洱海入湖河流弥苴河下游氮磷季节性变化特征及主要影响因素

入湖河流携带污染物对洱海水环境的影响日益明显,对洱海入湖水量最大的河流——弥苴河下游水体氮磷进行了连续采样分析,以期为河口湿地建设和水质改善提供基础数据.结果表明:1)弥苴河水质介于地表水Ⅲ-Ⅴ类之间,主要污染物为氮和磷,其中总氮平均浓度为1.17 mg/L,最高浓度达到2.00 mg/L;总磷平均浓度为0.06 mg/L;2)弥苴河下游总氮、总磷浓度丰水期高于枯水期,并呈现出季节性变化规律;3)弥苴河下游水体总氮、总磷年均浓度远高于洱海水体总氮、总磷年均浓度,其中总氮高出2.10倍,总磷高出2.90倍;4)弥苴河下游河段非点源污染占据主导地位.     

洱海底泥特性对七种沉水植物生长的影响

为研究洱海底泥特性对沉水植物生长的影响，采用不同比例洱海底泥与湖岸土壤掺混形成五种基质，并分别移栽苦草、黑藻、微齿眼子菜、马来眼子菜、光叶眼子菜、穿叶眼子菜和狐尾藻，进行为期70d的室外生长实验，结果表明不同基质对几种植物的影响具种间差异。（1）在基质为50%深层底泥+50%湖岸土壤（碳、氮、磷含量分别为31.59、0.334和0.095 mg/g）时，苦草、马来眼子菜和光叶眼子菜的株高最大；基质为100%深层底泥（碳、氮、磷含量分别为37.88、0.803和0.149 mg/g）时，黑藻、微齿眼子菜、穿叶眼子菜和狐尾藻的株高最大；（2）基质为100%深层底泥时，苦草、黑藻、微齿眼子菜、马来眼子菜和光叶眼子菜生物量增加最多且相对生长速率最大；基质为100%浅层底泥（碳、氮、磷含量分别为77.37、5.691和0.136 mg/g）时，穿叶眼子菜生物量增加最多，相对生长速率最大；狐尾藻在基质为50%浅层底泥+50%深层底泥（碳、氮、磷含量分别为49.27、2.005和0.131 mg/g）时生物量增加最多，相对生长速率最大；（3）基质为100%湖岸土壤（碳、氮、磷含量分别为22.06、0.327和0.231 mg/g）时，7种沉水植物均生长缓慢，生物量增加较少。综上所述，中营养底泥（碳、氮、磷含量分别为31.59-49.27、0.334-2.005和0.095-0.131 mg/g）更适合沉水植物生长，底泥中过高或过低营养都不利于沉水植物生长。     

西太湖河网区恢复与退化河岸带湿地生态及水环境功能比较

对西太湖平原河网区严重退化的河岸带湿地进行恢复的基础上,开展了恢复后湿地和退化湿地内植物群落物种多样性,生物量,植物干物质体内氮、磷含量,湿地水体中悬浮物含量、氮、磷营养物质浓度以及恢复后湿地内反硝化作用等湿地生态和水环境功能方面的比较研究。结果表明:(1)已恢复湿地群落结构趋于完整,物种多样性指数值较高,一般在1.7～3之间,均匀度在0.5～0.9之间波动。退化湿地物种丰富度低,多样性指数值较低,分别在0.3～1.5(H′),0.15～0.65(J)之间波动。在水花生(Alternanthera philoxeroides)入侵的群落内,种类稀少,且分布极不均匀,仅在0.3～1(H′)和0.1～0.3(J)之间波动,群落结构严重退化。(2)在植物生长旺盛期,1m2湿地内的芦苇(Phragmites communis)、香蒲(Typha orientalis)及茭草(Zizania latifolia)分别为42,18株和17株。其在湿地内的生物量分别为:4692,5142kg和2182kg;(3)上述物种单位干物质中的氮、磷平均含量分别达到2.88mg/g和2.09mg/g;沉水植物作为滨岸带湿地群落结构的重要组成部分,不仅具有高的生物量,而且吸收氮磷能力强,单位干物质氮、磷含量分别达到7.27mg/g和4.14mg/g;(4)植物对水体及沉积物中可溶性氮、磷的有效吸收以及颗粒态磷的自然沉降作用,使得上游来水中的总氮浓度流经湿地时,降至0.15～0.89mg/L之间,平均下降了85%;总磷浓度则由进水时的0.248～0.598mg/L,降至出水时的0.002～0.083mg/L;(5)滨岸带湿地对河水中悬浮物的有效拦截、滞留和吸附作用,使得入湖河水中的悬浮物含量降低了90%以上;(6)在高温缺氧的环境中,滨岸带湿地表现出较强的反硝化作用,且由河心向河岸逐渐增强的趋势。近河心处测定的N2O通量为0.034～0.068之间,到河岸处升至0.046～0.089。反硝化作用是削减水体中氮负荷的有效途径。     

水生植物腐烂分解对水质的影响

对6种水生植物进行64d的腐烂分解试验,对比不同水生植物腐烂分解过程中水体营养盐浓度的变化.结果表明:6种水生植物的腐烂分解速率差别较大,浮叶植物分解速度最快,沉水植物次之,挺水植物最慢.不同水生植物腐解过程对水质影响不同,并与植物生物量密度相关.挺水植物芦苇腐解过程中的水体化学需氧量、总氮和总磷浓度最低;在茭草分解后期,水体化学需氧量和总氮浓度上升,水质变差.浮叶植物荇菜和莲腐解过程中,水体化学需氧量和总氮浓度高于其他植物.沉水植物菹草和狐尾藻腐解过程中,水体铵态氮、硝态氮和总磷浓度最高.对于同一种植物,不同生物量密度处理下,主要水质指标变化趋势相似.适量的植物残体的存在可以有效促进水体氮、磷等营养元素的循环,一定程度上去除硝态氮,降低水体氮负荷.     

鄱阳湖湿地两种优势植物叶片C、N、P动态特征

2011年2—6月在鄱阳湖南矶湿地国家级自然保护区逐月测定了灰化苔草(Carex cinerascens)、南荻(Triarrhena lutarioriparia)叶片C、N、P含量及其地上生物量,以阐明鄱阳湖湿地优势植物C、N、P含量及化学计量比动态特征与控制因子,探讨湿地养分利用与限制状况。结果表明：1）两种优势植物叶有机碳含量变化范围分别为365.3—386.6 mg/g和352.6—393.2 mg/g,平均值(?标准差)分别为(375.5?17.4) mg/g和(371.7?12.5) mg/g;叶N含量分别为6.96—17.59 mg/g和5.50—20.68 mg/g,平均值分别为(11.35?1.40) mg/g和(11.54?0.84) mg/g;叶P含量变化范围为0.65—2.14 mg/g和0.57—2.25 mg/g,平均含量为(1.56?0.69) mg/g和(1.55?0.68) mg/g。两种植物C:N、C:P、N:P平均值分别为37.65、413.60、9.62和41.05、410.29、9.57,C、N、P及其化学计量比种间差异不显著(P>0.05)。2）气温与地上生物量是N、P及其化学计量比季节变化的主要控制因子,气温和生物量对两种优势植物叶片氮、磷含量的影响要高于对叶有机碳含量的影响。3）植物C:N、C:P与地上生物量变化趋势基本一致,显示N、P养分利用效率随植物的快速生长而提高;根据两种优势植物及土壤N、P含量与化学计量比来判断,研究区植物更多地受氮限制。     

广西澄江喀斯特湿地沉水植物碳、氮、磷化学计量特征

沉水植物是水生生态系统中重要的初级生产者。当前有关沉水植物生态化学计量特征的研究主要集中在非喀斯特区,而在喀斯特区的相关研究较为缺乏。因此,以广西澄江喀斯特典型湿地中的7种沉水植物为研究对象,分析沉水植物地上部分及底泥的碳（C）、氮（N）、磷（P）化学计量特征。结果显示,（1）7种沉水植物地上部分总碳、总氮和总磷平均含量最大值均出现在小茨藻（Najas minor）分别为（325.4±5.01） g/kg、（33.07±1.59） g/kg和（3.79±0.16） g/kg;C ： N、C ： P、N ： P平均值分别为10.14±0.18、96.23±3.56和9.47±0.32,C ： N最大值（11.89±0.54）出现在苦草（Vallisneria natans）,C ： P和N ： P最大值（113.27±18.14和11.13±1.63）均出现在穗花狐尾藻（Myriophyllum spicatum）。（2）底泥有机碳、总氮、总磷、碱解氮和速效磷含量平均值分别为（15.05±0.56） g/kg、（2.06±0.08） g/kg、（0.58±0.01） g/kg、（162.53±9.16） mg/kg和（21.73±0.86） mg/kg,有机碳、总氮、总磷、碱解氮和速效磷的平均含量最大值均出现在小茨藻（N.minor）分别为（18.54±1.04） g/kg、（2.55±0.25） g/kg、（0.66±0.03） g/kg、（214.82±32.05） mg/kg和（26.37±3.31） mg/kg;底泥的C ： N、C ： P和N ： P平均值分别是7.33±0.14、25.7±0.72和3.53±0.09,C ： N、C ： P和N ： P最大值分别出现在金鱼藻（Ceratophyllum demersum）（7.45±0.32）、小茨藻（N. minor）（28.29±1.29）和黑藻（Hydrilla verticillata）（3.89±0.25）。（3）沉水植物的地上部分总碳与底泥的有机碳和总氮均呈显著负相关性;沉水植物的地上部分总氮与底泥有机碳、碱解氮、C ： P均呈显著正相关性;沉水植物地上部分C ： N、C ： P均与底泥有机碳、总氮、总磷、碱解氮、C ： P、N ： P呈显著负相关,表明不同喀斯特湿地沉水植物和底泥之间的养分耦联性不同。本研究为喀斯特湿地生态系统生态化学计量学研究提供理论依据。     

天鹅湖湿地微生物群落PCR-DGGE分析

为了解天鹅湖湿地微生物群落结构组成及动态变化,本研究对不同季节环境因子进行了检测,并采用PCR-DGGE技术分析了天鹅湖湿地青龙湖位点不同季节微生物群落结构多样性。环境因子检测结果表明:天鹅湖湿地p H值介于6.76~6.89之间,总氮含量范围在3.4~5.8 mg/kg,9月总氮含量最高,达到5.8 mg/kg,12月含量最低,为3.4 mg/kg。总磷含量差异不大,范围介于0.14~0.19 mg/kg,其中9月份相对较高,达到0.19 mg/kg。PCR-DGGE结果显示:3个季节样品中微生物群落结构较为健全,物种丰富度高,群落结构相似性在76.9%~89.3%之间,季节间微生物群落结构存在差异。DGGE条带测序显示湿地优势类群与α-变形菌门(Alpha Proteobacterium)相似性最高,达到98%。     

富营养废水中碳氮磷浓度对布朗葡萄藻总脂和总烃的影响

以经过二次过滤的富营养化鱼塘养殖污水为培养液,添加外源的碳、氮、磷元索,研究了污水中不同的外源无机碳、总氮和总磷浓度对布朗葡萄藻(Botryococcus braunii)生物量、总脂和总烃含量的影响.结果表明:(1)以NaHCO3作为碳源,布朗葡萄藻的生物量和总脂含量在外源无机碳浓度为5～10 mg/L时最高,总烃含量在外源无机碳浓度为15mg/L时最高.(2)以KNO3作为氮源,布朗葡萄藻的生物量在总氮浓度为15mg/L时最高,总脂含量在总氮浓度为2mg/L时最高,总烃含量在总氮浓度为20mg/L时最高.(3)以KH2 PO4作为磷源,布朗葡萄藻生物量在总磷浓度为2mg/L时最高,总脂含量和总烃含量在总磷浓度为1.5 mng/L时最高.     

抚仙湖不同类型岸带沉水植物分布及水体氮磷特征

2005年6-7月,通过野外断面采样并结合水质分析,对抚仙湖不同类型岸带沉水植物分布及水体氮磷特征进行了调查.结果表明:抚仙湖岸带分为4个类型,岩石陡峭岸带、冲积平缓岸带、河口岸带以及湖湾岸带;沉水植物种类分布最多出现在河口型岸带和湖湾岸带,均为9种;最低出现在岩石陡峭岸带,为5种;沉水植物生物量最高出现在冲积平缓岸带,为8 300 g·m-2,最低出现在岩石陡峭岸带,为2416 g·m-2;沉水植物分布最深为A1岩石陡峭岸带,平均水深8.5 m,最深达到11.0 m;最浅为B2冲积平缓岸带,平均水深1.9 m,最深为6 m;岸带水体综合氮磷指标,以D2湖湾岸带最高,总氮、总磷分别达到5.34和0.145 mg·L-;最低为A2岩石陡峭岸带,总氮、总磷分别为0.87和0.015 mg·L-1抚仙湖沉水植物对水体氮磷的吸收固定总量约为总氮为5.28×104 kg,总磷为7 500 kg.     

5种湿地植物对生活污水净化效果研究

选择芦苇、水葱、千屈菜、扁秆藨草、长苞香蒲5种常见湿地植物分别构建人工湿地小试系统,测试它们对生活污水的净化作用,分析污水净化过程中氮磷元素在植物体内和土壤中的动态分布,以明确各湿地植物对污水的综合净化能力。结果表明:（1）5种湿地植物对生活污水中的CODcr、铵态氮、总氮和总磷的去除效果显著高于无植物对照;随处理时间延续,污水中CODcr、铵态氮、总氮和总磷浓度均呈下降趋势,定植土壤中的总氮和总磷含量呈先上升后下降的变化趋势。（2）5种植物能够吸收污水中89.7%～97.9%的磷元素,污水中剩余的磷元素量较少（2.1%～10.3%）;污水中氮元素在植物体、污水和土壤中分布因植物种类不同有很大差别,芦苇、水葱和长苞香蒲可吸收污水中76.1%～83.4%的大部分氮元素,而千屈菜和扁秆藨草对氮元素吸收量较少（分别为45.0%和46.8%）,在污水和土壤中滞留的氮元素较多（21.7%～31.6%）。（3）5种植物对氮元素的积累能力显著大于对磷元素的积累能力;5种湿地植物对生活污水的综合净化能力差异较大,从强到弱依次为芦苇＞长苞香蒲＞水葱＞扁秆藨草＞千屈菜。     

Nutrient concentration ratios and co-limitation in South African grasslands

*Assessing plant nutrient limitation is a fundamental part of understanding grassland dynamics. The ratio of concentrations of nitrogen (N) and phosphorus (P) in vegetation has been proposed as an index of the relative limitation of biomass production by N and P, but its utility has not been tested well in grasslands. *At five sites in Kruger National Park, South Africa, across soil and precipitation contrasts, N and P were added in a factorial design to grass-dominated plots. *Although the N:P ratio of unfertilized vegetation across all sites (5.8) would have indicated that production was N-limited, aboveground production was consistently co-limited by N and P. Aboveground production was still greater in plots fertilized with N and P than in those fertilized with just N, but the N:P ratio did not exceed standard thresholds for P limitation in N-fertilized vegetation. Comparisons among sites showed little pattern between site N:P ratio and relative responses to N and P. *When combined with results from other grassland fertilization studies, these data suggest that the N:P ratio of grasses has little ability to predict limitation in upland grasslands. Co-limitation between N and P appears to be much more widespread than would be predicted from simple assumptions of vegetative N:P ratios.     

C:N:P stoichiometry in soil: is there a “Redfield ratio” for the microbial biomass?

Well-constrained carbon:nitrogen:phosphorus (C:N:P) ratios in planktonic biomass, and their importance in advancing our understanding of biological processes and nutrient cycling in marine ecosystems, has motivated ecologists to search for similar patterns in terrestrial ecosystems. Recent analyses indicate the existence of “Redfield-like” ratios in plants, and such data may provide insight into the nature of nutrient limitation in terrestrial ecosystems. We searched for analogous patterns in the soil and the soil microbial biomass by conducting a review of the literature. Although soil is characterized by high biological diversity, structural complexity and spatial heterogeneity, we found remarkably consistent C:N:P ratios in both total soil pools and the soil microbial biomass. Our analysis indicates that, similar to marine phytoplankton, element concentrations of individual phylogenetic groups within the soil microbial community may vary, but on average, atomic C:N:P ratios in both the soil (186:13:1) and the soil microbial biomass (60:7:1) are well-constrained at the global scale. We did see significant variation in soil and microbial element ratios between vegetation types (i.e., forest versus grassland), but in most cases, the similarities in soil and microbial element ratios among sites and across large scales were more apparent than the differences. Consistent microbial biomass element ratios, combined with data linking specific patterns of microbial element stoichiometry with direct evidence of microbial nutrient limitation, suggest that measuring the proportions of C, N and P in the microbial biomass may represent another useful tool for assessing nutrient limitation of ecosystem processes in terrestrial ecosystems.     

Relationship between the Relative Limitation and Resorption Efficiency of Nitrogen vs Phosphorus in Woody Plants

Most previous studies have ascribed variations in the resorption of a certain plant nutrient to its corresponding environmental availability or level in tissues, regardless of the other nutrients’ status. However, given that plant growth relies on both sufficient and balanced nutrient supply, the nutrient resorption process should not only be related to the absolute nutrient status, but also be regulated by the relative limitation of the nutrient. Here, based on a global woody-plants dataset from literature, we test the hypothesis that plants resorb proportionately more nitrogen (or phosphorus) when they are nitrogen (or phosphorus) limited, or similar proportions of nitrogen (N) and phosphorus (P) when co-limited by both nutrients (the relative resorption hypothesis). Using the N:P ratio in green foliage as an indicator of nutrient limitation, we found an inverse relationship between the difference in the proportionate resorption of N vs P and this foliar N:P ratio, consistent across species, growth-forms, and vegetation-types globally. Moreover, according to the relative resorption hypothesis, communities with higher/lower foliar N:P (more likely P/N limited) tend to produce litter with disproportionately higher/lower N:P, causing a worsening status of P/N availability; this positive feedback may somehow be counteracted by several negative-feedback mechanisms. Compared to N, P generally shows higher variability in resorption efficiency (proportion resorbed), and higher resorption sensitivity to nutrient availability, implying that the resorption of P seems more important for plant nutrient conservation and N:P stoichiometry. Our findings elucidate the nutrient limitation effects on resorption efficiency in woody plants at the global scale, and thus can improve the understanding of nutrient resorption process in plants. This study also suggests the importance of the foliar N:P ratio as a key parameter for biogeochemical modeling, and the relative resorption hypothesis used to deduce the critical (optimal) N:P ratio for a specific plant community.     

Nutrient limitation in species‐rich Calthion grasslands in relation to opportunities for restoration in a peat meadow landscape

Questions: Which nutrient(s) limit(s) vegetation productivity in Calthion grasslands? Is phosphorus release a bottleneck for restoration of species‐rich Calthion grasslands on rewetted dairy meadows? Location: Three species‐rich Calthion grasslands in the Western Peat District in the Netherlands. Methods: We conducted a field fertilization experiment with nitrogen (N), phosphorus (P) and potassium (K) in three existing Calthion grasslands to evaluate the potential for restoration on rewetted dairy meadows. Responses of above‐ground biomass, tissue nutrient concentrations and nutrient ratios were determined after 2 yr of fertilization. Results: Biomass increased with fertilization with N‐only and K‐only but did not react to P‐only additions. Comparisons of tissue nutrient concentrations and nutrient ratios also gave indications of N and K limitation. Conclusions: The strong P release expected after rewetting should not necessarily interfere with restoration of Calthion communities on rewetted dairy meadows. It is concluded that for successful restoration management measures should focus on reducing N and/or K availability. Potassium might be an overlooked bottleneck in the restoration of species‐rich grasslands.     

Variation in nitrogen and phosphorus concentrations of wetland plants

The use of nutrient concentrations in plant biomass as easily measured indicators of nutrient availability and limitation has been the subject of a controversial debate. In particular, it has been questioned whether nutrient concentrations are mainly species' traits or mainly determined by nutrient availability, and whether plant species have similar or different relative nutrient requirements. This review examines how nitrogen and phosphorus concentration and the N:P ratio in wetland plants vary among species and sites, and how they are related to nutrient availability and limitation. We analyse data from field studies in European non-forested wetlands, from fertilisation experiments in these communities and from growth experiments with wetland plants. Overall, the P concentration was more variable than the N concentration, while variation in N:P ratios was intermediate. Field data showed that the N concentration varies more among species than among sites, whereas the N:P ratio varies more among sites than among species, and the P concentration varies similarly among both. Similar patterns of variation were found in fertilisation experiments and in growth experiments under controlled nutrient supply. Nutrient concentrations and N:P ratios in the vegetation were poorly correlated with various measures of nutrient availability in soil, but they clearly responded to fertilisation in the field and to nutrient supply in growth experiments. In these experiments, biomass N:P ratios ranged from 3 to 40 and primarily reflected the relative availabilities of N and P, although N:P ratios of plants grown at the same nutrient supply could vary three-fold among species. The effects of fertilisation with N or P on the biomass production of wetland vegetation were well related to the N:P ratios of the vegetation in unfertilised plots, but not to N or P concentrations, which supports the idea that N:P ratios, rather than N or P concentrations, indicate the type of nutrient limitation. However, other limiting or stressing factors may influence N:P ratios, and the responses of individual plant species to fertilisation cannot be predicted from their N:P ratios. Therefore, N:P ratios should only be used to assess which nutrient limits the biomass production at the vegetation level and only when factors other than N or P are unlikely to be limiting.     

Biomass production of the last remaining fen with Saxifraga hirculus in Switzerland is controlled by nitrogen availability

Olde Venterink H. and Vittoz P. 2008. Biomass production of the last remaining fen with Saxifraga hirculus in Switzerland is controlled by nitrogen availability. Bot. Helv. 118: 165 – 174. For conservation management of endangered plants it is important to know which nutrient(s) control growth of the vegetation, because maintenance of low nitrogen (N), phosphorus (P) or potassium (K) availability requires different management measures. The aim of this study was to determine the type of nutrient limitation for the vegetation in the last remaining site with Saxifraga hirculus in Switzerland, using nutrient ratios in the aboveground vegetation as an indicator. We made vegetation relevees, collected biomass of the vascular plants, and took soil samples in three plots at this site. The biomass was very low (152–231 g m ^-2), and all three plots were clearly N-limited with N:P ratios of 7– 8. Soil extractable N concentrations were generally low, and P and K concentrations were moderate to high, which was consistent with the indicated N limitation. Hence conservation management first of all needs to prevent N-enrichment, and needs to avoid increased mineralization rates through drainage, or the accumulation of N in the system from atmospheric deposition. Therefore N output seems required through for instance grazing or mowing. The current grazing management seems to function well, since total aboveground biomass is very low and S. hirculus has a high abundance in this last remnant. Submitted 5 June 2008; Accepted 14 October 2008 Subject editor: Sonja Wipf     

Plant responses to nutrient addition and predictive ability of vegetation N:P ratio in an austral fen

1. Previous studies of the N:P ratio in wetland plants have been carried out in northern hemisphere wetlands where atmospheric nitrogen deposition is higher. There is little research on foliar N:P ratio as a potential indicator of nutrient limitation in vegetation communities in southern hemisphere wetlands. This study aimed to redress this knowledge gap and answer the following questions: how well does the plant tissue nitrogen to phosphorus (N:P) ratio predict wetland plant community nutrient limitation, as indicated by vegetation standing stocks and below-ground biomass, in southern hemisphere fens? Secondly, what are the impacts of realistic upper levels of farm nutrient run-off on natural montane fen vegetation?
2. Low (35 kg ha⁻¹ year⁻¹) and high (70 kg ha⁻¹ year⁻¹) levels of nitrate-N or ammonium-N with and without P (20 kg ha⁻¹ year⁻¹) were added to 81 vegetation plots over a period of 2.75 years. Species composition, plant nutrient status, and above-ground live vegetation standing stocks were assessed after 3 years, and below-ground biomass after 2 years.
3. Plant tissue analysis suggested the community was N limited or N and P co-limited; we found greater standing stocks of vegetation in plots treated with 70 kg ha⁻¹ year⁻¹ ammonium-N, indicating N limitation. No difference between other treatments was found in above-ground standing stocks or below-ground biomass. Plant species cover increased in both high N treatments, consistent with N limitation. These changes in plant species cover were accompanied by significant decreases in species richness in both high N treatments. Native species dominated the vegetation and this was unaffected by nutrient addition (90% cover).
4. This is one of the first studies to test and find support for the N:P ratio in southern hemisphere wetlands. Observed declines in species richness after N fertilisation in an N-limited fen suggests increased N may pose risks to austral wetlands. Responses by plant communities (changes in composition, biomass) to lower levels of nutrient addition may require longer periods of fertilisation to be apparent in slow growing ecosystems.

     

Water pollution control by aquatic vegetation of treatment wetlands

Supplying polluted river water to nature reserves in The Netherlands often leads to eutrophication of the reserve. The eutrophication can be caused directly by the high nutrient input (external eutrophication) or indirectly by altering nutrient availability due to changes in nutrient desorption or mineralization. This paper investigates the potential of a ditch system that is tested for its potential to improve the water quality of polluted river water prior to supplying to the wet meadow reserve De Meije in The Netherlands. Concentrations of the macro-ions chloride, sulphate, calcium and bicarbonate in the polluted river water were much higher than original background values, measured in the reserve. During transport of the river water through the ditch system, no decline was observed in the concentrations of these macro-ions. The phosphorus concentration, however, decreased along the flow path and was significantly negatively correlated with the distance from the inlet point. High phosphorus removal occurred in a stretch of the ditch system where submerged and free floating species such as Fontinalis antipyretica and Lemna trisulca were dominant. The N: P ratio of F. antipyretica was especially low (N : P < 5) at sampling stations where high phosphorus concentrations were measured. The high N: P ratio indicated a luxury consumption of phosphorus. With decreasing phosphorus concentrations, the N: P ratio of F. antipyretica increased to a maximum of N: P = 25. The nutrient budget of the ditch system showed that supply of river water was the main input of phosphorus (12 kg P) whereas the main inputs of nitrogen of the ditch system were atmospheric deposition (66 kg N) and leaching from the wet meadows (44 kg N). For both nutrients, harvesting the aquatic vegetation in September was the main removal mechanism from the ditch system with 92 kg of nitrogen (80% of the annual input N) and 14 kg of phosphorus (95% of the annual P input) removed. It was concluded that the ditch system with aquatic vegetation could successfully remove nutrients from polluted river water. The concentrations of macro-ions, however, are not influenced by the ditch systems and internal eutrophication due to changes in adsorption or mineralization may still occur.     

Plant nitrogen and phosphorus limitation in 98 North American grassland soils

The availability of nutrients is a critical determinant of ecological dynamics in grasslands, but the relationships between soil resource availability and nutrient limitation across ecosystems are not clear. To better understand how soil nutrient availability determines nutrient limitation in vegetation, we grew the same species of grass (Schizachyrium scoparium) in 98 North American grassland soils and fertilized them factorially with nitrogen (N) and phosphorus (P). On average adding N, P, and the two nutrients together increased biomass relative to unfertilized plants by 81%, 22%, and 131%, respectively. Plants grown on low-P soils were not primarily limited by P. Instead, these plants were colimited by N and P, while plants grown on high-P soils were primarily limited by N and only secondarily limited by P. Limitation was not predicted by total soil N. The preponderance of colimitation between N and P on low-P soils suggests that low P availability alters the N cycle to constrain supplies to plants such that N and P are made available in proportion to their demand by plants.     

Interaction Effects of Ambient UV Radiation and Nutrient Limitation on the Toxic Cyanobacterium <Emphasis Type="Italic">Nodularia spumigena</Emphasis>

Nodularia spumigena is one of the dominating species during the extensive cyanobacterial blooms in the Baltic Sea. The blooms coincide with strong light, stable stratification, low ratios of dissolved inorganic nitrogen, and dissolved inorganic phosphorus. The ability of nitrogen fixation, a high tolerance to phosphorus starvation, and different photo-protective strategies (production of mycosporine-like amino acids, MAAs) may give N. spumigena a competitive advantage over other phytoplankton during the blooms. To elucidate the interactive effects of ambient UV radiation and nutrient limitation on the performance of N. spumigena, an outdoor experiment was designed. Two radiation treatments photosynthetic active radiation (PAR) and PAR +UV-A + UV-B (PAB) and three nutrient treatments were established: nutrient replete (NP), nitrogen limited (−N), and phosphorus limited (−P). Variables measured were specific growth rate, heterocyst frequency, cell volume, cell concentrations of MAAs, photosynthetic pigments, particulate carbon (POC), particulate nitrogen (PON), and particulate phosphorus (POP). Ratios of particulate organic matter were calculated: POC/PON, POC/POP, and PON/POP. There was no interactive effect between radiation and nutrient limitation on the specific growth rate of N. spumigena, but there was an overall effect of phosphorus limitation on the variables measured. Interaction effects were observed for some variables; cell size (larger cells in −P PAB compared to other treatments) and the carotenoid canthaxanthin (highest concentration in −N PAR). In addition, significantly less POC and PON (mol cell⁻¹) were found in −P PAR compared to −P PAB, and the opposite radiation effect was observed in −N. Our study shows that despite interactive effects on some of the variables studied, N. spumigena tolerate high ambient UVR also under nutrient limiting conditions and maintain positive growth rate even under severe phosphorus limitation.