Effect of experimental phosphorus enrichment on oligotrophic tropical marshes in Belize,Central America

Sedges from genus Eleocharisdominate extensive wetlands in the sugar cane growing areas of the Caribbean. Correlative data suggest that macrophytes in these wetlands are phosphorus limited. To determine effects of increased P input that can be expected, e.g. from agricultural runoff, a common sugar cane fertilizer was applied to representative plots in four marl-based and four peat-based marshes. The plots were located in the proximity of patches of Typha domingensis, which has been reported to be able to outcompete Eleocharis under nutrient rich conditions. Responses to the fertilizer treatment were documented as changes in: Plant height, density, biomass, net primary production, nutrient resorption, decomposition, plant and soil nutrient concentrations, percent cover of cyanobacterial mats, and potential colonization by Typha. Additions of phosphorus significantly increased plant density and height and, consequently, the aboveground net primary production. Phosphorus resorption efficiency following senescence was independent of fertilizer addition in Eleocharis but decreased in Typha from the fertilized plots. Phosphorus resorption proficiency was lower in fertilized plots for both Typha and Eleocharis. Decomposition of litter and cellulose assays was significantly faster in fertilized plots. No spontaneous establishment of Typha occurred in the fertilized plots, but survival of transplanted Typha was higher in fertilized plots than in controls. Increased plant density in fertilized plots led to elimination of a key component of these ecosystems, the nitrogen fixing cyanobacterial mats.     

Heterotrophic microbial activities and nutritional status of microbial communities in tropical marsh sediments of different salinities: the effects of phosphorus addition and plant species

Oligotrophic, phosphorus (P) limited herbaceous wetlands of northern Belize are being impacted by P loading from fertilizer runoff. P enrichment causes a shift in autotroph communities from a microphyte (cyanobacterial mats, CBM) to macrophyte (Eleocharis spp., Typha domingensis) dominated system. To document potential effects of P, salinity, and macrophyte species on the heterotrophic microbial community nutritional status (represented especially by specific phospholipids fatty acids and specific respiration rate), biomass and activities, we took soil samples from established P enrichment plots in replicated marshes of two salinity levels. P addition increased microbial biomass carbon (C), nitrogen (N) and P, as well as soil nutrient transformation rates (nitrogenase activity, N mineralization and immobilization, methanogenesis). The effect of plant species (Eleocharis vs Typha sites) was generally lower than the effect of P addition (CBM vs Eleocharis sites) and was most evident at the low salinity sites, where Eleocharis dominated plots had enhanced nitrogenase activity and P microbial immobilization. Salinity reduced the overall rates of microbial processes; it also weakened the positive effect of both P addition and plant species on microbial activities. Lastly, the amount of N stored in microbial cells, likely in form of osmoprotective compounds, was enhanced by salinity.     

Response of root and sediment phosphatase activity to increased nutrients and salinity

Wetlands of northern Belize, distributed along a salinity gradient, are strongly phosphorus limited and dominated largely by three species of emergent macrophytes: Eleocharis cellulosa, Cladium jamaicense, and Typha domingensis. We assessed changes in root and sediment phosphatase activities of each species to simultaneous changes of nutrients (N, P) and salinity in a mesocosm experiment. Phosphorus and nitrogen treatment effects on both root and sediment phosphatase were highly significant for all the species, while salinity significantly affected root phosphatase activity in Cladium only. All species showed a significant negative correlation between root phosphatase activity and increasing tissue P content until a threshold of 0.2% P, 0.15% P and 0.12% P in Eleocharis, Cladium and Typha, respectively. There was also a significant negative correlation between soil available P and root and sediment phosphatases in all species. Activity of root phosphatases of Eleocharis and Typha were positively correlated with root tissue N. Both root and sediment phosphatases of all three species were positively correlated with soil available N. The strongest (positive) correlation was found between phoshatase activites and N/P ratios. The results confirmed that these systems are P-limited and that extracellular phosphatases respond to P enrichment by decreasing their activities. Expression of root phosphatase activity by dry root weight, sediment volume, or whole plant biomass gave very different relative results across nutrient treatments and species, suggesting that root phosphatase activities need to be interpreted in a wider context that considers root density.     

Hydrologic,vegetation, and substrate characteristics of floating marshes in sediment-rich wetlands of the Mississippi river delta plain,Louisiana, USA

Floating marshes occur over 70% of the western Terrebonne Basin, Louisiana, USA, freshwater coastal wetlands. They are of several types: A free-floating thick-mat (45–60 cm) marsh dominated by Panicum hemitomon and Sagittaria lancifolia; a thick mat marsh dominated by Panicum hemitomon and Sagittaria lancifolia that floats part of the year, but whose vertical floating range is damped compared to adjacent water; and an irregularly-floating thin mat (< 30 cm) dominated by Eleocharis spp. in the spring and Ludwigia leptocarpa and Bidens laevis in the summer and fall. Floating mats must be almost entirely organic in order to be buoyant enough to float. The western Terrebonne wetlands receive large winter/spring supplies of suspended sediments from the Atchafalaya River. Even though sediment concentrations in the adjacent bayou are as high as 100 mg l^–1, the Panicum hemitomon/Sagittaria lancifolia free-floating marsh probably receives no over-surface sediments since it floats continuously. The bulk density data of the damped-floating marsh, however, suggest some mineral sediment input, probably during winter when this marsh is submerged. These two types of floating marsh could not have developed in the present sediment regime of the Atchafalaya River, but as long as they remain floating can continue to exist. Thin floating mats are found in areas receiving the least sediment (<20 mg 1^–1 suspended sediment concentration in adjacent bayous). This low sediment environment probably made possible their formation within the past 20 years. They may represent a transitional stage in mat succession from (1) existing thick-mat floating marsh to a degrading floating marsh, or (2) a floating marsh developing in shallow open water.Corresponding editor: D. Whigham     

Wetland plant decomposition under different nutrient conditions: what is more important, litter quality or site quality?

Plants in nutrient poor environments are often characterized by high nutrient resorption resulting in poor litter quality and, consequently, slow decomposition. We used oligotrophic, P-limited herbaceous wetlands of northern Belize as a model system, on which to document and explain how changes in nutrient content along a salinity gradient affect decomposition rates of macrophytes. In 2001 we established a nutrient addition experiment (P, N, and N&P) in 15 marshes of a wide range of water conductivities (200–6000 μS), dominated by Eleocharis spp. To determine what is more important for decomposition, the initial litter quality, or site differences, we used reciprocal litter placement and cellulose decomposition assay in a combined “site quality” and “litter quality” experiment. Our prediction of the positive effects of P-enrichment on decomposition rate due to both the quality of litter and the site was confirmed. The site effect was stronger than the litter quality although both were highly significant. Strong site quality effect was apparently the result of more active decomposer community in P-enriched plots as supported by finding of higher microbial biomass in litter decomposing there. The strong effect of site quality on decomposition was further confirmed by the cellulose assay. The cellulose decomposition was significantly slower at high salinity sites indicating lower decomposer microbial activity. Litter nutrient N and P content and nutrient ratios were well correlated with decomposition with the best fit found for log C/P. At C/P mass ratio of >4000 decomposition processes were extremely slow. We hypothesize that in a long run, the increased decomposition will compensate the increase in primary production resulting from increased nutrient loading and there will be no differences in accumulation of organic material between the controls and nutrient enriched plots.     

Invasion by <Emphasis Type="Italic">Aegilops triuncialis</Emphasis> (Barb Goatgrass) Slows Carbon and Nutrient Cycling in a Serpentine Grassland

Invasive plant species alter plant community composition and ecosystem function. In the United States, California native grasslands have been displaced almost completely by invasive annual grasses, with serpentine grasslands being one of the few remaining refugia for California grasslands. This study examined how the invasive annual grass, Aegilops triuncialis, has altered decomposition processes in a serpentine annual grassland. Our objectives were to (1) assess howA. triuncialis alters primary productivity and litter tissue chemistry, (2) determine whether A. triuncialis litter is more recalcitrant to decomposition than native litter, and (3) evaluate whether differences in the soil microbial community in A. triuncialis-invaded and native-dominated areas result in different decomposition rates of invasive and/or native plant litter. In invaded plant patches, A. triuncialis was approximately 50% of the total plant cover, in contrast to native plant patches in which A. triuncialis was not detected and native plants comprised over 90% of the total plant cover. End-of-season aboveground biomass was 2-fold higher in A. triuncialis dominated plots compared to native plots; however, there was no significant difference in belowground biomass. Both above- and below-ground plant litter from A. triuncialis plots had significantly higher lignin:N and C:N ratios and lower total N, P, and K than litter from native plant plots. Aboveground litter from native plots decomposed more rapidly than litter from A. triuncialis plots, although there was no difference in decomposition of belowground tissues. Soil microbial community composition associated with different soil patch types had no effect on decomposition rates. These data suggest that plant invasion impacts decomposition and nutrient cycling through changes in plant community tissue chemistry and biomass production.     

Groundwater shapes sediment biogeochemistry and microbial diversity in a submerged Great Lake sinkhole

For a large part of earth's history, cyanobacterial mats thrived in low‐oxygen conditions, yet our understanding of their ecological functioning is limited. Extant cyanobacterial mats provide windows into the putative functioning of ancient ecosystems, and they continue to mediate biogeochemical transformations and nutrient transport across the sediment–water interface in modern ecosystems. The structure and function of benthic mats are shaped by biogeochemical processes in underlying sediments. A modern cyanobacterial mat system in a submerged sinkhole of Lake Huron (LH) provides a unique opportunity to explore such sediment–mat interactions. In the Middle Island Sinkhole (MIS), seeping groundwater establishes a low‐oxygen, sulfidic environment in which a microbial mat dominated by Phormidium and Planktothrix that is capable of both anoxygenic and oxygenic photosynthesis, as well as chemosynthesis, thrives. We explored the coupled microbial community composition and biogeochemical functioning of organic‐rich, sulfidic sediments underlying the surface mat. Microbial communities were diverse and vertically stratified to 12 cm sediment depth. In contrast to previous studies, which used low‐throughput or shotgun metagenomic approaches, our high‐throughput 16S rRNA gene sequencing approach revealed extensive diversity. This diversity was present within microbial groups, including putative sulfate‐reducing taxa of Deltaproteobacteria, some of which exhibited differential abundance patterns in the mats and with depth in the underlying sediments. The biological and geochemical conditions in the MIS were distinctly different from those in typical LH sediments of comparable depth. We found evidence for active cycling of sulfur, methane, and nutrients leading to high concentrations of sulfide, ammonium, and phosphorus in sediments underlying cyanobacterial mats. Indicators of nutrient availability were significantly related to MIS microbial community composition, while LH communities were also shaped by indicators of subsurface groundwater influence. These results show that interactions between the mats and sediments are crucial for sustaining this hot spot of biological diversity and biogeochemical cycling.     

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.     

Leaf decomposition in an experimentally acidified stream channel

Decomposition of Alnus rugosa and Myrica Gale leaves immersed in artificial stream channels fed by a small headwater creek was followed over a three month period. At the end of experiment, remaining weights of both leaf types confined in litter bags were significantly higher after immersion in experimentally acidified water (pH 4.0) than when immersed in control water (pH 6.2–7.0). For both types of leaves and for all sampling times, there was generally no difference in the C:N ratios between leaves in acidified and those in control water. In control water, oxygen uptake by microorganism on A. rugosa leaves was significantly higher after 46 days of immersion, whereas differences between treatments appeared only after 69 days for M. Gale leaves. Transfer of A. rugosa leaves from acid to control water led to a rapid increase in microbial activity; this increased activity was reflected in a fast weight loss of the leaves. For both leaf types, total numbers of macroinvertebrates were usually higher in litter bags immersed in control water. Macroinvertebrates colonizing the litter bags were mainly collector-gatherers: Chironomidae were numerically dominant in control leaf packs whereas Oligochaeta dominated in acid leaf packs. Macroinvertebrate biomass in M. Gale litter was higher in control than in acidified water, which contrasted with macroinvertebrate biomass in A. rugosa leaf packs which was not significantly different between treatments. Macroinvertebrate contribution to the breakdown of leaf litter was thus considered less important than the microbial contribution. This study demonstrated that decomposition of leaf litter in acidic headwater streams can be seriously reduced, mainly as a result of a lower microbial activity.     

Nitrogen sequestration capacity of two salt marshes from the Tagus estuary

The role of salt marshes as nitrogen sink is examined taking into consideration the seasonal variation of above and belowground biomass of Spartina martima and Halimione portulacoides in two marshes from Tagus estuary, Pancas and Corroios, and the degradation rates of belowground litter. Total nitrogen was determined in plant components, decomposing litter and sediment. Biomass was higher in Corroios, the saltier marsh, with 7190 g m⁻² y⁻¹ dw of S. maritima and 6593 g m⁻² y⁻¹ dw of H. portulacoides and the belowground component contributed to 96% and 90% of total biomass, respectively. In the other marsh, Pancas, belowground biomass contributed to 56% and 76% of total biomass for S. maritima and H. portulacoides, respectively. Litterbag experiment showed that between 25% and 50% of nitrogen is lost within the first month and remained relatively constant in the next four months. Slower decomposition is observed in sediments with higher nitrogen concentration (max. 0.7% N in the saltier marsh). Higher concentrations of N were found in the sediment upper layers. Considering the sediment-root system, most of the nitrogen is stored in the sediment compartment and only about 1–4% of the total N was found in the roots. Considering these results, Tagus salt marshes act as a sink for nitrogen.     

Soil CO<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript> efflux and fungal and bacterial biomass in a plantation and a secondary forest in wet tropics in Puerto Rico

We examined the effects of root and litter exclusion on the rate of soil CO₂ efflux and microbial biomass using trenching and tent separation techniques in a secondary forest (SF) and a pine (Pinus caribaea Morelet) plantation in the Luquillo Experimental Forest in Puerto Rico. Soil surface CO₂ efflux was measured using the alkali trap method at 12 randomly-distributed locations in each treatment (control, root exclusion, litter exclusion, and both root and litter exclusion) in the plantation and the SF, respectively. We measured soil CO₂ efflux every two months and collected soil samples at each sampling location in different seasons to determine microbial biomass from August 1996 to July 1997. We found that soil CO₂ efflux was significantly reduced in the litter and root exclusion plots (7-year litter and/or root exclusion) in both the secondary forest and the pine plantation compared with the control. The reduction of soil CO₂ efflux was 35.6% greater in the root exclusion plots than in the litter exclusion plots in the plantation, whereas a reversed pattern was found in the secondary forest. Microbial biomass was also reduced during the litter and root exclusion period. In the root exclusion plots, total fungal biomass averaged 31.4% and 65.2% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 24% and 8.3% lower than the control plots in the plantation and the secondary forest, respectively. In the litter exclusion treatment, total fungal biomass averaged 69.2% and 69.7% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 48% and 50.1% lower than the control plots in the plantation and the secondary forest, respectively. Soil CO₂ efflux was positively correlated with both fungal and bacterial biomass in both the plantation the secondary forest. The correlation between soil CO₂ efflux and active fungal biomass was significantly higher in the plantation than in the secondary forest. However, the correlation between the soil CO₂ efflux and both the active and total bacterial biomass was significantly higher in the secondary forest than in the plantation in the day season. In addition, we found soil CO₂ efflux was highly related to the strong interactions among root, fungal and bacterial biomass by multiple regression analysis (R² > 0.61, P < 0.05). Our results suggest that carbon input from aboveground litterfall and roots (root litter and exudates) is critical to the soil microbial community and ecosystem carbon cycling in the wet tropical forests.     

Emergent macrophytes in phosphorus limited marshes: do phosphorus usage strategies change after nutrient addition?

Two strategies for phosphorus (P) economy in P-limiting environment are conservation of use and enhanced acquisition. Using two wetland macrophytes as an example, we show how these strategies change when the P-limitation is removed. Phosphorus resorption and activities of root phosphatases were evaluated over 4 years in Eleocharis cellulosa Torr. and Typha domingensis Pers. from nutrient addition experiment (P, N, N&P, control) established in 15 P limited marshes of Belize. We hypothesized that after P addition both species will increase tissue P content and decrease P resorption efficiency and root phosphatase activity. Initially high phosphorus resorption efficiency, PRE, significantly decreased in Eleocharis 2 years after the first nutrient addition, while no significant decrease was recorded for Typha. Even more dramatic was 5- to 6-fold increase in P in senescent tissues of Eleocharis as compared to less than 2-fold increase in Typha. Root phosphatase activity was high for both species from control plots. After P addition, Eleocharis showed 35% to 70% decrease in enzyme activity correlated to availability of inorganic P in sediments. Eleocharis and Typha employ the “conservation of use” strategy when growing in P limited oligotrophic marshes. In addition, Eleocharis is also using the “enhanced acquisition” strategy. These strategies change when the P limitation is removed but the response varies between the two species and thus changes in the proportion of these two species in a community may result in differences in ecosystem processes such as decomposition.     

Diurnal and seasonal variations of nitrogen fixation and photosynthesis in cyanobacterial mats

In situ measurements of nitrogenase activity and photosynthesis were performed simultaneously in cyanobacterial mats of intertidal sand flats of the Southern North Sea. Two types of cyanobacterial mats, which differed in species composition and biomass content, were investigated. The measurements were done monthly during 3 years to detect seasonal variations of nitrogen fixation and photosynthesis. Diurnal variations were investigated as well. The results showed that (i) freshly colonized sediment with the cyanobacteriumOscillatoria limosa as the dominant organism revealed the highest specific nitrogenase activities (ii) nitrogenase activities were highest in spring and summer, when mat development was initiated and (iii) diurnal fluctuations of nitrogenase activity indicated that it occurred temporally separated from oxygenic photosynthesis.     

Coupling between benthic biomass of Microcystis and phosphorus release from the sediments of a highly eutrophic lake

Variations in microbial biomass and activity in the sediments of hypereutrophic Lake Vallentunasjön were followed during a period of 5 years. The data were compared to the calculated release of phosphorus from the sediments during the same period. A strong co-variation was found between biomass of Microcystis, heterotrophic bacterial activity in the sediments and internal phosphorus loading. These parameters exhibited mainly a declining trend during the investigation period. A pronounced stability of the sediment chemistry, including the fractional composition of the sediment phosphorus, during the studied period indicates that microbial activity affected the phosphorus release from the sediments. Calculations of the percentage of sediment bacteria that was associated to the mucilage of Microcystis colonies imply, together with the specific bacterial production, that Microcystis in the sediment stimulates bacterial production. In the highly phosphorus-saturated sediments of Lake Vallentunasjön this would ultimately lead to an increased release of phosphorus from the sediment. Lake Vallentunasjön does not follow the common pattern of recovery after reduction of external phosphorus loading. The large biomasses and long survival of Microcystis in the sediment are probably important reasons for the delayed recovery of the lake.     

Decomposition of emergent macrophytes in a Wisconsin marsh

Loss of both dry weight and nutrients during decomposition was measured using litter bags, both in a natural marsh and in controlled experiments. At 348 days dry weight remains of Typha latifolia, Sparganium eurycarpum, Scirpus fluviatilis shoot litter in the marsh were 47.5, 26.9, 51.4% respectively, and for the rootrhizome litter were 59.1, 42.1, 27.8% (Scirpus > Sparganium > Typha). Under controlled conditions both temperature and type of water produced significant effect on dry weight loss of Typha leaves. Sterilization and antibiotics effectively inhibited the growth and activities of decomposers. Initial weight, N, P, Ca, and Mg losses resulted chiefly from leaching. These elements accumulated in spring and summer; N exhibited the highest accumulation. In the laboratory, N accumulation occurred within 15 days, as a result of microorganisms inhabiting the litter. Increase in P, Ca, Mg in later stages of decomposition were attributed to microorganisms, epiphytes, and precipitation from solution. High C : N ratios and relatively low P, Ca, Mg in original standing crop may be the cause of low herbivore consumption, whereas the relative increases in N, P, Ca, Mg in decomposed litter provide a more nutrient-rich substrate for detritivores. Much of the nutrient uptake in the annual cycle is via microbial and detritivore growth rather than by macrophyte producers.     

Long-term effects of prescribed fire on Cladium jamaicense crantz and Typha domingensis pers. densities

The importance of fire to the maintenance of herbaceous plant communities in Florida wetland ecosystems is widely acknowledged. However, despite the acceptance of fire as a natural and necessary disturbance, ecosystem responses to fire in these systems are still poorly understood. Of particular concern is the effect of fire on the dynamics of plant communities dominated by Cladium jamaicense Crantz and Typha domingensis Pers. High nutrient levels, primarily phosphorus, and prolonged hydroperiods have been associated with Typha expansion into Cladium dominated communities. Recent studies suggest that fire is a disturbance that may play a facilitative role in this process. The objective of this study was to monitor the long-term effects of a single prescribed fire on Cladium and Typha densities in a freshwater marsh in Florida. Transects located at two burned sites and one unburned site were sampled prior to and annually for four years following a prescribed, lightning-season fire. There was a significant increase (P < 0.01) in Typha at both burn sites for two years after the fire. However, this increase was temporary since Typha density declined to pre-burn levels in the third and fourth years post-burn. Cladium density at the burned sites either increased or remained unchanged throughout the study period. When the control site unexpectedly burned in the fourth year of the study, density changes of Typha were similar to those observed at the original burn sites. Overall, we did not see any lasting changes in Cladium and Typha as a result of the fires, even though soil nutrient levels and hydroperiods were within levels documented to enhance Typha expansion.     

Decomposition rates and phosphorus concentrations of purple Loosestrife (Lythrum salicaria) and Cattail (Typha spp.) in fourteen Minnesota wetlands

Purple Loosestrife is rapidly displacing native vegetation in North American wetlands. Associated changes in wetland plant communities are well understood. Effects of Loosestrife invasion on nutrient cycling and decomposition rates in affected wetlands are unknown, though potentially of significance to wetland function. We used litter bag methods to quantify decomposition rates and phosphorus concentrations of purple Loosestrife (Lythrum salicaria) and native cattails (Typha spp.) in fourteen Minnesota wetlands. A 170-day study that began in autumn modeled decomposition of Loosestrife leaves. Loosestrife stems andTypha shoots that had overwintered and fragmented were measured in a 280- day study that began in spring. In general, Loosestrife leaves decomposed most rapidly of the three;Typha shoots decomposed faster than Loosestrife stems. Significant decay coefficients (k-values) were determined by F-testing single exponential model regressions of different vegetation types in the fourteen wetlands. Significant decay coefficients were:k = 2.5 × 10⁻³ and 4.32 × 10⁻³ for all Loosestrife leaves (170 d);k = 7.2 × 10⁻⁴ and 1.11 × 10⁻³ for overwintered Loosestrife stems (280-d) andk = 7.9 × 10⁻⁴, 1.42 × 10⁻³ and 2.24 × 10⁻³ for overwinteredTypha shoots (280-d). Phosphorus concentrations of plant tissue showed an initial leaching followed by stabilization or increase probably associated with microbial growth. Loosestrife leaves had twice the phosphorus concentration of Loosestrife stems andTypha shoots. Our results indicate that conversion of wetland vegetation from cattails to Loosestrife may result in significant change in wetland function by altering timing of litter input and downstream phosphorus loads. Conversion of a riverine, flow- through wetland fromTypha to Loosestrife may effectively accelerate eutrophication of downstream water bodies. Impacts of Loosestrife invasion must be considered when wetlands are managed for wildlife or for improvement of downstream water quality.     

氮添加对川西高寒灌丛凋落枝化学计量特征及养分归还的影响

氮(N)是陆地生态系统初级生产力的重要限制因子,大气N沉降的增加将会对植物的化学元素含量和生物量产生重要影响,进而影响凋落物的化学计量特征及其养分归还。高寒灌丛是陆地生态系统的重要组成部分,但有关N沉降对高寒灌丛凋落物尤其是凋落枝的化学元素和生物量的研究还较为缺乏,难以深入揭示N沉降对高寒灌丛土壤碳(C)和养分循环的影响机理。基于此,以青藏高原东部地区的优势高寒灌丛类型—窄叶鲜卑花(Sibiraea angustata(Rehd.) Hand.-Mazz.)灌丛为研究对象,连续4年人工模拟N沉降,分析了凋落枝C、N、磷(P)、木质素和纤维素化学计量特征及其归还量对不同N添加浓度(0、20、50、100 kg hm^-2 a^-1)的响应趋势。结果表明：(1)N添加对凋落枝C、N含量无显著性影响(P>0.05),而对P、木质素和纤维素含量有显著性影响(P<0.05),但不同年份间的影响趋势不一致;(2)4年的N添加并未改变凋落枝的C/N、N/P,但显著降低了凋落枝的木质素/N(第3年)、C/P(第1年和第4年)和C/N/P(第1年);(...     

Confocal Laser Scanning Microscopy Image Analysis for Cyanobacterial Biomass Determined at Microscale Level in Different Microbial Mats

We recently published a new method based on determining cyanobacterial biomass by confocal laser scanning microscopy image analysis (CLSM-IA) (Solé et al., Ultramicrosc 107:669–673, 2007). CLSM-IA allows biomass calculation for microorganisms of a small size, since the limit of the technique’s resolution is that generated by a voxel, the smallest unit of a three-dimensional digital image, equivalent to 1.183 × 10⁻³ mgC/cm³ of sediment. This method is especially suitable for the quantitative analysis of a large number of CLSM images generated from benthic sediments in which complex populations of cyanobacteria are abundant, such as microbial mats. In order to validate the new CLSM approach, mats with varying structural characteristics were studied. We have grouped them into three types: Microcoleus mats (laminated), sandy mats (nonlaminated and composed of well-sorted quartz sands), and oil-polluted mats. In this work, we applied CLSM-IA in natural [the Ebro delta and Sant Jordi colony (Spain), Salins-de-Giraud and Etang de Berre (France), and Orkney Islands (Scotland)] and artificial [mesocosms (Israel)] microbial mats. A total of 4,103 confocal images were obtained in order to determine total and individual cyanobacteria biomass profiles, at microscale level. The data presented in this paper show the efficacy of the method, as it can be applied to highly diverse mat samples.     

放牧对鄂尔多斯高原油蒿草场生物量及植被-土壤碳密度的影响

以围封保护和自由放牧油蒿草场为研究对象,通过野外调查与室内分析,研究了围封和放牧条件下沙地草场生物量和植被-土壤碳密度。结果表明:(1)自由放牧使油蒿群落中植物种类增加,但降低了植物群落盖度。自由放牧不仅导致油蒿草场地上、地下总生物量降低,也使得油蒿地上、地下生物量占群落地上、地下总生物量的比例减小。生长季自由放牧样地凋落物生物量显著大于围封保护样地(P0.05);(2)围封保护样地植被碳密度大于自由放牧样地,土壤碳密度却小于自由放牧样地,但两个样地间差异不显著(P0.05);(3)油蒿草场90%以上的碳储存于土壤中,围封保护样地和自由放牧样地油蒿草场土壤碳密度占植被-土壤系统碳密度的91%、93%;(4)围封保护油蒿草场碳密度为2.29 kg/m2,自由放牧油蒿草场碳密度为2.68 kg/m2,两个样地间差异不显著,自由放牧对油蒿草场碳密度影响不大。