships between RGR and nutrient ratios. These differences possibly reflect differences in the capacity for P storage and biomass allocation patterns. In general, our data support recent theoretical predictions regarding the relationship between RGR and C : N : P stoichiometry, but they also show that species with different life forms differ in the relationships among RGR and C : N : P stoichimetries.     

Effect of stress and time for recovery on the amount of compensatory growth after grazing

Summary We tested the hypothesis that the amount of compensatory growth after defoliation is affected by the level of stress at which plants grow when defoliated and by the length of time for recovery. Growth response to defoliation went from partial compensation when plants were growing at high relative growth rates (RGR) to overcompensation when plants were more stressed and growing at low RGR. Defoliation released plants from the limitation imposed by the accumulation of old and dead tissue and this release overrode the negative effect of biomass loss. Compensatory growth resulted from a higher RGR aboveground that was not associated with a reduction in RGR belowground. Time available for recovery had a major impact on the outcome of defoliation. With a short time for recovery, RGR was decreased by defoliation because an immediate increase in net assimilation rate was overridden by a reduction in the ratio of leaf area to plant weight. After defoliation, this ratio increased quickly due to a larger allocation to leaf growth and lower leaf specific weights, resulting in higher RGR. We conclude that the compensatory response to grazing depends on the type and level of stress limiting growth. Allocation and physiological responses to stress may positively or negatively affect the response to grazing and, simultaneously, grazing may alleviate or aggravate the effects of different types of stress.     

Growth and dry-mass partitioning in tomato as affected by phosphorus nutrition and light

We studied the effects of phosphorus (P) and light on the physiological and morphological components of growth of young tomato plants (Lycopersicon esculentum Mill. cv. Capita). The importance of dry‐mass partitioning and starch accumulation in explaining the effects of P limitation on growth was examined more closely. Plants were grown at a wide range of exponential P supply rates (between 70 and 320 mg g^?1 d^?1) and one free‐access treatment (1 mm ). Two light levels (70 and 300 µmol m^?2 s^?1) were applied. Growth response coefficients (GRCs) were calculated to address the importance of different growth parameters in explaining relative growth rate (RGR). At both light levels, net assimilation rate (NAR) was more important than leaf area ratio (LAR) in explaining the effects of P on growth as indicated by GRCs. At less severe P limitation, LAR became more important and NAR less important. Dry‐mass partitioning to both roots and leaves played a minor role in determining the effects of P limitation on growth as indicated by low GRCs. The increase in starch at mild P limitation showed that the assimilate supply was not limiting. At severe P limitation, the rate of photosynthesis was decreased, as suggested by the decrease in starch accumulation.     

Growth rate and biomass partitioning of wildtype and low-gibberellin tomato (Solanum lycopersicum) plants growing at a high and low nitrogen supply

The growth-promoting effects of gibberellins (GAs) on plants are well documented, but a complete growth analysis at the whole plant level on plants with an altered GA biosynthesis has never been reported. In the present work, the relative growth rate (RGR), biomass partitioning and morphological parameters of wildtype (Wt) tomato ( Solanum lycopersicum L. cv. Moneymaker) plants were compared with those of isogenic ( gib ) mutants with a reduced biosynthesis of gibberellins. GA deficiency reduced RGR and specific leaf area (SLA, leaf area per unit leaf mass) and increased the net assimilation rate (NAR, the rate of biomass increment per unit leaf area). Despite the free access to nitrogen in the rooting medium, the low-GA mutants had a much higher root mass ratio (RMR, the root mass per unit plant biomass) than the Wt, suggesting that the mutants were disturbed in their growth response to nitrate supply. The experiment was repeated at a low exponential nitrate supply, which forced all plants to grow at the same low RGR. The persistence of the differences in RMR at low N-supply indicated that the high RMR of the mutants was a direct effect of low GA, which was independent of nitrate supply. Because the low N-supply increased the RMRs of all genotypes to the same extent, the response of RMR to N-supply does not seem to depend on GA. Although many of the traits of the slow growing GA mutants were very similar to those of inherently slow growing plant species from unproductive habitats, gibberellins are unlikely to be a main determinant of a plant's potential RGR.     

The effect of nitrogen nutrition on growth and biomass partitioning of annual plants originating from habitats of different nitrogen availability

Summary The hypothesis was tested that faster growth of nitrophilic plants at high nitrogen (N) nutrition is counterbalanced by faster growth of non-nitrophilic plants at low N-nutrition. Ten annual plant species were used which originated from habitats of different N-availability. The species' preference for N was quantified by the N-number of Ellenberg (1979), a relative measure of nitrophily. The plants were cultivated in a growth cabinet at five levels of ammonium-nitrate supply. At low N-supply, the relative growth rate (RGR) was independent of nitrophily. At high N-supply, RGR tended to be higher in nitrophilic than in non-nitrophilic species. However, the response of RGR to N-supply was strongly and positively correlated with the nitrophily of species. Increasing N-supply enhanced partitioning to leaf weight per total biomass (LWR) and increased plant leaf area per total biomass (LAR). Specific leaf weight (SLW) and LWR were both higher in non-nitrophilic than in nitrophilic species at all levels of N-nutrition. NAR (growth per leaf area or net assimilation rate) increased with nitrophily only under conditions of high N-supply. RGR correlated positively with LAR, irrespective of N-nutrition. Under conditions of high N-supply RGR correlated with SLW negatively and with NAR positively.     

Evidence that abscisic acid does not regulate a centralized whole-plant response to low soil-resource availability

It has been suggested that abscisic acid (ABA) regulates a centralized response of plants to low soil resource availability that is characterized by decreased shoot growth relative to root growth, decreased photosynthesis and stomatal conductance, and decreased plant growth rate. The hypothesis was tested that an ABA-deficient mutant of tomato (flacca; flc) would not exhibit the same pattern of down-regulation of photosynthesis, conductance, leaf area and growth, as well as increased root/shoot partitioning, as its near isogenic wild-type in response to nitrogen or water deficiency, or at least not exhibit these responses to the same degree. Plants were grown from seed in acid-washed sand and exposed to control, nutrient stress, or water stress treatments. Additionally, exogenous ABA was sprayed onto the leaves of a separate group of flc individuals in each treatment. Growth analysis, based on data from frequent harvests of a few individuals, was used to assess the growth and partitioning responses of plants, and gas exchange characteristics were measured on plants throughout the experiment to examine the response of photosynthesis and stomatal conductance. Differences in growth, partitioning and gas exchange variables were found between flc and wild-type individuals, and both nutrient and water treatments caused significant reductions in relative growth rate (RGR) and changes in biomass partitioning. Only the nutrient treatment caused significant reductions in photosynthetic rates. However, flc and wild-type plants responded identically to nutrient and water stress for all but one of the variables measured. The exception was that flc showed a greater decrease in the relative change in leaf area per unit increase of plant biomass (an estimate of the dynamics of leaf area ratio) in response to nutrient stress—a result that is opposite to that predicted by the centralized stress response model. Furthermore, addition of exogenous ABA to flc did not significantly alter any of the responses to nutrient and water stress that we examined. Although it was clear that ABA regulated short-term stomatal responses, we found no evidence to support a pivotal role for ABA, at least absolute amounts of ABA, in regulating a centralized whole-plant response to low soil resource availability.     

不同林龄油松(Pinus tabulaeformis)人工林植物、凋落物与土壤C、N、P化学计量特征

在陕西省北部延安市境内子午岭林区,采用时空互代的方法选取9、23、33、47年生油松(Pinus tabuliformis)人工林为研究对象,比较油松不同器官(叶、枝、干、根)、凋落物及土壤C、N、P含量及其比值的差异,探讨它们随林龄的变化及其相互间的关系,以期为油松人工林的生产、改善和林木生长环境的调节提供参考。结果表明:除根中C含量在林龄间差异不显著外,其它器官C、N、P含量及其比值在林龄间均差异显著且随林龄增加变化趋势不尽相同。9、23、33、47年生油松林C、N、P含量及N∶P比值均在叶中最高;C∶N比值均在干中最高,根中次之;C∶P比值均在干中最高,其它器官大小次序不一。除33年生油松林叶中N∶P比值大于14外,其它各器官各林龄N∶P比值均小于14,且N∶P比值随林龄先增加后减少,故可判断油松在该区域受N限制较为严重,且随林龄的增加受N限制的情况有所缓解。不同林龄土壤和凋落物C、N、P含量及其比值差异显著,且后者均大于前者。土壤与凋落物C、P含量及C∶N、C∶P、N∶P比值随林龄增加变化趋势完全一致,表明土壤与凋落物之间有着密切的关系。叶片与凋落物N、P含量及C∶N、C∶P、N∶P比值之间显著相关,表明凋落物的养分承自植物叶片,二者之间关系紧密;植物和土壤的C、N、P含量之间均不存在显著相关性,说明土壤C、N、P供应量对乔木叶片C、N、P含量影响不大。     

The role of mineral nutrients in the increased growth response of tomato plants in solarized soil

Soil solarization is a non-chemical disinfestation technique that frequently promotes plant growth in the absence of known major pathogens, a phenomenon termed increased growth response (IGR). The effect of solarization on plant nutrients and their role in the IGR was studied with tomato plants grown in solarized or non-solarized (control) sandy soil, under controlled conditions. Solarization considerably increased the soil concentrations of water extractable N, K, Ca, Mg and Na at most sites, whereas Cl and DTPA extractable Mn, Zn, Fe and Cu were decreased by the treatment. Plant growth and specific leaf area were enhanced in solarized as well as in N-supplemented control soil. In tomato plants grown in solarized soil, concentrations of most nutrients in the xylem sap, including N, were increased compared to the control, whereas Cl and SO4 levels decreased. The most significant increase in leaf nutrient concentration caused by soil solarization was recorded for N. Furthermore, leaf N concentration was highly and positively correlated with shoot growth. The concentration of Cu increased in leaves from the solarization vs. the control treatment, whereas that of SO4 and Cl decreased, the latter presumably below the critical toxicity level. The correlation between shoot growth and leaf concentration was positive for Cu and inverse for Cl and SO4. In conclusion, we found that soil solarization significantly affects nutrient composition in tomato plants, and provided strong evidence that N, and eventually also Cl, play a major role in IGR.     

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.     

Reprint of: Drivers of within-tree leaf trait variation in a tropical planted forest varying in tree species richness

In plant ecology, community-weighted trait means are often used as predictors for ecosystem functions. More recently, also within-species trait variation has been confirmed to contribute to ecosystem functioning. We here go even further and assess within-individual trait variation, assuming that every leaf in a plant individually adjusts to its micro-environment. Using forest plots varying in tree species richness (Sardinilla experiment, Panama), we analysed how leaf traits within individual trees vary along the vertical crown gradient. Furthermore, we tested whether niche partitioning in mixed stands results in a decrease of within-species leaf trait variation and whether niche partitioning can be also observed at the level of individual trees. We focused on leaf traits that describe the growth strategy along the conservative-acquisitive spectrum of growth. We found a decrease in within-species variation of specific leaf area (SLA) with increasing neighbourhood species richness. Both sampling height and local neighbourhood richness contributed to explaining within-species leaf trait variation, which however, varied in importance among different species and traits. With increasing sampling height, leaf dry matter content (LDMC), carbon to nitrogen ratio and lignin content increased, while leaf nitrogen concentration (leaf N), SLA, cellulose and hemicellulose decreased. Variation in leaf N decreased with increasing neighbourhood species richness, while the magnitude of within-individual variation of most traits was unaffected by neighbourhood species richness. Our results suggest an increased niche partitioning with increasing species richness both in a plant community and at the level of individual plants. Our findings highlight the importance of including within-individual trait variation to understand biodiversity-ecosystem functioning relationships.     

Drivers of within-tree leaf trait variation in a tropical planted forest varying in tree species richness

In plant ecology, community-weighted trait means are often used as predictors for ecosystem functions. More recently, also within-species trait variation has been confirmed to contribute to ecosystem functioning. We here go even further and assess within-individual trait variation, assuming that every leaf in a plant individually adjusts to its micro-environment. Using forest plots varying in tree species richness (Sardinilla experiment, Panama), we analysed how leaf traits within individual trees vary along the vertical crown gradient. Furthermore, we tested whether niche partitioning in mixed stands results in a decrease of within-species leaf trait variation and whether niche partitioning can be also observed at the level of individual trees. We focused on leaf traits that describe the growth strategy along the conservative-acquisitive spectrum of growth. We found a decrease in within-species variation of specific leaf area (SLA) with increasing neighbourhood species richness. Both sampling height and local neighbourhood richness contributed to explaining within-species leaf trait variation, which however, varied in importance among different species and traits. With increasing sampling height, leaf dry matter content (LDMC), carbon to nitrogen ratio and lignin content increased, while leaf nitrogen concentration (leaf N), SLA, cellulose and hemicellulose decreased. Variation in leaf N decreased with increasing neighbourhood species richness, while the magnitude of within-individual variation of most traits was unaffected by neighbourhood species richness. Our results suggest an increased niche partitioning with increasing species richness both in a plant community and at the level of individual plants. Our findings highlight the importance of including within-individual trait variation to understand biodiversity-ecosystem functioning relationships.     

How do nutrients drive growth?

The relationship between plant nutrient concentration and relative growth rate (RGR) was studied under non-steady state conditions using data from a new N interruption experiment with young lettuce plants grown hydroponically in the glasshouse. RGRs estimated from the fit of a versatile growth model were shown to decline curvilinearly with plant N concentration as N deficiency increased. Similar curvilinear relationships were also derived when the same model was used to reanalyse data for N, P and K interruption treatments from other experiments previously published in the literature. These results clearly indicate that the rate of remobilisation of nutrient reserves varies with the nutrient status of the plant. This contrasts with the linear relationships observed where the changes in plant N concentration occurred solely as a response to increasing plant age, or when plants were grown under steady state conditions with constant relative nutrient addition rates. These differences in the pattern of response provide strong evidence to support the hypothesis that the form of the relationship between RGR and plant nutrient concentration can vary depending upon whether a plant's external supplies or internal reserves of a particular nutrient are more limiting.     

Growth in epiphytic bromeliads: response to the relative supply of phosphorus and nitrogen

Insufficient nitrogen (N) and phosphorus (P) frequently limit primary production. Although most nutrient studies on vascular epiphytes have focused on N uptake, circumstantial evidence suggests that P rather than N is the most limiting element for growth in this plant group. We directly tested this by subjecting a total of 162 small individuals of three bromeliad species ( Guzmania monostachia , Tillandsia elongata , Werauhia sanguinolenta ) to three N and three P levels using a full-factorial experimental design, and determined relative growth rates (RGR) and nutrient acquisition over a period of 11 weeks. Both N and P supply had a significant effect on RGR, but only tissue P concentrations were correlated with growth. Uptake rates of N and P, in contrast, were not correlated with RGR. Increased nutrient supply led to an up to sevenfold increase in tissue P concentration compared to natural conditions, while concentrations of N hardly changed or even decreased. All treatment combinations, even at the lowest experimental P supply, led to decreased N:P ratios. We conclude that P is at least as limiting as N for vegetative function under natural conditions in these epiphytic bromeliads. This conclusion is in line with the general notion of the prevalence of P limitation for the functioning of terrestrial vegetation in the tropics.     

樟子松人工林原产地与不同自然降水梯度引种地土壤和植物叶片生态化学计量特征

降水格局是影响陆地生态系统结构和过程的重要环境要素,尤其对于干旱/半干旱地区,降水变化是植物生长驱动的关键生态因子。目前,针对降水变化对陆地生态系统C、N、P等元素生物地球化学循环过程影响开展了大量研究。然而,关于沙地樟子松重要引种地科尔沁沙地自然降水梯度下沙地樟子松人工林土壤、植物生态化学计量特征的研究未见报道。因此,本研究以樟子松原产地红花尔基和引种地科尔沁沙地自然降水梯度下4个典型沙地樟子松人工林为对象,研究樟子松引种地降水变化对土壤(0—10,10—20 cm和20—40 cm)和植物(1年和2年生叶)生态化学计量特征的影响。研究结果发现:(1)与红花尔基原产地樟子松人工林相比,科尔沁沙地引种的樟子松人工林土壤C、N、P元素含量显著降低;(2)科尔沁沙地自西向东,随降水量增加,沙地樟子松人工林土壤C、N、P含量以及C∶P和N∶P表现为逐渐增加趋势,而土壤C∶N呈减少趋势;(3)随着降水量增加,樟子松叶C含量呈下降趋势,叶N含量和N∶P比值呈增加趋势,植物叶P含量无一致性规律;(4)樟子松叶片P含量与土壤C、N、P含量呈极显著正相关关系,而叶片C和N含量与土壤C、N、P含量无显著相关性。研究表明,沙地樟子松引种地科尔沁沙地土壤C、N、P养分比较缺乏,且随着降水增加土壤N养分限制降低,而土壤P养分限制增加。本研究从生态化学计量特征角度,为今后开展科尔沁沙地不同降水梯度条件下引种樟子松人工林提供理论依据。     

氮肥添加对青藏高原高寒草甸6个群落优势种生态化学计量学特征的影响

以青藏高原高寒草甸为研究对象, 通过人工氮肥添加试验, 研究6个群落优势种在不同施氮(N)水平下叶片碳(C)、N、磷(P)元素含量的变化以及生态化学计量学特征。结果表明: 自然条件下, 6个物种叶片N、P质量浓度存在显著的差异, 表现为: 黄花棘豆(Oxytropis ochrocephala)最高, 为24.5和2.51 g·kg^-1, 其叶片N含量低于而P含量高于我国其他草地的豆科植物; 其余5个物种叶片N、P质量浓度分别为11.5-18.1和1.49-1.72 g·kg^-1, 嵩草(Kobresia myosuroides)叶片N含量最低, 垂穗披碱草(Elymus nutans)叶片P含量最低, 与我国其他区域的研究结果相比, 其叶片N和P含量均低于我国其他草地非豆科植物。随氮素添加量的增大, 6种群落优势种叶片的C和P含量保持不变; 其他5种植物叶片N含量显著增加, 黄花棘豆叶片N含量保持不变。未添加氮肥时, 6种植物叶片N:P为7.3-11.2, 说明该区植物生长更多地受N限制。随N添加量的增加, 除黄花棘豆外, 其他5种植物叶片N:P大于16, 表现为植物生长受P限制。综合研究表明, 青藏草原高寒草甸植物叶片N含量较低, 植物受N影响显著, 但不同物种对N的添加反应不同, 豆科植物黄花棘豆叶片对N添加不敏感, 其他5个物种叶片全N含量随着N添加量的升高而增加, 该研究结果可为高寒草甸科学施肥提供理论依据。     

Effects of N addition rates on the productivity ofPicea Sitchensis,Thuja plicata,andTsuga heterophylla seedlings

Seedlings ofPicea sitchensis, Thuja plicata andTsuga heterophylla were supplied N hydroponically at one of four exponentially increasing rates of addition (0.09, 0.07, 0.05, or 0.025 gN^-1 day^-1) for up to 3 months in a naturally illuminated glasshouse. Relative growth rates (RGR) were analyzed as a function of N uptake, the allocation of assimilated N to foliage (LNFR), foliar N concentrations (N_la) and met assimilation rates (NAR), which were combined to estimate N productivity (RGR per unit whole-plant N concentration). Nitrogen accumulation, biomass and N partitioning and RGR and its components varied with species in response to the different N regimes.T. heterophylla had the lowest maximum wholeplant N concentrations (wpN) and specific absorption rates for N and exhibited the least plasticity in root: shoot ratios as wpN increased from 11–21 mg g^-1. In all species, RGR increased linearly with wpN, while LNFR increased curvilinearly. Foliar N (N_la) increased linearly with wpN and NAR increased linearly with N_la. The RGRs ofT. heterophylla were highest at wpNs up to 18 mg g^-1, a result of higher foliar N use efficiencies (NAR/N_la). However, RGR increased more with wpN inT. plicata andP. sitchensis. Although LNFR increased with wpN in all species, foliar N use efficiency declined, possibly due to an increased partitioning of foliar soluble N to non-photosynthetic compounds. Thus, in each species, N productivity did not increase above intermediate levels of wpN: 14 mg g^-1 inT. heterophylla, 16 mg g^-1 inP. sitchensis and 17 mg g^-1 inT. plicata.     

A hierarchical analysis of the interactive effects of elevated CO<Subscript>2</Subscript> and water availability on the nitrogen and transpiration productivities of velvet mesquite seedlings

In this study we apply new extensions of classical growth analysis to assess the interactive effects of elevated CO₂ and differences in water availability on the leaf-nitrogen and transpiration productivities of velvet mesquite (Prosopis velutina Woot.) seedlings. The models relate transpiration productivity (biomass gained per mass of water transpired per day) and leaf-nitrogen productivity (biomass gain per unit leaf N per day) to whole-plant relative growth rate (RGR) and to each other, allowing a comprehensive hierarchical analysis of how physiological and morphological responses to the treatments interact with each other to affect plant growth. Elevated CO₂ led to highly significant increases in N and transpiration productivities but reduced leaf N per unit leaf area and transpiration per unit leaf area, resulting in no net effect of CO₂ on the RGR of seedlings. In contrast, higher water availability led to an increase in leaf-tissue thickness or density without affecting leaf N concentration, resulting in a higher leaf N per unit leaf area and consequently a higher assimilatory capacity per unit leaf area. The net effect was a marginal increase in seedling RGR. Perhaps most important from an ecological perspective was a 41% reduction in whole-plant water use due to elevated CO₂. These results demonstrate that even in the absence of CO₂ effects on integrative measures of plant growth such as RGR, highly significant effects may be observed at the physiological and morphological level that effectively cancel each other out. The quantitative framework presented here enables some of these tradeoffs to be identified and related directly to each other and to plant growth.