The consequences of lower nitrogen availability in autumn for internal nitrogen reserves and spring growth of <Emphasis Type="Italic">Calamagrostis epigejos</Emphasis>

The building and use of internal N stores in the grass Calamagrostis epigejos was investigated in context of complex ecological study focused on mechanisms underlying competitive ability of this highly successful invasive species. Induced changes in nitrogen availability in the course of two subsequent vegetation seasons were used as a tool for finding (i) to what extent high N availability in substrate is important for building N reserves in autumn that support spring regrowth and, (ii) if contrasting contents of N storage compounds may result in differences in growth in the next season. Plants were grown in solely inorganic substrate and received a nutrient solution containing 5 mol m⁻³ of NH₄NO₃. The nitrogen supply was reduced in a low nitrogen (LN) treatment to 0.25 mol m⁻³ in August whereas in high nitrogen (HN) treatment remained high till December. During the following growing season were plants from both treatments grown at the low N supply (0.25 mol m⁻³). An increase in the content of N storage compounds was observed from September to December in both treatments. Plants in the LN treatment showed significantly lower total N content and also N allocated to mobilizable reserves (20–50% of HN plants), namely due to a smaller accumulation of amino acids and soluble protein in autumn. External nitrogen availability in autumn is hence highly important for building N reserves in this species. A major portion of the nitrogen stored in HN plants during winter was taken up from growth medium in late autumn, whereas translocation from senescing shoots dominated in LN treatment. During the winter about 50% of N in plants was permanently present in shoots bearing several frost resistant green leaves. Spring regrowth was accompanied by a fast decrease of both total N and the content of N storage compounds in both treatments. Amino acids were identified as the most prominent source of mobilizable N during spring regrowth. Development of leaf area in LN plants was significantly slower in March and April than in HN plants namely due to smaller number of tillers and green leaves per plant. Low N availability in autumn, therefore, may result in restrictions of plant growth and development in the following season.     

Periphyton production and grazing by chironomids in Alderfen Broad, Norfolk

The standing crops of periphyton were measured on dead Typha stems and glass rods in a small, eutrophic lake from February to November. Chironomid larvae were also counted on the Typha stems; very few were present on the glass rods. The standing crop of periphyton on the Typha stems fell from 1.8 mg cm⁻² in early April to nearly zero in November. On the glass rods the periphyton reached a peak of 1.93 mg cm⁻² in late May and 1.94 mg cm⁻² in July, thereafter falling to a steady level of 1.6 mg cm⁻². The population of chironomids showed a peak in late May and then declined. The alimentary canals of chironomids collected fromTyphastems contained diatoms and filamentous algae so it appeared that chironomids were grazing down the periphyton. Chironomids moved on to theTyphastems in spring and returned to the mud in autumn. The periphyton is a richer source of essential amino acids than the mud, so that a movement from mud to reedstems in spring may increase the rates of growth and metamorphosis of the larvae. A crude production estimate gave a net primary production of periphyton of 170 mg dry wt m⁻² day⁻¹; the periphyton, however, would have contained bacteria and many small animals as well as algae.     

Standing crop and mineral content of reed,Phragmites australis (Cav.) Trin. exSteudel,in Sweden—Management of reed stands to maximize harvestable biomass

The mean above-ground biomass of reed,Phragmites australis, in closed South Swedish stands was found to be 1 kg dry weight. m^?2 in August. Leaves, which are shed in the autumn in contrast to culms that remain standing, represent 26% of the total shoot weight. Because part of the culm will be covered by water, ice and snow 0.5 kg dry weight. m^?2 is available for winter harvest. Nutrient concentrations in shoots decrease throughout summer and winter. Although part of the maximal summer standing stock of N, P and K is lost in shed leaves, 55%, 75% and 80%, respectively, can potentially be recycled to rhizomes. Nitrogen fertilization and removal of standing litter in winter can increase above-ground biomass production in reed stands. Reed culms, cut in winter with agricultural machinery or amphibious harvesters, have been tested as a fuel for heating purposes in Sweden     

Expansion of <Emphasis Type="Italic">Phragmites australis</Emphasis> (Cav.) Trin. ex Steud. (Common Reed) into <Emphasis Type="Italic">Typha</Emphasis> spp. (Cattail) Wetlands in Northwestern Indiana,USA

Expansion of Phragmites australis (Cav.) Trin. ex Steud. (common reed) into stands of Typha spp. (cattail; Typha australis L. and T. x glauca) is common in the wetlands of northwestern Indiana (USA). To understand this phenomenon better, we investigated the production of shoot sprouts and proportional allocation of biomass as well as a potential role for the water table in the relative dominance of each species. The reduction in sprouts from rhizomes upon vegetative expansion of Phragmites appeared to be the most likely process causing the decline of Typha. The latter had a shoot density of 39/m² in plots without Phragmites, but this dropped to 13 shoots m⁻² in plots that had been invaded by Phramites. Such a decline was likely caused by reduced reserves; e.g., the belowground biomass of Typha decreased from 11.3 g m⁻² without Phragmites to 8.1 g m⁻² with Phragmites. The latter also reduced its belowground biomass but not its shoot density in the presence of Typha. The mean weight of Phragmites shoots was 2.9 g, and nearly all produced inflorescences. Meanwhile, Typha failed to develop spadices despite its shoots having a greater biomass (7 g). This suggests that Phragmites is more efficient than Typha in shoot growth. Springtime flooding appeared to promote the sprout of Typha shoots from shallow rhizomes (≈18 cm below the soil surface), whereas the shoot density of Phragmites showed no correlation with water level in that season. Deep-rooted Phragmites (≈39 cm) occurred on both high and low water-table sites, whereas the shallow-rooted Typha was limited to only the former. Phragmites will likely continue its expansion, by vegetative sprouts from rhizomes, into Typha wetlands.     

Underground organs ofPhragmites communis,their growth,biomass and net production

The present paper sums up the knowledge obtained from the study of growth periodicity in the underground organs ofPhragmites communis Trin. and from the analyses of differentPhragmites stands in three regions of Czechoslovakia. A period of intense growth ofPhragmites rhizomes was recorded in summer. Spring (end of April and beginning of May) and autumn (mainly September) seem to be the periods of most active root growth. During July and August, accumulation of reserve material takes place both in new and old rhizomes. In the stands investigated, the biomass ofPhragmites rhizomes varied from 2 kg/m² to 5 kg/m², and root dry weight from 0.08 kg/m² to 3.6 kg/m². The ratio of underground to total aboveground dry weight was highly variable (1.0 to 9.9). The estimated annual net rhizome production ofPhragmites, in two different stand, was 30% (?akvický fishpond) and 60% (Nesyt fishpond) of the seasonal maximum above-ground biomass.     

Changes in soil carbon and nitrogen cycling along a 72-year wildfire chronosequence in Michigan jack pine forests

We investigated the changes in soil processes following wildfire in Michigan jack pine (Pinus banksiana) forests using a chronosequence of 11 wildfire-regenerated stands spanning 72 years. The objective of this study was to characterize patterns of soil nutrients, soil respiration and N mineralization with stand development, as well as to determine the mechanisms driving those patterns. We measured in situ N mineralization and soil respiration monthly during the 2002 growing season and used multiple regression analysis to determine the important factors controlling these processes. Growing-season soil respiration rates ranged from a low of 156 g C/m² in the 7-year-old stand to a high of 254 g C/m² in the 22-year-old stand, but exhibited no clear pattern with stand age. In general, soil respiration rates peaked during the months of July and August when soil temperatures were highest. We used a modified gamma function to model a temporal trend in total N mineralization (total N mineralization = 1.853−0.276 × age × e ^{−0.814 × age}; R ² = 0.381; P = 0.002). Total N mineralization decreased from 2.8 g N/m² in the 1-year-old stand to a minimum value of 0.5 g N/m² in the 14-year-old stand, and then increased to about 1.5 g N/m² in mature stands. Changes in total N mineralization were driven by a transient spike in N turnover in the mineral soil immediately after wildfire, followed by a gradual accrual of a slow-cycling pool of N in surface organic horizons as stands matured. Thus, in Michigan jack pine forests, the accumulation of surface organic matter appears to regulate N availability following stand-replacing wildfire.     

Life cycle and production of Skwala parallela (Frison) (Plecoptera: Perlodidae) in a Colorado montane stream

The life cycle and production of Skwala parallela, a perlodid stonefly, was investigated in a third-order Colorado montane stream. The species exhibited a univoltine life cycle with a distinct cohort. Small nymphs appeared in May. Rapid growth was exhibited throughout summer and autumn. During winter, growth slowed somewhat but was continuous until April. Maximum density of 34 nymphs/m² occurred in July. Based upon the instantaneous growth method, annual production was 395.3 mg/m² or 3.95 kg/ha dry weight with a P/B ratio of 4.4     

Harvesting rate of the termite,Drepanotermes tamminensis (Hill) within native woodland and shrubland of the Western Australian wheatbelt

The Western Australian termite,Drepanotermes tamminensis (Hill), harvests various plant materials according to biomass availability. The main litter components harvested by this termite in a woodland dominated byEucalyptus capillosa are bark and leaves of the major tree species, while in shrubland dominated byAllocasuarina campestris, shoots of this species are taken. Harvesting mainly occurs during the autumn (April–May) and spring (September–October) seasons. The commencement and duration of harvesting appears to depend partly on weather conditions, with harvesting taking place at temperatures between 15 and 25°C after periods of rain. This species of termite harvests approximately 15.6 g m⁻² year⁻¹ and 3.2 g m² year⁻¹ (dry weight of plant material) in the woodland and shrubland, respectively.     

Biomass harvest of invasive Typha promotes plant diversity in a Great Lakes coastal wetland

Ecological and financial constraints limit restoration efforts, preventing the achievement of desired ecological outcomes. Harvesting invasive plant biomass for bioenergy has the potential to reduce feedback mechanisms that sustain invasion, while alleviating financial limitations. Typha × glauca is a highly productive invasive wetland plant that reduces plant diversity, alters ecological functioning, its impacts increase with time, and is a suitable feedstock for bioenergy. We sought to determine ecological effects of Typha utilization for bioenergy in a Great Lakes coastal wetland by testing plant community responses to harvest‐restoration treatments in stands of 2 age classes and assessing community resilience through a seed bank study. Belowground harvesting increased light penetration, diversity, and richness and decreased Typha dominance and biomass in both years post‐treatment. Aboveground harvesting increased light and reduced Typha biomass in post‐year 1 and in post‐year 2, increased diversity and richness and decreased Typha dominance. Seed bank analysis revealed that young stands (<20 years) had greater diversity, richness, seedling density, and floristic quality than old stands (>30 years). In the field, stand‐age did not affect diversity or Typha dominance, but old stands had greater Typha biomass and slightly higher richness following harvest. Harvesting Typha achieved at least 2 desirable ecological outcomes: reducing Typha dominance and increasing native plant diversity. Younger stands had greater potential for native recovery, indicated by more diverse seed banks. In similar degraded wetlands, a single harvest of Typha biomass would likely result in significant biodiversity and habitat improvements, with the potential to double plant species richness.     

氮素添加和功能群去除对糙隐子草和大针茅根系特征的影响

由于草地不合理利用,中国北方草原严重退化,导致生态系统结构性破坏甚至功能性紊乱。随着草原退化,中能值功能群植物糙隐子草会取代高能值功能群植物大针茅成为群落中优势度最大的植物。为了解大针茅和糙隐子草在氮素添加以及去除功能群的情况下根系特征的变化趋势,选取锡林郭勒盟典型草原中的退化草原群落,分别开展两项实验,实验1:氮素添加梯度实验(0、10.5、17.5、28 gN/m~2),实验2:同时进行功能群的去除和氮素的添加,功能群去除实验(将植物分为高能值植物功能群、中能值植物功能群、低能值植物功能群3个功能群;每个处理分别剪除另外两个功能群,留取单一功能群),并同时进行氮素添加实验(0、17.5 gN/m~2)。在进行两年的实验处理后,通过Delta-T SCAN根系分析仪测量大针茅和糙隐子草根系的长度、直径、面积、体积指标。分析植物根系对氮素添加的响应,以及功能群去除是否改变这两种植物对氮素添加的响应格局。实验1研究结果表明:在受到其他功能群的竞争压力下,大针茅根系长度、面积、体积均在高氮素添加(28 gN/m~2)情况下显著增加,糙隐子草根系直径和体积在中氮素添加(17.5 gN/m~2)的情况下显著高于其他3个处理,退化样地中土壤氮素的增加,促使大针茅根系主要通过增加根系长度扩大在土壤中的空间分布,而促使糙隐子草主要通过增粗生长来扩大土壤分布空间。实验2研究结果表明:功能群的去除,中氮素添加对根系的影响,只有糙隐子草的根系直径显著增加。综合来看,功能群去除实验对大针茅的根系长度和面积有影响,对糙隐子草的根系长度、直径和面积有影响。功能群去除处理与氮素添加的交互作用对大针茅根系没有影响,对糙隐子草的根系直径和体积有影响。     

Seasonal changes in the photosynthetic response ofMercurialis perennis plants from different light regime conditions

Curves relating net photosynthetic rate to irradiance [P(I) curve relation] were estimated and analysed inMercurialis perennis L. plants stemming from three forest (spruce, beech and ash) stands with different tree leaf canopy development and different light regime. The saturating irradiance (I_s) reached the highest values in plants of all three stands in spring (spruce forest: 438 W m⁻², beech forest: 440 W m⁻² and ash forest: 367 W m⁻²), it declined sharply in the middle of the growing season (283, 285 and 297 W m^-2, respectively) and this I_s level persisted until autumn. A pronounced dynamics in plants from spruce and beech forests made itself manifest also in the adaptation (I_a) and compensating (I_c) irradiances, respectively. After a sudden decline in summer, values in autumn were close to those of the vernal season. The most pronounced parameter, which optimally expressed the adaptation ofMercurialis perennis to various light conditions, was the photosynthetic efficiency (α) calculated as the slope of the linear part of the curve relating net photosynthetic rate to irradiance. At the time of the highest P_{N sat.} value in course of the growing season (August) (spruce forest: 100, beech forest: 98.7 and ash forest: 85.8 μg CO₂ m⁻² s⁻¹), R_D was in its minimum; in autumn P_{N sat.} reached the lowest values which corresponded to the most intensive R_D. It was found thatMercurialis perennis plants stemming from forest stands with different light conditions did not make use equally of the altering light conditions in the course of the growing season. By the underlying analysis of P(I) curves this rhizomatous perennial herb (geophyte) may be characterized as a shade tolerant species.     

The nitrogen cycle in lodgepole pine forests,southeastern Wyoming

Storage and flux of nitrogen were studied in several contrasting lodgepole pine (Pinus contorta spp.latifolia) forests in southeastern Wyoming. The mineral soil contained most of the N in these ecosystems (range of 315–860 g · m^–2), with aboveground detritus (37.5–48.8g · m^–2) and living biomass (19.5–24.0 g · m^–2) storing much smaller amounts. About 60–70% of the total N in vegetation was aboveground, and N concentrations in plant tissues were unusually low (foliage = 0.7% N), as were N input via wet precipitation (0.25 g · m^–2 · yr^–1), and biological fixation of atmospheric N (<0.03 g · m^–2 · yr^–1, except locally in some stands at low elevations where symbiotic fixation by the leguminous herbLupinus argenteus probably exceeded 0.1 g · m^–2 · yr^–1).Because of low concentrations in litterfall and limited opportunity for leaching, N accumulated in decaying leaves for 6–7 yr following leaf fall. This process represented an annual flux of about 0.5g · m^–2 to the 01 horizon. Only 20% of this flux was provided by throughfall, with the remaining 0.4g · m^–2 · yr^–1 apparently added from layers below. Low mineralization and small amounts of N uptake from the 02 are likely because of minimal rooting in the forest floor (as defined herein) and negligible mineral N (< 0.05 mg · L^–1) in 02 leachate. A critical transport process was solubilization of organic N, mostly fulvic acids. Most of the organic N from the forest floor was retained within the major tree rooting zone (0–40 cm), and mineralization of soil organic N provided NH₄ for tree uptake. Nitrate was at trace levels in soil solutions, and a long lag in nitrification was always observed under disturbed conditions. Total root nitrogen uptake was calculated to be 1.25 gN · m^–2 · yr^–1 with estimated root turnover of 0.37-gN · m^–2 · yr^–1, and the soil horizons appeared to be nearly in balance with respect to N. The high demand for mineralized N and the precipitation of fulvic acid in the mineral soil resulted in minimal deep leaching in most stands (< 0.02 g · m^–2 · yr^–1). These forests provide an extreme example of nitrogen behavior in dry, infertile forests.     

The biology of the ground beetle Harpalus rufipes in a strawberry field in Northumberland

Harpalus rufipes (Degeer) was studied in a strawberry plot in Northumberland from 1973 to 1978 by pitfall trapping, and in the laboratory. Adults were active from April until November. Overwintered male beetles predominated at the beginning of each season until May, followed by overwintered females in June and July. Newly emerged, mainly female, beetles were active from August onwards. Overwintered females matured during early summer and laid eggs in August with a fecundity of 10–15 eggs/female. In the laboratory about 30% of beetles survived from one breeding season to the next. First- and second-instar larvae were caught in pitfall traps in autumn; in the laboratory they made approximately vertical burrows in which they stored seeds taken from the soil surface. Third-instar larvae fed on these seeds and were not active on the surface. Preferred seeds were those of grasses and Chenopodium album L. Larvae were usually aggregated in the soil at densities of 3–20/m².     

Denitrification in the periphyton associated with plant shoots and in the sediment of a wetland system supplied with sewage treatment plant effluent

Seasonal variation in denitrification and major factors controlling this process were determined in sediment, microbial communities attached to plant shoots (periphyton) and in the water of a Phragmites and an Elodea-dominated stand of a constructed wetland system between May 1997 and February 1998. The wetland was supplied with effluent from a sewage treatment plant. The denitrification rate in periphyton on plants shoots (expressed per shoot area) was always considerably higher than in the sediment and varied with the chlorophyll-a content of the periphyton in the course of the year. The algae in the periphyton provided attachment surfaces and probably also organic compounds to the denitrifying bacteria. Decreases in periphyton biomass and denitrification rate in the Phragmites and Elodea-dominated stands during the growing season were associated with enhanced shading by Phragmites shoots or a floating layer of macro-algae and Lemna spp., respectively. Light availability and the denitrification rate of periphyton increased again after the Phragmites shoots were cut in October. Nitrate appeared to limit the denitrification rate in the sediment. Periphyton denitrification rates were mostly lower on Elodea shoots than on Phragmites shoots, in spite of the higher living algal biomass on Elodea shoots. This difference was associated with lower nitrate concentrations in the Elodea-dominated stand. In the two stands, the daily denitrification rates in periphyton on shoots of Phragmites australis (44.4–121 mg N m^–2 stand area d^–1) and Elodea nuttallii (14.8–33.1 mg N m^–2 d^–1) were clearly more important than rates in the sediment (0.5–25.5 mg N m^–2 d^–1) or the water (0.4–3.9 mg N m^–2 d^–1). The presence of few bacteria attachment sites or low organic carbon availability possibly resulted in low denitrification rates in the water. Denitrification appeared to be a major process in nitrate removal from the through-flowing water in this wetland system.     

外源氮输入对黄河口新生湿地植物-土壤系统硫分布特征的影响

选择黄河口北部滨岸高潮滩的碱蓬湿地为研究对象,基于野外原位氮(N)输入模拟试验,研究了不同氮输入梯度下(N0,无氮输入;N1,低氮输入,9.0 gN m~(-2)a~(-1);N2,中氮输入,12.0 gN m~(-2)a~(-1);N3,高氮输入,18.0 gN m~(-2)a~(-1))碱蓬湿地植物-土壤系统全硫(TS)分布特征的差异。结果表明,外源N输入明显改变了湿地土壤TS含量的分布状况。随着N输入量的增加,除表层TS含量变化不明显外,其他土层均呈增加趋势。不同氮输入处理下植物各器官的TS含量整体均表现为叶茎根,叶是硫的主要累积器官。尽管氮输入处理并未改变植被的硫分配格局以及其地上与地下之间的硫养分供给关系,但其为适应不同养分环境可进行自身生长特性及养分分配的调整,且这种调整在N2处理下表现的尤为明显。随氮输入量的增加,不同氮处理下植物-土壤系统的S储量整体呈增加趋势,但土壤S储量的增幅远低于植物亚系统S储量的增幅以及N供给的增幅,说明N、S之间的养分供给存在不同步性。研究发现,未来黄河口N养分负荷增加情况下,碱蓬湿地植物-土壤系统的S生物循环速率不但可能会加速,而且N、S养分之间也可能形成一个正反馈机制,并将有利于维持新生湿地的稳定与健康。     

重庆缙云山两种林分土壤呼吸对模拟氮沉降的季节响应差异性

氮沉降对土壤呼吸的影响仍然存在着争论,需要进一步研究。选择重庆缙云山的马尾松林和柑橘林开展了氮添加实验,分别设置3个氮添加水平(低氮T_5:20 g N m~(-2)a~(-1),中氮T_(10):40 g N m~(-2)a~(-1)和高氮T_(15):60 g N m~(-2)a~(-1))和对照(T_0:0 g N m~(-2)a~(-1))共4个水平的处理,各林分每个处理各9次重复,每个处理量分4次,在每个季度开始各施1次。采用ACE(Automated Soil CO_2 Exchange Station,UK)自动土壤呼吸监测系统测定两林分土壤表层(0—10 cm)的呼吸、温度和湿度,分别在当年的7月、9月、11月、第2年的1月、2月、3月、5月、6月各连续测定4d,每天(8:00—18:00)4次,以揭示两种林分土壤呼吸对模拟氮沉降的季节动态响应及其差异性。结果表明:(1)柑橘林与马尾松林林下土壤表层呼吸表现出一致的季节变化动态趋势:夏季春季秋季冬季,但柑橘林土壤呼吸显著高于马尾松林(P0.05)。(2)总体上氮沉降抑制了2种林分土壤表层呼吸,而N沉降量大抑制程度越高。只在冬季土壤湿度低的马尾松林下氮沉降促进了土壤呼吸。(3)土壤温度与土壤呼吸有极显著的正相关指数关系(P0.01),而土壤水分与土壤呼吸有显著的二次模型拟合关系,但均受到氮沉降量处理的影响。综合分析表明,在亚热带山区2类森林下的典型案例结果支持氮沉降抑制土壤呼吸的认识。     

Influence of floristic composition on the net primary production and dry matter turnover in a tropical grassland

Plant biomass, net primary productivity and dry matter turnover were studied in a grassland situated in a tropical monsoonal climate at Kurukshetra, India (29°58′N, 76°51′E). Based on differences in vegetation in response to microrelief, three stands were distinguished on the study site. The stand I was dominated by Sesbania bispinosa, stand II represented mixed grasses and stand III was dominated by Desmostachya bipinnata. Floristic composition of the three stands revealed the greatest number of species on stand II (75). The study of life form classes indicated a thero-cryptophytic flora. The biomass of live shoots in all the three stands attained a maximum value in September (424–1921 g m^-2) and below ground plant biomass in November (749–1868 g m^-2). The annual above ground net primary production was greatest on stand I (2143 g m^-2) and lowest on stand II (617 g m^-2). The rate of production was highest during the rainy season (15.34 to 3.18 g m^-2 day^-2). Below ground net production ranged from 1592 to 785 g m^-2 y^-2 and the rates were high in winter and summer seasons. Total annual net primary production was estimated to be 3141, 1403, 2493 and 2134 g m^-2 on stands I, II, III and on the grassland as a whole, respectively. The turnover of total plant biomass plus below ground biomass indicated almost a complete replacement of phytomass within the year. The system transfer functions showed greater transfer of material from total net primary production to the shoot compartment during rainy season and to the root compartment during winter and summer seasons.     

Cutting regime affects the amount and allocation of symbiotically fixed N in green manure leys

Cutting strategy effect on N₂ fixation and distribution of fixed N above and below ground in red clover (Trifolium pratense L.) and mixed red clover/perennial ryegrass (Lolium perenne L.) green manure leys was quantified in field experiments including in situ mezotrons and microplots. Symbiotically fixed N in clover, transfer of fixed N to grass in the mixed stands and the fate of ¹⁵N contained in mulch were estimated by isotope dilution. Below ground clover-derived N was estimated by leaf labelling. Total N₂ fixation was estimated by correcting fixed N in plant shoots with plant-derived N below ground and recycled N from mulch. The total N₂ fixation was larger in harvested and mulched stands (average 45 g?m^?2) than in the intact stands (32 g?m^?2). Of the fixed N, 53% (intact), 46% (harvested) and 60% (mulched) was found below ground. The average recycling of N in mulch was 21% and contributed 13.7% (pure clover) and 2.2% (mixed) of the clover N in the regrowth. Recycling of N did not decrease N₂ fixation in the mulched compared with harvested stands. The results indicate that cutting regime should be considered when estimating total amounts of N fixed by green manure leys.     

Macrophyte-related shifts in the nitrogen and phosphorus contents of the different trophic levels in a biomanipulated shallow lake

Lake Zwemlust, a small highly eutrophic lake, was biomanipulated without reducing the external nutrient loading, and the effects were studied for four years. In this paper we pay special attention to the shifts in relative distribution of nitrogen and phosphorus in the different trophic levels and to the changes in growth limitation of the autotrophs.Despite of the high external nutrient loads to the lake (ca 2.4 g P m^–2 y^–1 and 9.6 g N m^–2 y^–1), the effects of biomanipulation on the lake ecosystem were pronounced. Before biomanipulation no submerged vegetation was present in the lake and P and N were stored in the phytoplankton (44% N, 47% P), fish (33% N, 9% P) and in dissolved forms (23% N, 44% P). P and N contents in sediments were not determined. In the spring and summer following the biomanipulation (1987), zooplankton grazing controlled the phytoplankton biomass and about 90% of N and P were present in dissolved form in the water. From 1988 onwards submerged macrophyte stands continue to thrive, reducing the ammonium and nitrate concentrations in the water below detection levels. In July 1989 storage of N and P in the macrophytes reached 86% and 80%, respectively. Elodea nuttallii (Planchon) St.John, the dominant species in 1988 and 1989, acted as sink both for N and P during spring and early summer, withdrawing up to ca 60% of its N and P content from the sediment. At the end of the year only part of the N and P from the decayed macrophytes (ca 30% of N and 60% of P) was recovered in the water phase of the ecosystem (chiefly in dissolved forms). The rest remained in the sediment, although some N may have been released from the lake by denitrification.In summer 1990 only 30% of the N and P was found in the macrophytes (dominant species Ceratophyllum demersum L.), while ca 30% of N and P was again stored in phytoplankton and fish.     

三峡三期蓄水后长江口海域浮游动物群落特征及影响因子

根据2010年8月、11月以及2011年5月3个航次、各次24个监测点的调查数据,分析了三峡工程三期蓄水后一个水文年内长江口浮游动物优势种、湿重生物量及丰度的变化,并用BIOENV筛选出影响浮游动物分布的关键环境因子。结果表明:长江口浮游动物春季绝对优势种为夜光虫(Noctiluca scientillans)与中华哲水蚤(Calanus sinicus),夏季绝对优势种为太平洋纺锤水蚤(Acartia pacifica steuer),秋季绝对优势种为针刺拟哲水蚤(Paracalanus aculeatus);浮游动物湿重生物量夏季(970.6 mg/m~3)秋季(613.8 mg/m~3)春季(571.5 mg/m~3),丰度夏季(783.5个/m~3)春季(691.3个/m~3)秋季(399.5个/m~3);影响浮游动物分布的关键环境因子为底层盐度、底层温度及底层硅酸盐。