Effects of CO2 Enrichment on Photosynthesis, Growth, and Biochemical Composition of Seagrass Thalassia hemprichii (Ehrenb.) Aschers

The effects of CO2 enrichment on various ecophysiological parameters of tropical seagrass Thalassia hemprichii(Ehrenb.)Aschers were tested.T.hemprichii,collected from a seagrass bed in Xincun Bay,Hainan island of Southern China,was cultured at 4 CO2(aq)concentrations in flow-through seawater aquaria bubbled with CO2.CO2 enrichment considerably enhanced the relative maximum electron transport rate(RETRmax)and minimum saturating irradiance(Ek)of T.hemprichii.Leaf growth rate of CO2enriched plants was significantly higher than that in unenriched treatment.Nonstructural carbohydrates(NSC)of T.hemprichii,especially in belowground tissues,increased strongly with elevated CO2(aq),suggesting a translocation of photosynthate from aboveground to belowground tissues.Carbon content in belowground tissues showed a similar response with NSC,while in aboveground tissues,carbon content was not affected by CO2 treatments.In contrast,with increasing CO2(aq),nitrogen content in aboveground tissues markedly decreased,but nitrogen content in belowground was nearly constant.Carbon: nitrogen ratio in both tissues were obviously enhanced by increasing CO2(aq).Thus,these results indicate that T.hemprichii may respond positively to CO2-induced acidification of the coastal ocean.Moreover,the CO2-stimulated improvement of photosynthesis and NSC content may partially offset negative effects of severe environmental disturbance such as underwater light reduction.     

环境胁迫对海草非结构性碳水化合物储存和转移的影响

非结构性碳水化合物在海草体内的代谢对植株的生长有重要影响。为更好地跟踪非结构性碳水化合物在海草响应环境胁迫中所起的作用,根据国内外最新文献,重点综述了光强、营养盐、盐度、海洋酸化、温度、硫化物和动物摄食等环境胁迫对海草非结构性碳水化合物储存和转移的影响。光限制和富营养化均降低非结构性碳水化合物的合成,并使之从地下根茎转移到叶;而海洋酸化却促进非结构性碳水化合物合成并向地下组织转移;盐度变化改变海草体内渗透压,需要非结构性碳水化合物的新陈代谢来维持;温度通过影响光合作用、呼吸作用、氮代谢来影响非结构性碳水化合物的合成与储存;而硫化物和动物摄食则分别通过抑制海草酶的活性和啃食海草光合组织,减少非结构性碳水化合物的合成和储存。同时指出了一些今后关于海草非结构性碳水化合物的重点研究方向:(1)海草不同生命阶段(种子休眠和萌发,发育,繁殖等)非结构性与结构性碳水化合物之间,以及可溶糖与淀粉之间的转化分配机制;(2)双环境因子或者多环境因子对海草非结构性碳水化合物的耦合作用;(3)非结构性碳水化合物作为海草床生态系统健康评价指标的研究与应用。     

Does Nutrient Availability Regulate Seagrass Response to Elevated CO<Subscript>2</Subscript>?

Future increases in oceanic carbon dioxide concentrations (CO_2(aq)) may provide a benefit to submerged plants by alleviating photosynthetic carbon limitation. However, other environmental factors (for example, nutrient availability) may alter how seagrasses respond to CO_2(aq) by regulating the supply of additional resources required to support growth. Thus, questions remain in regard to how other factors influence CO_2(aq) effects on submerged vegetation. This study factorially manipulated CO_2(aq) and nutrient availability, in situ, within a subtropical seagrass bed for 350 days, and examined treatment effects on leaf productivity, shoot density, above- and belowground biomass, nutrient content, carbohydrate storage, and sediment organic carbon (C_org). Clear, open-top chambers were used to replicate CO_2(aq) forecasts for the year 2100, whereas nutrient availability was manipulated via sediment amendments of nitrogen (N) and phosphorus (P) fertilizer. We provide modest evidence of a CO₂ effect, which increased seagrass aboveground biomass. CO_2(aq) enrichment had no effect on nutrient content, carbohydrate storage, or sediment C_org content. Nutrient addition increased leaf productivity and leaf N content, however did not alter above- or belowground biomass, shoot density, carbohydrate storage, or C_org content. Treatment interactions were not significant, and thus NP availability did not influence seagrass responses to elevated CO_2(aq). This study demonstrates that long-term carbon enrichment may alter the structure of shallow seagrass meadows, even in relatively nutrient-poor, oligotrophic systems.     

不同CO2浓度和N肥水平对粳稻茎鞘非结构性碳水化合物含量与积累的影响

为了解CO2浓度升高和N肥水平对水稻茎鞘内非结构性碳水化合物(NSC)含量和积累量的影响,利用开顶式气室(OTC),以常规粳稻"南粳9108"为试验材料,设置3个CO2浓度水平:对照T0(背景大气)、T0+120μmol·mol-1(T1)和T0+200μmol·mol-1(T2)。在OTC内采用盆栽方式,设置3个氮(N)肥水平:10 g N·m^-2(N1)、20 g N·m^-2(N2)和30g N·m^-2(N3)。分别于水稻抽穗期、灌浆期(抽穗后20 d)和成熟期对地上部分各器官生物量、茎鞘NSC含量以及顶部四张叶片的N含量进行分析。结果表明:CO2浓度升高对抽穗期叶N含量总体无显著影响,但显著降低灌浆期N2和N3水平的叶N含量;CO2浓度升高对抽穗期茎鞘NSC含量和积累量无显著影响,抽穗期置换到高CO2浓度环境使灌浆期茎鞘NSC积累显著增加,置换到低CO2浓度环境使NSC积累显著减少。同一CO2浓度条件下,NSC含量和积累量均为N1>N2>N3,且N1处理均显著高于N3处理,CO2浓度升高和N水平的交互作用对灌浆期茎鞘NSC含量影响显著。水稻产量在不同CO2浓度水平间无显著差异,但随施氮水平的提高而增加。抽穗期与灌浆期水稻茎鞘NSC含量和积累量与茎鞘干重呈极显著正相关,与叶N含量呈极显著负相关;叶N衰减越慢,灌浆期水稻茎鞘NSC残留比(RNSC)越低;结实率和产量与RNSC呈显著负相关,RNSC越大,茎鞘NSC转移的越少,结实率和产量越低。     

Evaluation of aboveground and belowground biomass recovery in physically disturbed seagrass beds

Several studies addressed aboveground biomass recovery in tropical and subtropical seagrass systems following physical disturbance. However, there are few studies documenting belowground biomass recovery despite the important functional and ecological role of roots and rhizomes for seagrass ecosystems. In this study, we compared the recovery of biomass (g dry weight m(-2)) as well as the biomass recovery rates in ten severely disturbed multi-species seagrass meadows, after the sediments were excavated and the seagrasses removed. Three sites were located in the tropics (Puerto Rico) and seven in the subtropics (Florida Keys), and all were originally dominated by Thalassia testudinum. Total aboveground biomass reached reference values at four out of ten sites studied, two in the Florida Keys and two in Puerto Rico. Total belowground biomass was lower at the disturbed locations compared to the references at all sites, apart from two sites in the Florida Keys where the compensatory effect of opportunistic species (Syringodium filiforme and Halodule wrightii) was observed. The results revealed large variation among sites in aboveground and belowground biomass for all species, with higher aboveground recovery than belowground for T. testudinum. Recovery rates for T. testudinum were highly variable across sites, but a general trend of faster aboveground than belowground recovery was observed. Equal rates between aboveground and belowground biomass were found for opportunistic species at several sites in the Florida Keys. These results indicate the importance of belowground biomass when assessing seagrass recovery and suggest that the appropriate metric to assess seagrass recovery should address belowground biomass as well as aboveground biomass in order to evaluate the full recovery of ecological services and functions performed by seagrasses. We point out regional differences in species composition and species shifts following severe disturbance events and discuss ecological implications of gap dynamics in multi-species seagrass meadows.     

Effects of light availability on growth, architecture and nutrient content of the seagrass Zostera noltii Hornem

The growth vs. irradiance response of the seagrass Zostera noltii from Cadiz Bay Natural Park (southwestern Spain) was characterised. Plants were exposed along 14 days to different light treatments (1%, 7%, 42% and 100% surface irradiance, SI), using shade screens in an outdoor mesocosm. Growth at 100% SI (1.6 mg DW plant⁻¹ day⁻¹) was lower than that at 42% SI (2.4 mg DW plant⁻¹ day⁻¹), suggesting photoinhibition. The minimum light requirement estimated was 0.8 mol photons m⁻² day⁻¹ (2% SI). Light availability affected the pattern of plant development and the overall plant growth. The contribution of the apical shoots to the aboveground production was nearly constant (c.a. 1.13 cm plant⁻¹ day⁻¹) regardless of the light level (except at 1% SI). In contrast, recruitment and growth of lateral shoots arising from the main rhizome axes accounted for the observed differences in aboveground growth. Rhizome branching was only observed at 42% SI. The possibility of a light threshold for rhizome branching could explain the seasonality of shoot recruitment, as well as the observed decrease in shoot density along depth (or light) gradients in seagrass meadows. Carbon demands at low irradiances (1% and 7% SI) were partially met by mobilization of carbohydrate reserves (sucrose in belowground and starch in aboveground parts). Plant nitrogen content decreased with increasing light, especially in belowground parts, reaching critical levels for growth.     

CO(2) enrichment enhances flag leaf senescence in barley due to greater grain nitrogen sink capacity

Senescence is a highly regulated process which is under genetic control. In monocarpic plants, the onset of fruit development is the most important factor initiating the senescence process. During senescence, a large fraction of plant nutrients is reallocated away from vegetative tissues into generative tissues. Senescence may therefore be regarded as a highly effective salvage mechanism to save nutrients for the offspring. CO(2) enrichment, besides increasing growth and yield of C(3) plants, has often been shown to accelerate leaf senescence. C(3) plants grown under elevated CO(2) experience alterations in their nutrient relations. In particular their tissue nitrogen concentrations are always lower after exposure to elevated CO(2). We used a monocarpic C(3) crop - spring barley (Hordeum vulgare cv. Alexis) - grown in open-top field chambers to test the effects of CO(2) enrichment on growth and yield, on nitrogen acquisition and redistribution, and on the senescence process in flag leaves, at two applications of nitrogen fertilizer. CO(2) enrichment (650 vs. 366 μmol mol(-1)) caused an increase both in biomass and in grain yield by 38% (average of the two fertilizer applications) which was due to increased tillering. Total nitrogen uptake of the crops was not affected by CO(2) treatment but responded solely to the N supply. Nitrogen concentrations in grains and straw were significantly lower (-33 and -24%) in plants grown at elevated CO(2). Phenological development was not altered by CO(2) until anthesis. However, progress of flag leaf senescence as assessed by chlorophyll content, protein content and content of large and small subunit of RubisCO and of cytochrome b559 was enhanced under elevated CO(2) concentrations by approximately 4 days. We postulate that CO(2) enhanced flag leaf senescence in barley crops by increasing the nitrogen sink capacity of the grains.     

Response of plant biomass to nitrogen addition and precipitation increasing under different climate conditions and time scales in grassland

生产力是草地生态系统重要的服务功能, 而生物量作为生态系统生产力的主要组成部分, 往往同时受到氮和水分两个因素的限制。在全球变化背景下, 研究草地生态系统生物量对氮沉降增加和降水变化的响应具有重要意义, 但现有研究缺乏对其在大区域空间尺度以及长时间尺度上响应的综合评估和量化。本研究搜集了1990-2017年间发表论文的有关模拟氮沉降及降水变化研究的相关数据, 进行整合分析, 探讨草地生态系统生物量对氮沉降和降水量两个因素的变化在空间和时间尺度上的响应。结果表明: (1)氮添加、增雨处理以及同时增氮增雨处理都能够显著地提高草地生态系统的地上生物量(37%, 41%, 104%)、总生物量(32%, 23%, 60%)和地上地下生物量比(29%, 25%, 46%)。单独增雨显著提高地下生物量(10%), 单独施氮对地下生物量影响不显著, 但同时增雨则能显著提高地下生物量(43%); (2)氮添加和增雨处理对草地生态系统生物量的影响存在明显的空间变异。在温暖性气候区和海洋性气候区的草地生态系统中, 氮添加对地上、总生物量及地上地下生物量比的促进作用更强, 而在寒冷性气候区和温带大陆性气候区的草地生态系统中, 则增雨处理对地下、总生物量的促进作用更强; (3)草地生态系统生物量对氮添加和增雨处理的响应也存在时间格局上的变化, 地下生物量随着氮添加年限的增加有降低的趋势, 地上、总生物量及地上地下生物量比则有增加的趋势。增雨年限的增加对总生物量没有明显的影响, 但持续促进地上生物量和地下生物量, 增加地上地下生物量比, 可见长期增氮、长期增雨对地上生物量的促进作用更明显。     

草地生态系统生物量在不同气候及多时间尺度上对氮添加和增雨处理的响应

生产力是草地生态系统重要的服务功能, 而生物量作为生态系统生产力的主要组成部分, 往往同时受到氮和水分两个因素的限制。在全球变化背景下, 研究草地生态系统生物量对氮沉降增加和降水变化的响应具有重要意义, 但现有研究缺乏对其在大区域空间尺度以及长时间尺度上响应的综合评估和量化。本研究搜集了1990-2017年间发表论文的有关模拟氮沉降及降水变化研究的相关数据, 进行整合分析, 探讨草地生态系统生物量对氮沉降和降水量两个因素的变化在空间和时间尺度上的响应。结果表明: (1)氮添加、增雨处理以及同时增氮增雨处理都能够显著地提高草地生态系统的地上生物量(37%, 41%, 104%)、总生物量(32%, 23%, 60%)和地上地下生物量比(29%, 25%, 46%)。单独增雨显著提高地下生物量(10%), 单独施氮对地下生物量影响不显著, 但同时增雨则能显著提高地下生物量(43%); (2)氮添加和增雨处理对草地生态系统生物量的影响存在明显的空间变异。在温暖性气候区和海洋性气候区的草地生态系统中, 氮添加对地上、总生物量及地上地下生物量比的促进作用更强, 而在寒冷性气候区和温带大陆性气候区的草地生态系统中, 则增雨处理对地下、总生物量的促进作用更强; (3)草地生态系统生物量对氮添加和增雨处理的响应也存在时间格局上的变化, 地下生物量随着氮添加年限的增加有降低的趋势, 地上、总生物量及地上地下生物量比则有增加的趋势。增雨年限的增加对总生物量没有明显的影响, 但持续促进地上生物量和地下生物量, 增加地上地下生物量比, 可见长期增氮、长期增雨对地上生物量的促进作用更明显。     

Response of plant biomass to nitrogen addition and precipitation increasing under different climate conditions and time scales in grassland北大核心CSCD

Aims Plant biomass accounts for the main part of grassland productivity. The productivity of grassland regarded as one of important ecosystem function is always co-limited by nitrogen and water availability, therefore, how grasslands respond to atmosphic nitrogen (N) addition and precipitation increasing need to be systematically and quantitatively evaluated at different climate conditions and temporal scales. Methods To investigate the impact of nitrogen addition and precipitation increasing on grassland biomass over climate conditions and temproal scales, a meta-analysis was conducted based on 46 papers that were published during 1990-2017 involving 1 350 observations. Important findings Results showed that: (1) N addtion, precipitation increasing and the combinations of these two treatments significantly increased the aboveground biomass (37%, 41%, 104%), total biomass (32%, 23%, 60%) and the ratio of aboveground biomass to belowground biomass (29%, 25%, 46%) in grassland ecosystem. Belowground biomass showed no response to single N addtion, but could be significantly enhanced together with increaseing precipitation; (2) The response of grassland biomass under these N addtion and the increasing of precipitation showed obvious spatial pattern under different climate conditions. The N addition tended to increase more aboveground biomass, total biomass and the ratio of aboveground biomass to belowground biomass under high sites with high mean annual air temperature (MAT) and mean annual precipitation (MAP) while precipitation increasing tended to simulate more belowground biomass and total biomass under low MAT and MAP sites; (3) In addition, the response of grassland biomass under these two global change index showed obvious temporal pattern. With the increase of duration of N addition, the belowgound biomass tended to decrease, while the aboveground biomass, total biomass and the ratio of aboveground biomass to belowground biomass tended to increase under N addition. With the increase of duration of precipitation manipulation, the total biomass showed no response to precipitation increasing, while aboveground biomass, belowground biomass and the ratio of aboveground biomass to belowground biomass tended to be enhanced. The results indicated that aboveground biomass was more likely to be enhanced than belowground biomass under N addition or precipitation increasing in the long term. © 2018 Editorial Office of Chinese Journal of Plant Ecology. All rights reserved.     

The short-term influence of herbivory near patch reefs varies between seagrass species

The coexistence of multiple species within a trophic level can be regulated by consumer preferences and nutrient supply, but the influence of these factors on the co-occurrence of seagrass species is not well understood. We examined the biomass and density responses of two seagrass species in the Florida Keys Reef Tract to grazing pressure near patch reefs, and evaluated how nutrient enrichment impacted herbivory dynamics. We transplanted Halodule wrightii (shoalgrass) sprigs into caged and uncaged plots in a Thalassia testudinum (turtlegrass) bed near a patch reef. Nutrients (N and P) were added to half of the experimental plots. We recorded changes in seagrass shoot density, and after three months, we measured above- and belowground biomass and tissue nutrient content of both species. Herbivory immediately and strongly impacted H. wrightii. Within six days of transplantation, herbivory reduced the density of uncaged H. wrightii by over 80%, resulting in a decrease in above- and belowground biomass of nearly an order of magnitude. T. testudinum shoot density and belowground biomass were not affected by herbivory, but aboveground biomass and leaf surface area were higher within cages, suggesting that although herbivory influenced both seagrass species, T. testudinum was more resistant to herbivory pressure than H. wrightii. Nutrient addition did not alter herbivory rates or the biomass of either species over the short-term duration of this study. In both species, nutrient addition had little effect on the tissue nutrient content of seagrass leaves, and N:P was near the 30:1 threshold that suggested a balance between N and P. The different impacts of grazing on these two seagrass species suggest that herbivory may be an important regulator of the distribution of multiple seagrass species near herbivore refuges like patch reefs in the Caribbean.     

Long-Term Effects of Adding Nutrients to an Oligotrophic Coastal Environment

Management of ecological disturbances requires an understanding of the time scale and dynamics of community responses to disturbance events. To characterize long-term seagrass bed responses to nutrient enrichment, we established six study sites in Florida Bay, USA. In 24 plots (0.25 m²) at each site, we regularly added nitrogen (N) and phosphorus (P) in a factorial design for 7 years. Five of the six sites exhibited strong P limitation. Over the first 2 years, P enrichment increased Thalassia testudinum cover in the three most P-limited sites. After 3 years, Halodule wrightii began to colonize many of the P-addition plots, but the degree of colonization was variable among sites, possibly due to differences in the supply of viable propagules. Thalassia increased its allocation to aboveground tissue in response to P enrichment; Halodule increased in total biomass but did not appear to change its aboveground: belowground tissue allocation. Nutrient enrichment did not cause macroalgal or epiphytic overgrowth of the seagrass. Nitrogen retention in the study plots was variable but relatively low, whereas phosphorus retention was very high, often exceeding 100% of the P added as fertilizer over the course of our experiments. Phosphorus retentions exceeding 100% may have been facilitated by increases in Thalassia aboveground biomass, which promoted the settlement of suspended particulate matter containing phosphorus. Our study demonstrated that low-intensity press disturbance events such as phosphorus enrichment can initiate a slow, ramped successional process that may alter community structure over many years.     

The role of leaf nitrogen content in determining turtlegrass (Thalassia testudinum) grazing by a generalized herbivore in the northeastern Gulf of Mexico

In shallow marine environments the variability in grazing on seagrasses has been hypothesized to be controlled, in part, by the nutritive quality (i.e., nitrogen content) of their leaves. The few existing studies of the relationship between leaf nitrogen content and seagrass grazing have all found a positive relationship between leaf nitrogen content and preference by selective vertebrate grazers (i.e., the bucktooth parrotfish, green sea turtles, and dugongs). However, most marine herbivores (both vertebrate and invertebrate) are thought to be extreme generalists with broad diets of variable nutritive quality (e.g., detritus, living plants, and animals), suggesting the currently held view on the role leaf nutrient content in explaining the variability of seagrass grazing is an oversimplification.In this study, we evaluated how leaf nitrogen content influenced grazing on turtlegrass by a generalist invertebrate herbivore (the pink sea urchin Lytechinus variegatus) in the northeastern Gulf of Mexico. Using a short-term laboratory test and a longer-term field experiment, we tested the hypothesis that leaf nitrogen content controls sea-urchin grazing on seagrass leaves. We hypothesized that if poor nutritive value of seagrasses is responsible for reduced rates of feeding, then increasing leaf nitrogen concentrations should lead to increased rates of seagrass consumption by sea urchins.In the field experiment, we significantly enriched seagrass leaf nitrogen concentrations (some 10-20% depending on month) in experimental plots with a commercial fertilizer and we manipulated grazing intensity by enclosing adult sea urchins at densities that bracketed the range of average densities observed in the region (i.e., 0, 10 and 20 individuals/m(2)). Comparisons of changes in aboveground seagrass production and biomass showed no evidence that sea urchins grazed significantly more in treatments where leaf nitrogen was enriched. Because the statistical power of our test to detect such differences was low and aboveground seagrass production varied significantly among treatments, we also used a mass balance equation to estimate sea urchin consumption of nitrogen-enriched and unenriched leaves. This showed that sea urchins compensated for low nitrogen levels in our unenriched treatments by eating more leaves than in treatments where leaf nitrogen was elevated. Using a laboratory test, we also found that sea urchins ate less nitrogen-enriched seagrass than unenriched seagrass. In combination, these results show that, in contrast to findings reported for vertebrate herbivores, sea urchins feed at higher rates when offered seagrass leaves of lower leaf nitrogen content, and that low levels of leaf nitrogen are not always an effective defense against herbivores.     

Effects of CO2 enrichment on whole-plant carbon budget of seedlings of Fagus grandifolia and Acer saccharum in low irradiance

Carbon exchange rates (CER) and whole-plant carbon balances of beech (Fagus grandifolia) and sugar maple (Acer saccharum) were compared for seedlings grown under low irradiance to determine the effects of atmospheric CO₂ enrichment on shade-tolerant seedlings of co-dominant species. Under contemporary atmospheric CO₂, photosynthetic rate per unit mass of beech was lower than for sugar maple, and atmospheric CO₂ enrich ment enhanced photosynthesis for beech only. Aboveground respiration per unit mass decreased with CO₂ enrichment for both species while root respiration per unitmass decreased for sugar maple only. Under contemporary atmoapheric CO₂, beech had lower C uptake per plant than sugar maple, while C losses per plant to nocturnal aboveground and root respiration were similar for both species. Under elevated CO₂, C uptake per plant was similar for both species, indicating a significant relative increase in whole-seedling CER with CO₂ enrich ment for beech but not for sugar maple. Total C loss per plant to aboveground respiration was decreased for beech only because increase in sugar maple leaf mass counterbalanced a reduction in respiration rates. Carbon loss to root respiration per plant was not changed by CO₂ enrichment for either species. However, changes in maintenance respiration cost and nitrogen level suggest changes in tissue composition with elevated CO₂. Beech had a greater net daily C gain with CO₂ enrichment than did sugar maple in contrast to a lower one under contemporary CO₂. Elevated CO₂ preferentially enhances the net C balance of beech by increasing photosynthesis and reducing respiration cost. In all cases, the greatest C lost was by roots, indicating the importance of belowground biomass in net C gain. Relative growth rate estimated from biomass accumulation was not affected by CO₂ enrichment for either species possibly because of slow growth under low light. This study indicates the importance of direct effects of CO₂ enrichment when predicting potential change in species distribution with global climate change.     

Response of carbon and nitrogen content in plants and soils to vegetation cover change in alpine Kobresia meadow of the source region of Lantsang, Yellow and Yangtze Rivers

We conducted this study in lightly and severely degraded Kobresia pygmaea meadow in Gande County, Qinghai Province of China. The purpose of this research was to compare carbon and nitrogen concentrations, content and dynamics of aboveground tissue, belowground roots and soil (0-40 cm) between lightly and severely degraded Kobresia meadow. The results showed that C and N concentrations and C:N ratio of the aboveground tissue were significantly higher in lightly degraded grassland than in severely degraded grassland. In addition, total carbon and nitrogen concentrations of the aboveground tissue were ranked in order of forbs > grasses > sedges in the same grassland type. Total carbon and nitrogen concentrations of belowground roots were significantly higher in severely degraded grassland than in lightly degraded grassland. Total carbon and nitrogen concentrations were higher in the aboveground tissue than in the belowground roots. Total soil organic carbon concentration in severely degraded grassland was significantly lower than that in lightly degraded grassland, and decreased with depth. C and N content per unit area was ranked in order of 0-40 cm soil depth > belowground roots > aboveground issue in the same grassland type. The total carbon content per unit area of aboveground tissue, roots and 0-40 cm soil depth declined by 7.60% after degradation from lightly (14669.2 g m⁻²) to severely degraded grassland (13554.3 g m⁻²), i.e., 0-40 cm soil depth declined by 4.10%, belowground roots declined by 59.97% and aboveground tissue declined by 15.39%. The nitrogen content per unit area of aboveground tissue, roots and 0-40 cm soil depth increased after degradation by 12.76% from lightly (3352.7 g m⁻²) to severely degraded grassland (3780.6 g m⁻²), i.e., 0-40 cm soil depth increased by 13.07%, belowground roots declined by 55.09% and aboveground tissue declined by 16.00%. As a result of grassland degradation, the total carbon lost by 11149 kg hm⁻², and the total nitrogen increased by 4278 kg hm⁻².     

No evidence for depletion of carbohydrate pools in Scots pine (Pinus sylvestris L.) under drought stress

The physiological mechanisms leading to Scots pine (Pinus sylvestris L.) decline in the dry inner alpine valleys are still unknown. Testing the carbon starvation hypothesis, we analysed the seasonal course of mobile carbohydrate pools (NSC) of Scots pine growing at a xeric and a dry-mesic site within an inner alpine dry valley (750 m a.s.l., Tyrol, Austria) during 2009, which was characterised by exceptional soil dryness. Although, soil moisture content dropped to ca. 10% at both sites during the growing season, NSC concentrations rose in all tissues (branch, stem, root) until the end of July, except in needles, where maxima were reached around bud break. NSC concentrations were not significantly different in the analysed tissues at the xeric and the dry-mesic site. At the dry-mesic site, NSC concentrations in the aboveground tree biomass were significantly higher during the period of radial growth. An accumulation of NSC in roots at the end of July indicates a change in carbon allocation after an early cessation in aboveground growth, possibly due to elevated belowground carbon demand. In conclusion, our results revealed that extensive soil dryness during the growing season did not lead to carbon depletion. However, even though carbon reserves were not exhausted, sequestration of carbohydrate pools during drought periods might lead to deficits in carbon supply that weaken tree vigour and drive tree mortality.     

Settlement preferences and early migration of the tropical sea cucumber Holothuria scabra

Settlement and post-settlement processes of the sea cucumber Holothuria scabra Jaeger were studied in the laboratory. Independent and paired choice experiments revealed that several substrates could induce metamorphosis into pentactulae, but that specific substrates favoured settlement. Leaves of seagrass Thalassia hemprichii, with or without their natural bio-film, yielded the highest settlement rates (4.8-10.5%). T. hemprichii was preferred as a settlement substrate over sand, crushed coral, several other plant species and artificial seagrass leaves with or without a bio-film. Only settlement on the seagrass, Enhalus acoroides, was similar to that recorded for T. hemprichii. In the absence of a substrate, the larvae delayed settlement for nearly 96 h and survival was less than 0.5%. Sand and crushed coral, either alone or together, induced settlement from <1.5% of the available larvae. The pentactulae found on sand, coral and in bare containers were 10-35% smaller than those on T. hemprichii leaves. Soluble extracts from T. hemprichii and E. acoroides successfully induced metamorphosis and settlement on clean plastic surfaces. Newly settled juveniles remained on the seagrass leaves for 4-5 weeks before migrating to sand at around 6 mm in length. Prior to this, the juveniles spent 4-5 days moving on and off the leaves. Once on the sand, the juveniles became deposit-feeders, but did not show the typical burrowing behaviour of older specimens until they reached around 11 mm in length. The larvae of H. scabra appear to actively select seagrass leaves, possibly through chemical detection. We hypothesise that larvae settling on seagrass have an increased chance of growth and survival because they are provided with a suitable substrate on which to grow, and a bridge to sand substrates as they become deposit-feeders.     

Effects of CO 2 enrichment on photosynthesis,growth, and nitrogen metabolism of the seagrass Zostera noltii

Seagrass ecosystems are expected to benefit from the global increase in CO ₂ in the ocean because the photosynthetic rate of these plants may be C_i‐limited at the current CO ₂ level. As well, it is expected that lower external pH will facilitate the nitrate uptake of seagrasses if nitrate is cotransported with H⁺ across the membrane as in terrestrial plants. Here, we investigate the effects of CO ₂ enrichment on both carbon and nitrogen metabolism of the seagrass Zostera noltii in a mesocosm experiment where plants were exposed for 5 months to two experimental CO ₂ concentrations (360 and 700 ppm). Both the maximum photosynthetic rate (P_m) and photosynthetic efficiency (α) were higher (1.3‐ and 4.1‐fold, respectively) in plants exposed to CO ₂‐enriched conditions. On the other hand, no significant effects of CO ₂ enrichment on leaf growth rates were observed, probably due to nitrogen limitation as revealed by the low nitrogen content of leaves. The leaf ammonium uptake rate and glutamine synthetase activity were not significantly affected by increased CO ₂ concentrations. On the other hand, the leaf nitrate uptake rate of plants exposed to CO ₂‐enriched conditions was fourfold lower than the uptake of plants exposed to current CO ₂ level, suggesting that in the seagrass Z. noltii nitrate is not cotransported with H⁺ as in terrestrial plants. In contrast, the activity of nitrate reductase was threefold higher in plant leaves grown at high‐CO ₂ concentrations. Our results suggest that the global effects of CO ₂ on seagrass production may be spatially heterogeneous and depend on the specific nitrogen availability of each system. Under a CO ₂ increase scenario, the natural levels of nutrients will probably become limiting for Z. noltii. This potential limitation becomes more relevant because the expected positive effect of CO ₂ increase on nitrate uptake rate was not confirmed.     

江河源区高山嵩草(Kobresia pygmaea)草甸植物和土壤碳、氮储量对覆被变化的响应

以青海省果洛州藏族自治州甘德县青珍乡高山嵩草Kobresia pygmaea草甸轻度退化草地和重度退化草地为研究对象,通过植物地上部分主要功能群(禾草类、杂类草、莎草类)、植物根系和土壤碳、氮浓度及储量动态研究,结果表明:高寒小嵩草草甸轻度退化草地地上部分主要功能群碳、氮浓度和C ∶ N比值明显高于重度退化草地的浓度.同一草地类型主要功能群比较,碳、氮浓度依次为杂类草>禾草类>莎草类;植物地上部分的碳、氮浓度明显高于地下根系的碳、氮浓度.重度退化草地植物根系碳、氮浓度高于轻度退化草地植物根系碳、氮浓度.重度退化草地土壤总有机碳浓度显著低于轻度退化草地土壤总有机碳浓度,随着土层的加深碳、氮浓度有减少的趋势.江河源区高山嵩草草甸的土壤有机碳、氮储量最大,植物根系碳、氮储量居中,植物地上部分碳、氮储量最小.重度退化草地总有机碳储量(13554.3 g/m2)较轻度退化草地储量(14669.2 g/m2)下降7.60%.其中,0～40cm土壤层碳储量下降4.10%,植物根系碳储量下降59.97%,植物地上部分碳储量下降15.39%;重度退化草地总氮储量(3780.6 g/m2)较轻度退化草地储量(3352.7 g/m2)高12.76%,其中,0～40cm土壤中总氮储量高13.07%,植物根系全氮储量下降55.09%,植物地上部分全氮下降16.00%.由于草地退化损失有机碳11149 kg/hm2,而全氮增加4278 kg/hm2.     

Rapid restoration of belowground structure and fauna of a seagrass habitat

Seagrass meadows are habitat for an abundance and diversity of animal life, and their continuing global loss has focused effort on their restoration. This restoration not only aims to re‐establish the structure of the seagrass, but also to restore its function, particularly as habitat. The success of seagrass restoration is typically measured by the recovery of aboveground structure, but this ignores the important role of the belowground component of seagrass ecosystems, which may not recover at the same rate, and is equally important for faunal communities. We quantify infaunal communities (abundance, richness, and composition) within expanding plots of restored seagrass, and relate their change to the recovery of belowground seagrass biomass and sediment properties. We found that infaunal abundance and composition converged on that found in natural seagrass within 2 years, while the overall infaunal richness was not affected by habitat. The carbon content of surface sediments also recovered within 2 years, although recovery of belowground biomass and sediment grain size took 4 to 6 years. These results suggest that the structure of recovering seagrass habitats may not need to attain that of natural meadows before they support infauna that is comparable to natural communities. This pace and effectiveness of recovery provides some optimism for future seagrass restoration.