Influence of Potamogeton crispus growth on nutrients in the sediment and water of Lake Tangxunhu

An incubation experiment was performed on Potamogeton crispus (P. crispus) using sediment collected from Lake Tangxunhu in the center of China, in order to determine the effects of plant growth on Fe, Si, Cu, Zn, Mn, Mg, P, and Ca concentrations in the sediments and overlying waters. After 3 months of incubation, Ca, Mg, and Si concentrations in the water column were significantly lower, and P and Cu concentrations were significantly higher than in unplanted controls. The effect of P. crispus growth on sediment pore waters and water-extractable elements varied. Concentrations of Ca, Mg, Si, Fe, Cu, and Zn were significantly higher, and P was significantly lower, than in pore waters of the control. Water-extracted concentrations of Fe, Mg, and Si in the sediments were lower, and P was higher, than in the control. Presence of P. crispus generally enhanced concentration gradients of elements between pore waters and overlying waters but not for P. The growth of P. crispus was associated with an increase in water pH and formation of root plaques, resulting in complex effects on the sediment nutritional status.

Phytoplankton biomass in relation to nutrients and zooplankton in thirty subtropical lakes adjacent to the Yangtze River, China

A total of 30 shallow lakes, located along the middle and lower reaches of the Yangtze River, were studied to assess the relative importance of nutrients and zooplankton biomass in determining the phytoplankton biomass in subtropical China. Zooplankton biomass and nutrients both varied greatly in these lakes. Factor analysis and multiple linear regression showed that phytoplankton biomass was positively correlated with TN, NH4+, NO3- and TP, while it did not show any negative relationship to zooplankton biomass. Meanwhile, the phytoplankton biomass showed contrary relationships to the mass ratio of TN/TP in spring and summer, suggesting that in nutrient-richer lakes the dominant phytoplankton species have different preferences for TN/TP ratio. The insignificant top-down control of phytoplankton biomass may be attributed to the dominance of small-sized crustaceans and low crustacean biomass resulting from cyanobacterial dominance and planktivorous fish predation as well as other factors. Thus, it is likely that nutrients were more important than zooplankton biomass in explaining the total variance of phytoplaDkton biomass in these subtropical lakes.

Daily dynamics of nutrients and chlorophyll a during a spring phytoplankton bloom in Xiangxi Bay of the Three Gorges Reservoir

We studied the daily dynamics of nutrients (total phosphorus [TP], total nitrogen [TN], and dissolved silicate [SiO2]) and chlorophyll a (chl a) during a spring bloom in Xiangxi Bay of the Three Gorges Reservoir in year 2005. According to the daily dynamics of chl a, the bloom occurred in two stages (23 February-25 March and 26 March-28 April). The concentration of SiO2 decreased at different layers of the water column with the development of the bloom. However, the decrease of SiO2 in the layers with high concentration of chl a was more dramatic than in the layers with low concentration of chl a. The concentration of TP was lowest value a few days after the peak of chl a during the first bloom period, and the lowest value of TN was found a few days after the peak of chl a during the second bloom period. Correlative analyses indicated that SiO2 and TP were limiting factors in the first bloom period, and SiO2 and TN were limiting factors in the second bloom period.

Dried field populations of Nostoc flagelliforme (Cyanophyceae) require exogenous nutrients for their photosynthetic recovery

The effects of nutrients on the photosynthetic recovery of Nostoc flagelliforme during re-hydration were investigated in order to see if their addition was necessary. Net photosynthesis was negligible in distilled water without nutrient-enrichment. Addition of K+ resulted in significant enhancement of net photosynthesis, whereas other nutrients (Fe3+, Mg2+, Na+, NO3-, PO43-, Cl-) and trace-metals (A(5)) showed little effect. The recovered net photosynthetic activity increased with the increased K+, and reached the maximum at concentrations above 230 mu M. Desiccation and re-hydration did not affect the dependence of photosynthetic recovery on K+. It was concluded that dried field populations of N. flagelliforme require exogenous addition of potassium for photosynthetic recovery and that growth may be potassium-limited in nature.

ROLE OF PHOSPHORUS CYCLING IN ALGAL METABOLISM AND ALGAL SUCCESSION IN LAKE DONGHU, CHINA

The role of phosphorus cycling in algal metabolism was studied in a shallow lake, Donghu, in Wuhan using the methods of measuring cell quota C, N and P, and calculating nutrients uptake rate by algal photosynthesis. The mean daily phosphorus uptake rate of phytoplankton varied between 0.04-0.11 and 0.027-0.053 g/m2/d in station I and station II respectively. The turnover time of phosphorus in phytoplankton metabolism ranged from 0.75-5.0 days during 1979-1986. The available P was 0.176 (+/- 0.156) g/m3 (mean +/- SD) in 1982 and 0.591 (+/- 0.24) g/m3 in 1986. The relationship between P/B ratio (Y) and TP (X: mg/l) was described by the following regression equation Y = 1.163 + 0.512logX (r = 0.731, P < 0.001). The dynamics of algal biomass and algal species succession were monitored as the indicators of environmental enrichment. The small-sized algae have replaced the blue-green algae as the dominant species during 1979-1986. The small-sized algae include Merismopedia glauca, Cryptomonas ovata, Cryptomonas erosa, several species Cyclotella. There has been drastic decrease in algal biomass and an obvious increase in P/B ratio. A nutrient competition hypothesis is proposed to explain the reason of the disappearance of blue-green algae bloom. The drastic change in algal size and the results in high P/B ratio (reaching a maximum mean daily ratio of 1.09 in 1986) may suggest a transition of algal species from K-selection to r-selection in Lake Donghu.

Competition between band gap and yellow luminescence in undoped GaN grown by MOVPE on sapphire substrate

Undoped GaN epilayer on c-face (0 0 0 1) sapphire substrate has been grown by metalorganic vapor-phase epitaxy (MOVPE) in a horizontal-type low-pressure two-channel reactor. Photoluminescence (PL) as a function of temperature and excitation intensity have been systematically studied, and the competition between near band gap ultraviolet (UV) and defect-related yellow luminescence (YL) has been extensively investigated, It is revealed that the ratio of the UV-to-YL peak intensities depends strongly on the excitation intensity and the measurement temperature. The obtained results have been analyzed in comparison with the theoretical predications based on a bimolecular model. (C) 2001 Elsevier Science B.V. All rights reserved.

水分梯度上不同灌木生长对干旱河谷土壤理化和生物学性质的影响

岷江上游干旱河谷区水土流失强烈，地质灾害频繁，生态环境十分脆弱，而土壤条件恶劣（水分不足和养分缺乏）是阻碍该区植被恢复的关键因子，因此研究水分和乡土灌木生长对土壤的影响对该区的生态恢复具有指导意义。本文通过定点模拟实验，选取三种优势豆科灌木为研究对象，分别是白刺花（Sophora davidii）、小马鞍羊蹄甲（Bauhinia faberi var. microphylla）和小雀花（Campylotropics polyantha），设置5 个水分梯度，分别为100%、80%、60%、40%和20%田间持水量（FC），对栽种植物与不种植物下土壤理化性质和酶活性进行测定分析，系统比较和研究了不同水分条件和不同乡土灌木生长对干旱河谷区土壤结构、养分循环、酶活性以及微生物量的影响。主要结果如下：1. 无论生长植物与否，土壤的毛管持水量和毛管孔隙度都随着水分含量的减少而降低，最大持水量、总孔隙度和容重变化不大，相应地，土壤中的非毛管孔隙随含水量的减少而升高。各水分条件下，种植植物的毛管持水量和毛管孔隙度低于无植物生长的土壤，非毛管孔隙度相应地高于无植物土壤。土壤含水量在100%－40% FC 时，三种豆科灌木的毛管持水量和毛管孔隙度存在差异，而20% FC 条件下，三种豆科灌木土壤的物理性质基本相同。2. 水分胁迫影响土壤中养分的矿化和积累，主要表现在降低了水溶性碳和铵态氮的含量，中等程度胁迫时（60% FC）促进了有机碳和硝态氮的富集，对速效钾和有效磷没有明显作用。种植豆科灌木后各水分梯度上都增加了有机碳、铵态氮、速效钾和有效磷的积累。增加程度上三种豆科灌木间有一定差异，对于土壤有机碳总量，种植白刺花和小马鞍羊蹄甲明显高于小雀花，同样的情况还出现在铵态氮和速效钾上，但是对于有效磷，种植小雀花后的增加程度则明显高于白刺花和小马鞍羊蹄甲。种植豆科灌木不仅增加了土壤养分的相对含量，也改变了其在水分梯度上的变化趋势及其变化幅度，这种作用主要体现在碳元素和氮元素上。3. 无植物生长时脲酶活性随水分含量的减少而升高，水分胁迫对磷酸酶和过氧化氢酶的作用不显著，蔗糖酶也保持在相对较高的水平。种植植物后，蔗糖酶、磷酸酶活性与无植物时相比有较大幅度的提高，种植白刺花的脲酶活性也升高，其升高的程度在不同水分含量时不同。种植植物还降低了酶活性在水分梯度上的变幅，使之在水分梯度间的差异显著性降低。脲酶活性在指示土壤性质改变方面是较敏感的指标，其它三种酶在不同植物间的差异不明显。4. 在无植物生长时，中等程度的水分胁迫（60% FC）提高了土壤微生物量碳含量，过高或过低的土壤水分均不利于微生物碳的积累。种植小马鞍羊蹄甲后微生物量碳在水分梯度上的变化趋势与无植物生长时一致，而种植白刺花和小雀花后微生物量碳随着水分含量的减少而降低。不同种类植物的微生物量碳在水分梯度上的变化特征也不同，100% FC 条件下三种植物间没有差异，80%和60% FC 条件下小马鞍羊蹄甲显著高于白刺花和小雀花，40%和20% FC 条件下白刺花和小马鞍羊蹄甲也显著高于小雀花，说明不同种类植物随着干旱胁迫程度的加深微生物量碳的降低幅度不同，在极度干旱时，白刺花和小马鞍羊蹄甲土壤依然保持了较高的微生物活性，而小雀花土壤微生物量则明显下降。The dry valley of the upper reaches of the Minjiang River is seriously degradedmountain ecosystem. It was endangered by extremely soil lost and frequentlygeological disaster. Previous studies showed that short of water and nutrients in soilwas the principal limiting factors of vegetation restoration in this area. The typical soiland three dominant leguminous shrubs Sophora davidii, Bauhinia faberi var.microphylla and Campylotropics polyantha in upper reaches of arid Minjiang Rivervalley were considered as experimental material. Two-month old seedlings of eachspecies were exposed to five water supplies (100%, 80%, 60%, 40% and 20% waterfield capacity (FC)) in a temperature and light-controlled greenhouse. Afterthree-month water treatment, soil physiochemical variables and soil microbialactivities were determined by conventional methods. The main results showed that:1. Soil capillary capacity and capillary porosity decreased along water supplyregimes in all treatments, while saturated water capacity, total porosity and bulkdensity kept in a relatively stable level, as a result, the non-capillary porosity andcapacity increased with decrease of water supply. Compared to non-planted soil, theplant-soil systems had a higher non-capillary porosity and capacity, suggestingappropriate oxygen was present in soil to maintain the living of microorganism. Soilof three type shrub species shared the same capillary capacity and capillary porosityunder 20% FC.2. Water soluble carbon and NH4+-N decreased in response to water stress, whiletotal organic carbon and NO3--N promoted by moderate water stress and inhibited by 100% and 20% FC. Total organic carbon, NH4+-N, rapidly available K and availableP increased after the planting of leguminous shrubs in five water supply regimescompared to non-planted soil. For TOC, NH4+-N and rapidly available K, thepromotion effect was higher in S. davidii and B. faberi var. microphylla than C.polyantha planted soil, while available P displayed the opposite side. The planting ofshrubs also reduced the variance of observed traits along water supply gradients.3. Drought stress increased urease activity in non-planted soil, while insignificantdifferences were observed in phosphatase and catalase activity among five watersupply regimes. The planting of leguminous shrubs facilitated the β-glucosidase andphosphatase activity compared to the non-planted soil. It also reduced the variance ofenzyme activity along water supply gradients. Urease was more sensitive to waterstress than other three enzymes.4. Soil water content significantly affected microbial biomass carbon andCmic:Corg. S. davidii and B. faberi var. microphylla showed more drought toleranceability than C. polyantha, attributing not only to their relatively smaller variance ofmicrobial biomass carbon along soil water supply gradients, but also to the highlevel of microbial activity under severe water stress. S. davidii and B. faberi var.microphylla benefited reproduction of soil microorganism at 60%-80% FC, whilesevere drought limited it due to the competition of water and nutrients between plantand soil microorganism.