Heterotrophic nitrogen fixation in oligotrophic tropical marshes: changes after phosphorus addition

In order to determine the impact of nutrient enrichment on phosphorus (P) limited wetlands, we established experimental P additions in marshes throughout northern Belize. P significantly increased macrophyte primary production, which led to the rapid elimination of cyanobacterial mats. The replacement of cyanobacterial mats by macrophytes constrained autotrophic nitrogen (N) fixation, increased the quantity, and changed the quality of organic matter input to the sediments. We predicted that the activity of sediment heterotrophic N fixers will be impacted by these alterations in carbon input. We used the acetylene reduction technique to measure potential (glucose amended) nitrogenase activity (NA) in sediments from controls and treatment plots that have been P enriched for four years and dominated either by Eleocharis cellulosa, or Typha domingensis for two years. NA in P-enriched plots was 2–3 orders of magnitude higher than NA in controls. NA was positively correlated with the soil reactive P, both total organic and microbial carbon, live root biomass, and total phospholipid fatty acids (PLFA) as an indicator of active microbial biomass. It was negatively correlated with the concentration of ammonium-N. Path analysis revealed that the indirect effect of P on NA through the root biomass was more important than the direct effect of P. NA of the upper sediment layer was consistently higher in Eleocharis than in Typha dominated plots, despite the higher litter input by Typha. We feel that the higher levels of lignin and phenolics occurring in Typha litter, relative to Eleocharis, constrained NA in Typha plots. Handling editor: Luis Mauricio Bini     

Dynamics of <Emphasis Type="Italic">Typha domingensis</Emphasis> spread in <Emphasis Type="Italic">Eleocharis</Emphasis> dominated oligotrophic tropical wetlands following nutrient enrichment

Recent agricultural intensification in tropical countries has led to increased nutrient input and eutrophication of wetland ecosystems. Higher nutrient levels often lead to changes of vegetation structure and, eventually, shift in species dominance and loss of ecosystem services. We studied the dynamics of species shift in a manipulative nutrient enrichment experiment (+N, +P, +N&P) in oligotrophic wetlands of northern Belize distributed along a salinity gradient. We monitored spread and biomass accumulation of an introduced single individual of Typha domingensis within a 4 years period. The focus was on speed of the spreading and the relative importance of neighbouring ramets in this process. Large differences were found between control and N addition plots versus P and N&P addition plots. The ramets planted in control and N plots died or barely survived, while ramets in P and N&P plots grew vigorously and almost completely outcompeted original vegetation represented by Eleocharis spp. Final numbers of ramets were 2 and 576 per 100 m² for control and N versus P and N&P plots. The filling dynamics of P-enriched plots of differing salinity changed in time. The spreading was delayed in low salinity plots compared to high and medium salinity plots, although it finally reached comparable rates and values. We attribute this delay to originally denser vegetation and less suitable soil conditions in low salinity plots than to a direct salinity effect. Eventually, the number of ramets stabilized and often even decreased, probably due to self-thinning. Spatiotemporal model extrapolating observed vegetative spread suggested that in P-enriched conditions, a clone originating from a single individual is able to cover 1 ha plot completely within 9 years. We conclude that P-enrichment strongly increases the possibility of fast takeover of Belizean wetlands by Typha domingensis. Eventually, such species change can highly increase potential larval habitat for malaria transmitting mosquitoes.     

Ecological effects of low-level phosphorus additions on two plant communities in a neotropical freshwater wetland ecosystem

We conducted a low-level phosphorus (P) enrichment study in two oligotrophic freshwater wetland communities (wet prairies [WP] and sawgrass marsh [SAW]) of the neotropical Florida Everglades. The experiment included three P addition levels (0, 3.33, and 33.3 mg P m^–2 month^–1), added over 2 years, and used in situ mesocosms located in northeastern Everglades National Park, Fla., USA. The calcareous periphyton mat in both communities degraded quickly and was replaced by green algae. In the WP community, we observed significant increases in net aboveground primary production (NAPP) and belowground biomass. Aboveground live standing crop (ALSC) did not show a treatment effect, though, because stem turnover rates of Eleocharis spp., the dominant emergent macrophyte in this community, increased significantly. Eleocharis spp. leaf tissue P content decreased with P additions, causing higher C:P and N:P ratios in enriched versus unenriched plots. In the SAW community, NAPP, ALSC, and belowground biomass all increased significantly in response to P additions. Cladium jamaicense leaf turnover rates and tissue nutrient content did not show treatment effects. The two oligotrophic communities responded differentially to P enrichment. Periphyton which was more abundant in the WP community, appeared to act as a P buffer that delayed the response of other ecosystem components until after the periphyton mat had disappeared. Periphyton played a smaller role in controlling ecosystem dynamics and community structure in the SAW community. Our data suggested a reduced reliance on internal stores of P by emergent macrophytes in the WP that were exposed to P enrichment. Eleocharis spp. rapidly recycled P through more rapid aboveground turnover. In contrast, C. jamaicense stored added P by initially investing in belowground biomass, then shifting growth allocation to aboveground tissue without increasing leaf turnover rates. Our results suggest that calcareous wetland systems throughout the Caribbean, and oligotrophic ecosystems in general, respond rapidly to low-level additions of their limiting nutrient.     

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.     

Effect of macrophyte community composition and nutrient enrichment on plant biomass and algal blooms

Submerged freshwater macrophytes decline with increasing eutrophication. This has consequences for ecosystem processes in shallow lakes and ponds as macrophytes can reduce algal blooms under eutrophic conditions. We hypothesize that the productivity of submerged vegetation, biomass change under eutrophication and the suppression of algal blooms may be affected by macrophyte community composition. To test our hypothesis, we established three macrophyte community types in 36 fishless experimental ponds: one dominated by the oligotrophic species Chara globularis, one dominated by the eutrophic species Potamogeton pectinatus and a diverse vegetation which became co-dominated by Elodea nuttallii and C. globularis, and we fertilized half of the ponds.The macrophyte communities produced different amounts of biomass and they responded differently to fertilization. The community dominated by Potamogeton produced the lowest overall biomass, but was not affected by nutrient addition. The communities dominated by Chara and co-dominated by Elodea and Chara produced more than four-fold the amount of biomass produced in Potamogeton communities under oligotrophic conditions, but were strongly negatively affected by nutrient addition.Phytoplankton abundance did not differ significantly among the plant community types, but showed large variation within community types. There was a significant negative relationship between spring macrophyte biomass and the probability of summer algal blooms. The occurrence of algal blooms coincided with low daphnid densities and high pH (>10).We conclude that the macrophyte community composition, characterized by the dominant species, strongly affected the amount of biomass production as well as the short-term response of the vegetation to nutrient enrichment. Macrophyte community composition had no direct effect on algal blooms, but can affect the occurrence of algal blooms indirectly as these occurred only in ponds with low (<100 g/m² DW) spring macrophyte biomass.     

Litter effects of two co-occurring alpine species on plant growth, microbial activity and immobilization of nitrogen

We measured the litter chemistry of two co‐dominant alpine species, Acomastylis rossii, a forb characterized by a low growth rate and N uptake capacity, and Deschampsia caespitosa, a grass characterized by a high growth rate and N uptake capacity, and examined the effect litter of these two species had on the growth of Deschampsia phytometers in a greenhouse. We also examined the influence of litter from the two species on microbial respiration and immobilization of N, in two separate laboratory microcosm experiments and in the field. We hypothesized that Acomastylis litter would reduce plant growth more than Deschampsia litter, corresponding with either 1) suppression of microbial activity and thus a decrease in N mineralization, or 2) by stimulation of microbial biomass and increasing microbial immobilization of N. Relative to Deschampsia litter, Acomastylis litter had higher total water soluble organic carbon (DOC), and higher total phenolic concentration. Deschampsia litter had 30 times higher carbohydrate (primarily glucose and fructose) concentrations than Acomastylis litter. Soil respiration, microbial biomass N, and consumption of DOC and N were higher with the Acomastylis litter treatment than the Deschampsia litter treatment in experimental microcosms, and both respiration and microbial biomass N were higher in field soils under canopies dominated by Acomastylis relative to those dominated by Deschampsia. These results indicate that phenolics in Acomastylis are a C source for soil microorganisms, rather than an inhibitor of microbial activity. Deschampsia phytometers grew significantly less, had higher root: shoot biomass ratios, and acquired less nitrogen in the Acomastylis litter treatment relative to the control and Deschampsia litter treatments. The results of these experiments indicate that Acomastylis litter influences soil N cycling by increasing microbial activity and N immobilization, which may influence N supply to neighboring plants. This mechanism has the potential to influence competitive interactions between Acomastylis and its neighbors.     

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.     

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.     

Nutrient impacts on epifaunal density and species composition in a subtropical seagrass bed

The capacity of epifauna to control algal proliferation following nutrient input depends on responses of both grazers and upper trophic level consumers to enrichment. We examined the responses of Thalassia testudinum (turtle grass) epifaunal assemblages to nutrient enrichment at two sites in Florida Bay with varying levels of phosphorus limitation. We compared epifaunal density, biomass, and species diversity in 2 m² plots that had either ambient nutrient concentrations or had been enriched with nitrogen and phosphorus for 6 months. At the severely P-limited site, total epifaunal density and biomass were two times higher in enriched than in unenriched plots. Caridean shrimp, grazing isopods, and gammarid amphipods accounted for much of the increase in density; brachyuran crabs, primary predatory fish, and detritivorous sea cucumbers accounted for most of the increase in biomass. At the less P-limited site, total epifaunal density and biomass were not affected by nutrient addition, although there were more caridean shrimp and higher brachyuran crab and pink shrimp biomass in enriched plots. At both sites, some variation in epifaunal density and biomass was explained by features of the macrophyte canopy, such as T. testudinum and Halodule wrightii percent cover, suggesting that enrichment may change the refuge value of the macrophyte canopy for epifauna. Additional variation in epifaunal density and biomass was explained by epiphyte pigment concentrations, suggesting that enrichment may change the microalgal food resources that support grazing epifauna. Increased epifaunal density in enriched plots suggests that grazers may be able to control epiphytic algal proliferation following moderate nutrient input to Florida Bay. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Differences in tolerance of pondweeds and charophytes to vertebrate herbivores in a shallow Baltic estuary

It has been suggested that herbivorous waterfowl may be important in shaping aquatic plant communities in shallow wetlands. As such, a shift from canopy forming pondweeds to bottom-dwelling charophytes in a formerly turbid pondweed dominated lake has been partly attributed to waterfowl herbivory. Here we study the separate and combined effects of both belowground herbivory in spring by whooper swans and Bewick ‘s swans, and grazing in summer by waterfowl and fish on the community composition in a shallow Baltic estuary during one year. The macrophyte community was dominated by charophytes (mainly Chara aspera) with Potamogeton pectinatus and Najas marina present as subdominants. Other species were rare. Both spring and summer herbivory had no effect on total plant biomass. However, P. pectinatus was more abundant in plots that were closed to spring and summer herbivores. N. marina was more abundant in grazed plots, whereas Chara spp. biomass remained unaffected. Probably belowground propagules of both C. aspera and P. pectinatus were consumed by swans but since C. aspera bulbils were numerous it may have compensated for the losses. P. pectinatus may not have fully recovered from foraging on tubers and aboveground biomass. Our results are in line with other studies in Chara dominated lakes, which found no effect of grazing on summer aboveground Chara biomass, whereas several studies report strong effects of herbivory in lakes dominated by P. pectinatus.     

Nitrogen limitation and microbial diversity at the treeline

Tree growth limitation at treeline has mainly been studied in terms of carbon limitation while effects and mechanisms of potential nitrogen (N) limitation are barely known, especially in the southern hemisphere. We investigated how soil abiotic properties and microbial community structure and composition change from lower to upper sites within three vegetation belts (Nothofagus betuloides and N. pumilio forests, and alpine vegetation) across an elevation gradient (from 0 to 650 m a.s.l.) in Cordillera Darwin, southern Patagonia. Increasing elevation was associated with a decrease in soil N‐NH₄⁺ availability within the N. pumilio and the alpine vegetation belt. Within the alpine vegetation belt, a concurrent increase in the soil C:N ratio was associated with a shift from bacterial‐dominated in lower alpine sites to fungal‐dominated microbial communities in upper alpine sites. Lower forested belts (N. betuloides, N. pumilio) exhibited more complex patterns both in terms of soil properties and microbial communities. Overall, our results concur with recent findings from high‐latitude and altitude ecosystems showing decreased nutrient availability with elevation, leading to fungal‐dominated microbial communities. We suggest that growth limitation at treeline may result, in addition to proximal climatic parameters, from a competition between trees and soil microbial communities for limited soil inorganic N. At higher elevation, soil microbial communities could have comparably greater capacities to uptake soil N than trees, and the shift towards a fungal‐dominated community would favour N immobilization over N mineralization. Though evidences of altered nutrient dynamics in tree and alpine plant tissue with increasing altitude remain needed, we contend that the measured residual low amount of inorganic N available for trees in the soil could participate to the establishment limitation. Finally, our results suggest that responses of soil microbial communities to elevation could be influenced by functional properties of forest communities for instance through variations in litter quality.     

Symbiont nitrogenase,alder growth,and soil nitrate response to phosphorus addition in alder (Alnus incana ssp. rugosa) wetlands of the Adirondack Mountains,New York State,USA

Speckled alder (Alnus incana ssp. rugosa) is a characteristic species of scrub-shrub 1-type wetlands, the second most common wetland type in major watersheds of the Adirondack Mountains in New York State. Speckled alder is an actinorhizal nitrogen fixer that relies heavily on N₂ over soil N and fixes substantial amounts of nitrogen in wetlands, resulting in little vegetation processing of anthropogenic N between alder-shrub wetlands and streams. Phosphorus (P) is an element that limits nitrogen fixation and plant growth. However, studies testing this hypothesis in the field, especially for actinorhizal plants, are very few. The objectives of this study were to evaluate the potential limitation of N fixation and growth in speckled alder by P, and to determine interactions between P fertilization and nitrate levels in riparian alder stands in a region that receives elevated N in atmospheric deposition. P fertilization significantly increased specific nitrogenase activity during the seasonal peak in early August. Nitrate concentrations were greater in reference plots compared to treatment plots, and phosphate concentrations were lower in reference plots compared to treatment plots over a period of 6 weeks in the growing season. There was a significant twig and foliar biomass response to P fertilization in the second year after fertilization, but no significant change in individual biomass or relative numbers of different sized nodules. Response of nitrogen fixation to P appears limited to a brief but significant increase in specific activity of nitrogenase late in the growing season, but P stimulated growth of above ground tissues 1 year following fertilizer application, and decreased resin-captured nitrate beneath riparian speckled alder. These results suggest that growth of alder and growth or activity of soil microbes, rather than nitrogen fixation, is P limited in riparian wetlands dominated by speckled alder, and that P controls nitrate leaching in these near-stream systems.     

Effects of experimental warming and nitrogen fertilization on soil microbial communities and processes of two subalpine coniferous species in Eastern Tibetan Plateau,China

Aim

This study aimed at predicting how sub-alpine coniferous ecosystems respond to global changes in the Eastern Tibetan Plateau by understanding soil microbial communities and activities, as well as variation in the quality and quantity of soil organic matter.

Methods

An experiment was conducted to examine soil microbial communities and their related soil processes in rhizospheric soil of two coniferous species that were exposed to two levels of temperature (unwarmed and infrared heater warming) and two levels of nitrogen (unfertilized and 25 g N m^?2 a^?1) from April 2007.

Results

Four-year night warming alone slightly affected the phospholipid fatty acid contents of the microbial community. However, the combination of nitrogen addition and soil warming significantly affected soil microbial composition while reducing the biomass of major microbial groups and the activities of most enzymes, especially in Abies faxoniana plots. The combination of warming and nitrogen addition increased soil labile C and N pools in Picea asperata plots and was beneficial for soil recalcitrant C, as well as for labile and total C and N pools in A. faxoniana plots.

Conclusion

Results indicated that future warming will slightly affect soil microbial communities and their related soil processes. However, warming combined with high nitrogen deposition will significantly constrain soil microbial biomass and enzyme activities, consequently increasing soil C and N pools in sub-alpine coniferous forests of this region.     

Microbial biomass and respiration responses to nitrogen fertilization in a polar desert

How microbial communities respond to increases in available nitrogen (N) will influence carbon (C) and nutrient cycles. Most studies addressing N fertilization focus on mid-latitude ecosystems, where complex aboveground–belowground interactions can obscure the response of the soil microbial community, and little is known about how soil microbial communities of polar systems, particularly polar deserts, will respond. The low C content and comparatively simpler (low biomass and biodiversity) soil communities of the McMurdo Dry Valleys of Antarctica may allow easier identification of the mechanisms by which N fertilization influences microbial communities. Therefore, we conducted a microcosm incubation using three levels of N fertilization, added in solution to simulate a pulse of increased soil moisture, and measured microbial biomass and respiration over the course of 4.5 months. Soil characteristics, including soil pH, conductivity, cation content, chlorophyll a, and organic C content were measured. Soils from two sites that differed in stoichiometry were used to examine how in situ C:N:P influenced the N-addition response. We hypothesized that negative influences of N enrichment would result from increased salinity and ion content, while positive influences would result from enhanced C availability and turnover. We observed that microbes were moderately influenced by N addition, including stimulation and inhibition with increasing levels of N. Mechanisms identified include direct inhibition due to N toxicity and stimulation due to release from N, rather than C, limitation. Our results suggest that, by influencing microbial biomass and activity, N fertilization will influence C cycling in soils with very low C content.     

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

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

Epiphytes of the St. Lucia Estuary and their response to water level and salinity changes during a severe drought

St. Lucia is the largest estuary in South Africa with extensive areas of submerged macrophytes that fluctuate rapidly in response to water level and salinity changes. Epiphytes associated with submerged macrophytes were sampled during a severe drought between November 2004 and October 2005 when very low water level and high, variable salinity characterised the estuary. Potamogeton pectinatus and Ruppia cirrhosa were the dominant submerged macrophytes throughout the estuary, with P. pectinatus occurring at relatively low salinity (∼10 ppt) and R. cirrhosa at higher salinity (9–33 ppt). Zostera capensis, normally the other dominant submerged macrophyte, was conspicuously absent under the prevailing conditions. Epiphytic biomass, estimated as chlorophyll a, varied greatly between sites and over the 12 month sampling period, ranging from 10.9 to 71.7 mg Chl a m⁻² leaf area for P. pectinatus and 16.9–165.0 mg Chl a m⁻² leaf area for R. cirrhosa. Epiphytic biomass was twice as high in the Southern Lake where R. cirrhosa occurred, probably because the dominant epiphytes were macroalgae. An assessment of the diatom species composition of the epiphytic community indicated the dominance of only six species throughout the estuary. Neither epiphytic biomass nor diatom species composition showed strong statistical relationships with the environmental variables measured and it is believed that biological factors may be more important than the physico-chemical environment in determining epiphyte biomass distribution. Because epiphyte biomass is dependent on the presence of host surface area it will only contribute substantially to overall system biomass and productivity when submerged macrophyte area cover is high. In the St. Lucia Estuary this will occur when the water level is high and the upper level of the salinity gradient does not increase above that of seawater.     

A water-centred framework to assess the effects of salinity on the growth and yield of wheat and barley

We conducted a field experiment in two alpine meadows to investigate the short-term effects of nitrogen enrichment and plant litter biomass on plant species richness, the percent cover of functional groups, soil microbial biomass, and enzyme activity in two alpine meadow communities. The addition of nitrogen fertilizer to experimental plots over two growing seasons increased plant production, as indicated by increases in both the living plant biomass and litter biomass in the Kobresia humilis meadow community. In contrast, fertilization had no significant effect on the amounts of living biomass and litter biomass in the K. tibetica meadow. The litter treatment results indicate that litter removal significantly increased the living biomass and decreased the litter biomass in the K. humilis meadow; however, litter-removal and litter-intact treatments had no impact on the amounts of living biomass and litter biomass in the K. tibetica meadow. Litter production depended on the degree of grass cover and was also influenced by nitrogen enrichment. The increase in plant biomass reflects a strong positive effect of nitrogen enrichment and litter removal on grasses in the K. humilis meadow. Neither fertilization nor litter removal had any impact on the grass biomass in the K. tibetica meadow. Sedge biomass was not significantly affected by either nutrient enrichment or litter removal in either alpine meadow community. The plant species richness decreased in the K. humilis meadow following nitrogen addition. In the K. humilis meadow, microbial biomass C increased significantly in response to the nitrogen enrichment and litter removal treatments. Enzyme activities differed depending on the enzyme and the different alpine meadow communities; in general, enzyme activities were higher in the upper soil layers (0–10 cm and 10–20 cm) than in the lower soil layers (20–40 cm). The amounts of living plant biomass and plant litter biomass in response to the different treatments of the two alpine meadow communities affected the soil microbial biomass C, soil organic C, and soil fertility. These results suggest that the original soil conditions, plant community composition, and community productivity are very important in regulating plant community productivity and microbial biomass and activity.     

Environmental factors affecting the balance of autotrophs versus heterotrophs in the microbial food web of temporary ponds

The relative abundance of autotrophic (A) and heterotrophic (H) organisms in the microbial food web of temporary ponds and its relationship to environmental variables were analysed. Four localities (43 temporary ponds) were selected for study. They were located in unaltered, protected basins without intensive agriculture or strong human pressure. Because the supply of allochthonous organic matter is expected to be higher than inorganic nutrient inputs in temporary ponds, we hypothesized that the microbial food web in these unaltered basins was dominated by heterotrophic organisms. Our results showed that the log A:H biomass ratio (A/H) was always negative, indicating strong dominance by heterotrophic organisms in these ponds. Moreover, A/H was negatively related to DOC concentrations, but this relationship was weaker in the locality with the highest DOC concentration. No significant relationships were found between A/H and the other environmental variables analysed, namely inorganic nutrients, fulvic acids, chlorophyll-a concentrations, macrophyte biomass or total zooplankton biomass. However, when the different groups of zooplankton were considered separately, A/H correlated negatively with cladoreran biomass and positively with harpacticoid biomass. Thus, temporary ponds differ from deeper ecosystems, in which the greater importance of autochthonous energy inputs leads to microbial food web that is more dominated by autotrophs.     

氮添加对荒漠草原植物群落组成与微生物生物量生态化学计量特征的影响

大气氮(N)沉降增加加速了土壤N循环, 引起微生物生物量碳(C):N:磷(P)生态化学计量关系失衡、植物种丧失和生态系统服务功能降低等问题。开展N添加下植物群落组成与微生物生物量生态化学计量特征关系的研究, 可为深入了解N沉降增加引起植物多样性降低的机理提供新思路。该文以宁夏荒漠草原为研究对象, 探讨了N添加下植物生物量和群落多样性的变化趋势, 分析了微生物生物量C:N:P生态化学计量特征独立及其与其他土壤因子共同对植物群落组成的影响。结果表明: N添加下猪毛菜(Salsola collina)生物量呈显著增加趋势, 牛枝子(Lespedeza potaninii)生物量呈逐渐降低趋势, 其他植物种生物量亦呈降低趋势但未达到显著水平; 沿N添加梯度, Shannon-Wiener多样性指数、Simpson优势度指数和Patrick丰富度指数均呈先略有增加后逐渐降低的趋势; N添加提高了微生物生物量N含量和N:P, 降低了微生物生物量C:N; 植物群落组成与微生物生物量N含量、微生物生物量C:N、微生物生物量N:P、土壤NO₃ ^--N浓度、土壤NH₄ ⁺-N浓度以及土壤全P含量有较强的相关关系; 微生物生物量C:N:P生态化学计量特征对植物种群生物量和群落多样性变化的独立解释力较弱, 但却与其他土壤因子共同解释了较大变差, 意味着N添加下微生物生物量C:N:P生态化学计量特征对植物群落组成的影响与其他土壤因子高度相关。     

Soil microbial eco-physiological response to nutrient enrichment in a sub-tropical wetland

Eutrophication in subtropical wetland ecosystems can lead to extensive displacements of vegetative communities and as a result changes in overall environmental conditions (loss of indigenous habitat, substrate quality, etc.). This has generated a demand for a set of sensitive indicator(s) that prelude these structural changes. The functional response of bacterial communities may indicate the effect and extent of the impact on the overall system. The effects of nutrient enrichment on the microbial community and its ecophysiology were measured in a subtropical marsh (Water Conservation Area 2a) in the northern Everglades, USA. We investigated the microbially mediated organic matter decomposition processes and nutrient cycling in three areas of the marsh, a nutrient enriched site, an intermediate site and a unimpacted (oligotrophic) site. We chose measures associated to the hydrolytic enzyme activities of alkaline phosphatase, β-glucosidase and aminopeptidase. We also monitored microbial biomass carbon (C), nitrogen (N) and phosphorus (P) and the associated elemental turnover rates (C, N and P). We found a significant (α = 0.05) spike in microbial biomass C, N, and P in the intermediate site. The elemental turnover rates (C, N and P) where significantly higher in the impacted and intermediate site when compared to the unimpacted site. The enzymatic profiles at the unimpacted site illustrate a system regulated for optimal use of P. In the intermediate zone between the overall P-limited and P-impacted areas, the nutrient inputs alleviates the stress imposed by the P-limitation. Microbial biomass increased dramatically without a decrease in the overall microbial metabolic efficiency. The metabolic coefficients (particularly q-Potentially Mineralizable P – qPMP and qCO₂) indicated that after the disturbance, the impacted areas in the Everglades are characterized by relatively open, inefficient nutrient cycles. The nonlinear shifts (threshold behavior) in microbial parameters indicate that microbial indicators function effectively as early warning signals.