Effects of hydrologic changes on aboveground production and surface water chemistry in two boreal peatlands in Alberta: Implications for global warming

Aboveground net primary production (NPP) and surface water chemistryvariables were monitored in a lacustrine sedge fen and a bog for four years.There were no significant differences in precipitation, mean growing seasonannual temperature, and number of growing degree days from 1991 to 1994. Themean annual water levels in the lacustrine sedge fen differed significantly,whereas they were similar in the bog during these four years. We measured 15surface water variables in the lacustrine sedge fen and the bog, and foundthat only two correlated significantly with water level fluctuations. In thelacustrine sedge fen, calcium correlated positively (r²= 0.56) and nitrate correlated negatively (r² =0.20) with water levels. In the bog, potassium correlated positively(r² = 0.88) and total dissolved phosphorus correlatednegatively (r² = 0.62) with water levels. The remainingchemical variables showed no significant correlations with water levelfluctuations. Net primary production of the different vegetation strataappeared to respond to different environmental variables. In the lacustrinesedge fen, graminoid production was explained to a significant degree bywater levels (r² = 0.53), whereas shrub production wasexplained to a significant degree by surface water chemistry variables, suchas nitrate (r² = 0.74) and total phosphorus(r² = 0.22). In the bog, temperature was the onlyvariable that explained moss production to a significant degree(r² = 0.71), whereas ammonium explained graminoidproduction (r² = 0.66) and soluble reactive phosphorusexplained shrub production to significant degrees (r² =0.71). There are few direct data on the impact of climatic warming in borealwetlands, although paleoecological and 2×CO₂ model datahave provided some indications of past and possibly future changes invegetation composition, respectively. Our results suggest that thelacustrine sedge fen may succeed to a bog dominated by Sphagnum spp. andPicea mariana, whereas the bog may succeed to an upland-type forestecosystem. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of nutrient loading on biogeochemical and microbial processes in a New England salt marsh

Coastal marshes represent an important transitional zone between uplands and estuaries. One important function of marshes is to assimilate nutrient inputs from uplands, thus providing a buffer for anthropogenic nutrient loads. We examined the effects of nitrogen (N) and phosphorus (P) fertilization on biogeochemical and microbial processes during the summer growing season in a Spartina patens (Aiton (Muhl.)) marsh in the Narragansett Bay National Estuarine Research Reserve on Prudence Island (RI). Quadruplicate 1 m² plots were fertilized with N and P additions, N-only, P-only, or no additions. N-only addition significantly stimulated bacterial production and increased pore water NH₄⁺ and NO₃⁻ concentrations. Denitrification rates ranged from 0 to 8 mmol m⁻² day⁻¹. Fertilization had no apparent effect on soil oxygen consumption or denitrification measured in the summer in intact cores due to high core-to-core variation. P fertilization led to increased pore water dissolved inorganic phosphorus (DIP) concentrations and increased DIP release from soils. In contrast the control and N-only treatments had significant DIP uptake across the soil-water interface. The results suggest that in the summer fertilization has no apparent effect on denitrification rates, stimulates bacterial productivity, enhances pore water nutrient concentrations and alters some nutrient fluxes across the marsh surface.     

Sediment nutrient accumulation and nutrient availability in two tidal freshwater marshes along the Mattaponi River, Virginia, USA

Sediment deposition is the main mechanism of nutrient delivery to tidal freshwater marshes (TFMs). We quantified sediment nutrient accumulation in TFMs upstream and downstream of a proposed water withdrawal project on the Mattaponi River, Virginia. Our goal was to assess nutrient availability by comparing relative rates of carbon (C), nitrogen (N), and phosphorus (P) accumulated in sediments with the C, N, and P stoichiometries of surface soils and above ground plant tissues. Surface soil nutrient contents (0.60–0.92% N and 0.09–0.13% P) were low but within reported ranges for TFMs in the eastern US. In both marshes, soil nutrient pools and C, N, and P stoichiometries were closely associated with sedimentation patterns. Differences between marshes were more striking than spatial variations within marshes: both C, N, and P accumulation during summer, and annual P accumulation rates (0.16 and 0.04 g P m^–2 year^–1, respectively) in sediments were significantly higher at the downstream than at the upstream marsh. Nitrogen:P ratios <14 in above ground biomass, surface soils, and sediments suggest that N limits primary production in these marshes, but experimental additions of N and/or P did not significantly increase above ground productivity in either marsh. Lower soil N:P ratios are consistent with higher rates of sediment P accumulation at the downstream site, perhaps due to its greater proximity to the estuarine turbidity maximum.     

Effects of fertilization and drought stress on tannin biosynthesis of Casuarina equisetifolia seedlings branchlets

Nutrient, water, and their interactions influence the allocation of investment by plants to resistance and tolerance traits. We used a completely crossed randomized-block design experiment to examine the independent and interactive effects of nutrients and water availability on tannin production of C. equisetifolia seedlings. The results showed that nitrogen and phosphorus fertilizer have significant effects on total phenolics (TP) and extractable condensed tannins (ECT) concentrations of branchlets. TP and ECT concentrations decreased with fertilizer addition and increased in arid condition. This pattern lends to support source-sink hypothesis such as the carbon-nutrient balance (CNB) hypothesis and the growth-differentiation balance (GDB) hypothesis. Soluble sugars or starch concentrations were both inversely related to TP concentrations. However, there was no significant correlation between them and ECT concentrations. In addition, chlorophyll concentration had a positive linear correlation with TP and no significant correlation with ECT. On the contrary, chlorophyll a/b ratios were negatively correlated with TP and positively correlated with ECT. The discrepancy of relationship between carbohydrates and TP or ECT showed that the biosynthetic routes of different tannins were different. In this study, no significant correlation between TP and N, or ECT and N, did not support protein competition model (PCM). TP:N and ECT:N ratios were higher in nutrient deficiency and arid conditions, which were one of the important nutrient conservation strategies for C. equisetifolia.     

Carbon,Nitrogen, Phosphorus,and Potassium Stoichiometry in an Ombrotrophic Peatland Reflects Plant Functional Type

Ombrotrophic bog peatlands are nutrient-deficient systems and important carbon (C) sinks yet the stoichiometry of nitrogen (N), phosphorus (P) and potassium (K), essential for plant growth and decomposition, has rarely been studied. We investigated the seasonal variation in C, N, P, and K concentrations and their stoichiometric ratios in photosynthetically active tissues of 14 species belonging to five plant functional types (PFTs) (mosses, deciduous trees/shrubs, evergreen shrubs, graminoids, and forb) at Mer Bleue bog, an ombrotrophic peatland in eastern Ontario, Canada. Although we observed variations in stoichiometry among PFTs at peak growing season, there was convergence of C:N:P:K to an average mass ratio of 445:14:1:9, indicating N and P co-limitation. Nitrogen, P, and K concentrations and stoichiometric ratios showed little seasonal variation in mosses, evergreens, and graminoids, but in forb and deciduous species were the largest in spring and decreased throughout the growing season. Variations in nutrient concentrations and stoichiometric ratios among PFTs were greater than seasonal variation within PFTs. Plants exhibit N and P co-limitation and adapt to extremely low nutrient availability by maintaining small nutrient concentrations in photosynthetically active tissues, especially for evergreen shrubs and Sphagnum mosses. Despite strong seasonal variations in nutrient availabilities, few species show strong seasonal variation in nutrient concentrations, suggesting a strong stoichiometric homeostasis at Mer Bleue bog.     

Calcium and iron as key drivers of brown moss composition through differential effects on phosphorus availability

Brown moss-dominated rich fens are characterized by minerotrophic conditions, in which calcium (Ca) and iron (Fe) concentrations show large variations. We examined the relative importance of Ca and Fe in relation to the occurrence of three typical brown moss species: Scorpidium scorpioides, Scorpidium cossonii, and Hamatocaulis vernicosus. Peat chemistry was examined in 24 stands of brown moss-dominated rich fens: 12 in the Netherlands and 12 in central Sweden. Ca and Fe turned out to be important drivers of brown moss composition. Fens dominated by Scorpidium scorpioides or Scorpidium cossonii were characterized by high pore water Ca-concentrations and total soil Ca-contents, but low P-availability. In these Ca-rich, but Fe-poor fens, foliar N:P ratios of vascular vegetation exceeded 20?g?g^?1, indicating phosphorus (P)-limitation due to Ca-P precipitation or low P-sorption capacity due to low Fe-levels. In contrast, fens dominated by Hamatocaulis vernicosus were characterized by high pore water Fe-concentrations and total soil Fe-contents, but also relatively high P-availability. N:P ratios in these fens were below 13.5?g?g^?1, indicating potential nitrogen (N)-limitation. We conclude that the relative roles of Ca and Fe, as related to the geohydrological conditions present, strongly determine the brown moss composition in rich fens through their differential effects on plant P-availability.     

Sediment and Water Nutrient Characteristics in Patches of Submerged Macrophytes in Running Waters

(1) The relative importance of sediments and water as nutrient sources for submerged macrophytes in running waters is poorly understood. Here we present water and sediment nutrient characteristics within macrophyte patches in Bavarian rivers. (2) No significant differences between early (June/July) and late summer (August/September) sediment nutrient characteristics could be detected within macrophyte patches. Therefore, a single sediment sample per macrophyte patch was considered to be sufficient for characterising nutrient concentrations during the main growing season in running waters. (3) Sediment TP (total phosphorus) is not a useful parameter for predicting trophic status in running waters. Sediment porewater SRP (soluble reactive phosphorus) concentration is not correlated to water body SRP or TP concentration; nor is it correlated with sediment TP content. Potamogeton coloratus, a oligotrophic species, is associated with low overlying and porewater SRP concentrations but high sediment TP content. Eutrophic species, such as Potamogeton pectinatus, are associated with low sediment TP. (4) It is hypothesized that Chara hispida primarily takes up sediment ammonia for nitrogen nutrition. (5) Nutrient characteristics of the water body and the sediment of eight macrophyte species in Bavarian rivers are described.     

In search of a hydrological explanation for vegetation changes along a fen gradient in the Biebrza Upper Basin (Poland)

The understanding of succession from rich fen to poorer fen types requires knowledge of changes in hydrology, water composition, peat chemistry and peat accumulation in the successional process. Water flow patterns, water levels and water chemistry, mineralisation rates and nutrient concentrations in above-ground vegetation were studied along a extreme-rich fen-moderate-rich fen gradient at Biebrza (Poland). The extreme-rich fen was a temporary groundwater discharge area, while in the moderate-rich fen groundwater flows laterally towards the river. The moderate-rich fen has a rainwater lens in spring and significant lower concentrations of calcium and higher concentrations of phosphate in the surface water. Mineralisation rates for N, P and K were higher in the moderate-rich fen. Phosphorus concentrations in plant material of the moderate-rich fen were higher than in the extreme-rich fen, but concentrations of N and K in plant material did not differ between both fen types. Water level dynamics and macro-remains of superficial peat deposits were similar in both fen types.We concluded that the differences observed in the moderate-rich and the extreme-rich fens were caused by subtile differences in the proportion of water sources at the peat surface (rainwater and calcareous groundwater, respectively). Development of an extreme-rich fen into a moderate-rich fen was ascribed to recent changes in river hydrology possibly associated with a change in management practices. The observed differences in P-availability between the fen types did not result in significantly different biomass. Moreover, biomass production in both fen types was primarily N-limited although P-availability was restricted too in the extreme-rich fen. Aulacomnium palustre, the dominant moss in the moderate-rich fen, might be favoured in competition because of its broad nutrient tolerance and its quick establishment after disturbance. It might outcompete low productive rich fen species which were shown to be N-limited in both fens. We present a conceptual model of successional pathways of rich fen vegetation in the Biebrza region.     

Phosphorus Retention at the Redox Interface of Peatlands Adjacent to Surface Waters in Northeast Germany

It is demanded currently in public discussions to rewet peatlands and re-establish their function as nutrient sinks. But due to high phosphorus (P) concentrations in the pore water of rewetted peatlands (40–420 μM) it is hypothesized that they can act as a surplus P source for adjacent surface waters and consequently support the eutrophication of such waters. Our detailed investigations of processes at the redox interface in four fens with different geochemical character show the dependence of P retention from the chemistry of the pore water. The precipitation of Fe(III) oxyhydroxide led to high retention of phosphorus and other substances such as DOC and sulphate in the eutrophic fens. When molar Fe/P ratios were larger than about 3 the initially high P concentrations in the anaerobic pore water (20–210 μM) decreased to concentrations below 1 μM under aerobic conditions. Thus, after rewetting high pore water concentrations of P do not automatically result in an increased P load to adjacent surface waters compared to pre-rewetting conditions. An enhanced P export to adjacent surface waters from eutrophic fens can be expected when the Fe/P ratio is smaller than 3 in the anaerobic pore water. In our investigations of natural, oligotrophic to mesotrophic fens the precipitation of Fe(III) oxyhydroxide was inhibited by the formation of stable dissolved Fe ∼ humic complexes. P retention in these fens was only related to the DOC concentrations at the redox interface, so that lower DOC concentrations concurred with higher P retention. The P equilibrium concentrations in an aerobic environment can be higher than that of eutrophic fens with Fe/P ratios larger than about 3 in the anaerobic pore water.     

Seasonal pattern in nutrient limitation and grazing control of the phytoplankton community in a non-stratified lake

1. The relative importance of zooplankton grazing and nutrient limitation in regulating the phytoplankton community in the non-stratified Lake Kvie, Denmark, were measured nine times during the growing season.
2. Natural phytoplankton assemblage bioassays showed increasing importance of nutrient limitation during summer. Growth rates at ambient nutrient concentrations were continually below 0.12 per day, while co-enrichment with nitrogen (N) and phosphorus (P) to above concentration-saturated conditions enhanced growth rates from May to the end of July.
3. Stoichiometric ratios of important elements in seston (C : N, C : P, N : P), in lake water (TN : TP), in external loading (TN : TP) and in internal loading (DIN : DIP) were measured to determine whether N or P could be the limiting nutrient. TN : TP molar ratio of both lake water, benthic fluxes and external loading suggested P limitation throughout the growing season. However, seston molar ratios suggested moderate P-deficiency only during mid-summer.
4. Abundance and community structure of the zooplankton varied considerably through the season and proved to be important in determining the responses of algal assemblages to grazing. High abundance of cladocerans and rotifers resulted in significant grazing impact, while cyclopoid copepods had no significant effect on the phytoplankton biomass.
5. Regeneration of ammonium and phosphate by zooplankton were periodically important for phytoplankton growth. A comparison of nutrient regeneration by zooplankton with nutrient inputs from sediment and external sources indicated that zooplankton may contribute significantly in supplying N and P for the growth of phytoplankton.     

The relationships of vegetation to surface water chemistry and peat chemistry in fens of Alberta,Canada

The relationships between vegetation components, surface water chemistry and peat chemistry from 23 fens in boreal Alberta, Canada, substantiate important differences along the poor to rich fen gradient. Each of the three fen types have their own characteristic species. The extreme-rich fens are characterized by Calliergon trifarium, Drepanocladus revolvens, Scirpus hudsonianus, S. cespitosus, Scorpidium scorpioides, and Tofieldia glutinosa. Moderate-rich fens are characterized by Brachythecium mildeanum, Carex diandra, Drepanocladus vernicosus, D. aduncus, and D. polycarpus. Poor fens are characterized by Carex pauciflora, Drepanocladus exannulatus, Sphagnum angustifolium, S. jensenii, and S. majus. Moderate-rich fens have fewer species in common with poor fens than with extreme-rich fens, while species richness is highest in the moderate-rich fens and lowest in poor fens. Variation in vascular plant occurrence appears to be more associated with nutrient levels, while bryophytes are more affected by changes in acidity and mineral elements. Based on chemical criteria, the three fen types are clearly separated by surface water pH, calcium, magnesium, and conductivity, but are less clearly differentiated by the nitrogen and phosphorus components of the surface waters. Moderate-rich fens are chemically variable both temporally and spatially, whereas poor fens and extreme-rich fens are more stable ecosystems. Whereas components of alkalinity-acidity are the most important factors that distinguish the three fen types in western Canada, nutrient concentrations in the surface waters generally do not differ appreciably in the three fen types.     

Seasonal phosphatase activities of mosses from Upper Teesdale,northern England

Abstract

Changes in tissue nutrient concentrations and surface phosphatase activities of eight moss species were measured over one year in terrestrial and semi-aquatic environments on Widdybank Fell, Upper Teesdale National Nature Reserve, northern England. Rates of phosphatase activity in apical regions of moss shoots differed markedly between species, but were generally greatest in the winter and least in the summer in most species. Mean values for phosphomonoesterase activity (µmol para-nitrophenol g^-1 d.wt h^-1) ranged from 18.2 for Polytrichum commune to 85.8 for Palustriella commutata var. falcata. Mean phosphodiesterase activity ranged from 3.1 for Polytrichum commune to 86.2 for Hylocomium splendens. In contrast, tissue nitrogen and phosphorus concentrations remained relatively constant throughout the year. Phosphatase activities were negatively correlated with tissue phosphorus concentration for several species, although few relationships were detected between ambient nutrient concentrations and phosphatase activity, tissue nitrogen, or tissue phosphorus concentration. These results demonstrate that phosphatase activities can provide a sensitive indicator of nutrient stress in terrestrial and semi-aquatic mosses, notably in the ectohydric Hylocomium splendens. However, further studies at sites with a wide range of nutrient levels are required to determine whether the technique can be used to indicate ambient nutrient status.     

Nutrient dynamics in small mesotrophic fens surrounded by cultivated land

In a typical Dutch polder landscape the effects of nutrient transport from cultivated grassland to mesotrophic fen communities were studied. In a comparative approach, biomass production and nutrient (N, P and K) uptake were determined monthly in four fens and a hayfield differeing in productivity and species composition. The interstitial ground water was sampled every two weeks for determinations of inorganic nutrient concentrations.The differences in productivity between the fens were clearly reflected in the amount of N, P and K taken up in the above-ground vegetation. N and P proved to be limiting plant growth in the fens, whereas K was the main limiting factor in the hayfield. The ground water welling up from the sandy bottom into the fens proved to be rich in ammonia (3–5 ppm). There are strong indications that this continual seepage leads to a considerable input of N into the fens but not to a higher productivity, as the ammonia is absorbed by the lowermost peat layers covering the sand.At this moment, the differences in productivity between the fens must be caused by differences in the rates of mineralization of the superficial peat layer. The degree of fixation of the floating vegetation mat, determining whether or not low water levels lead to an aerated soil top layer, is important in this respect. Within a period of decades, however, the continuous inflow of ammonia may eventually cause an increase in the productivity and a change in the species composition of the fens.     

Influence of temperature and soil nitrogen and phosphorus availabilities on fine-root productivity in tropical rainforests on Mount Kinabalu,Borneo

We investigated how temperature and nutrient availability regulate fine-root productivity in nine tropical rainforest ecosystems on two altitudinal gradients with contrasting soil phosphorus (P) availabilities on Mount Kinabalu, Borneo. We measured the productivity and the nutrient contents of fine roots, and analyzed the relationships between fine-root parameters and environmental factors. The fine-root net primary productivity (NPP), total NPP, and ratio of fine-root NPP to total NPP differed greatly among the sites, ranging from 72 to 228 (g m^?2 year^?1), 281–2240 (g m^?2 year^?1), and 0.06–0.30, respectively. A multiple-regression analysis suggested a positive effect of P availability on total NPP, whereas fine-root NPP was positively correlated with mean annual temperature and with P and negatively correlated with N. The biomass and longevity of fine roots increased in response to the impoverishment of soil P. The carbon (C) to P ratio (C/P) of fine roots was significantly and positively correlated with the P-use efficiency of above-ground litter production, indicating that tropical rainforest trees dilute P in fine roots to maintain the C allocation ratio to these roots. We highlighted the mechanisms regulating the fine-root productivity of tropical rainforest ecosystems in relation to the magnitude of nutrient deficiency. The trees showed C-conservation mechanisms rather than C investment as responses to decreasing soil P availability, which demonstrates that the below-ground systems at these sites are strongly limited by P, similar to the above-ground systems.     

A function of cyanobacterial mats in phosphorus-limited tropical wetlands

Cyanobacterial mats are important components of oligotrophic wetland ecosystems in the limestone-based regions of the Caribbean. Our goals were to: (1) Estimate the biomass and primary production of cyanobacterial mats, quantify the extent of nitrogen fixation and measure the activity of alkaline phosphatase (APA) in representative marshes of northern Belize; (2) Record changes in these variables following nutrient additions. The mat biomass ranged from 200 to 700 g m^–2 AFDM, with the epipelon contributing up to 87% of the total. Tissue nitrogen was similar in all marshes (1.1–1.5%), while tissue phosphorus was extremely low (0.0055–0.0129%) and well correlated with the N:P ratio in water. Nitrogen fixation expressed as nitrogenase activity was high in some marshes (17.5 nmol C₂H₄ cm^–2 h^–1) and low (< 5 nmol C₂H₄ cm^–2 h^–1) in others depending mainly on the proportion of heterocyst-forming cyanobacteria (Nostocales, Stigonematales) in the mat. Alkaline phosphatase activity was positively correlated with the N:P ratio of the mat. Experimental addition of phosphorus resulted in significant increase in primary production and nitrogen fixation while it suppressed the APA activity. The presented data clearly showed that oligotrophic marshes of northern Belize are strongly P limited. Increased input of phosphorus would profoundly change their structure and functions.     

Persistence of high elevation fens in the Southern Rocky Mountains,on Grand Mesa,Colorado, U.S.A.

Small headwater fens at high elevations exist in the dry climatic regime of western Colorado, despite increasing demands for water development since the 1800’s. Fens on Grand Mesa have accumulated plant material as peat for thousands of years due to cold temperatures and consistently saturated soils. The peatlands maintain unique plant communities, wildlife habitat, biodiversity, and carbon storage. We located and differentiated 88 fens from 15 wet meadows and 2 marshes on Grand Mesa. Field work included determining vegetation, soils, moisture regimes, and impacts from human activities. All fens were groundwater-supported systems that occurred in depressions and slopes within sedimentary landslide and volcanic glacial till landscapes. Fens occupied 400 ha or less than 1 % of the 46,845 ha research area and ranged in size from 1 to 46 ha. Peat water pH in undisturbed sites ranged from 4.3 to 7.1. Most fens had plant communities dominated by sedges (Carex) with an understory of brown mosses. Variation in vegetation was controlled by stand wetness, water table level, organic C, conductivity (EC), and temperature °C. Fen soils ranged from 13.6 to 44.1 % organic C with a mean of 30.3 %. Species diversity in fens was restricted by cold short growing seasons, stressful anaerobic conditions, and disturbance. Multivariate analysis was used to analyze relationships between vegetation, environmental, and impact variables. Stand wetness, water table level, OC, electrical conductivity (EC), and temperature were used to analyze vegetation variance in undisturbed fens, wet meadows, and marshes. Vegetation composition in impacted fens was influenced by flooding, sedimentation, stand wetness, water table level, OC, EC, and temperature. Hydrologically modified fens supported 58 plant species compared to 101 species in undisturbed fens. Analysis of historical 1936–2007 aerial photographs and condition scalars helped quantify impacts of human activities in fens as well as vegetation changes. Fourteen fens had evidence of peat subsidence, from organic soil collapse, blocks of peat in the margins, soil instability, and differences in surface peat height between the fen soil surface and the annually flooded soil surface. Of 374 ha of fens in the Grand Mesa study area, 294 ha (79 %) have been impacted by human activities such as ditching, drainage, flooding, or vehicular rutting. Many fens had little restoration potential due to severe hydrological and peat mass impacts, water rights, or the cost of restoration.     

Ecophysiological mechanisms characterising fen and bog species: focus on variations in nitrogen uptake traits under different soil–water pH

Although the productivity and nitrogen (N)-use traits of mire plants differ dramatically between fens and bogs, soil N richness does not necessarily differ, whereas the soil–water pH is distinctly lower in bogs than in fens. The ecophysiological mechanisms underlying these relations are unclear. To assess the relative availability of N forms in relation to soil–water pH, we focused on the net N uptake rate per unit root weight (NNUR), glutamine synthetase activity and nitrate reductase activity, and performed reciprocal transplant experiments with the seedlings of fen (Carex lyngbyei) and bog (C. middendorffii) sedge species in intact habitat sites. The soil–water pH was clearly lower at the bog site, but the NH₄ ⁺, NO₃ ⁻ or dissolved organic-N concentrations did not differ between the fen and bog sites. The activity of both enzymes for inorganic-N assimilation did not differ among the sites and species. However, the fen species grown at bog sites showed a drastic decrease in the NNUR, suggesting a suppression of organic-N uptake. The bog species showed no NNUR difference between the sites. These results indicate that inorganic-N availability does not differ between the two habitats, but organic-N availability is lowered in a low-pH bog, particularly in the case of fen species. Therefore, the relative availability of N forms shows species-specific variations that depend on the differences in the soil–water pH of root zone, even at similar N richness, which would play a key role in plant distribution strategies in relation to the fen-bog gradient.     

石漠化地区苔藓结皮对土壤养分及生态化学计量特征的影响

苔藓结皮是石漠化生态系统的重要地表覆被物，但其在土壤养分累积和元素循环过程中的作用尚不明确。以我国贵州典型喀斯特高原峡谷石漠化区-花江大峡谷两岸不同等级石漠化生境下的苔藓结皮及其覆被土壤为研究对象，研究了苔藓结皮覆被对土壤养分及生态化学计量特征的影响。结果表明：（1）苔藓结皮层养分含量显著高于下层土壤，结皮覆被土壤有机碳（SOC）、全氮（TN）、全磷（TP）、全钾（TK）、碱解氮（AN）、速效磷（AP）、速效钾（AK）平均含量分别为25.95、3.05、1.00、5.10 g/kg和189.61、1.59、275.10 mg/kg，较无结皮覆被的裸土分别增加46.08%、26.50%、53.62%、20.25%、25.24%、110.47%和83.76%。（2）苔藓结皮覆被土壤C/N、C/K、P/K显著高于裸土，且随土层加深而递减；N/P显著低于裸土，且随土层加深而升高。（3）苔藓结皮覆被土壤养分恢复指数为33.16%-72.48%，呈现随石漠化等级升高而增加的趋势，中度和强度石漠化阶段较无石漠化阶段分别增加83.26%和118.58。本研究表明苔藓结皮能有效促进土壤养分累积，加速石漠化土壤养分恢复进程，可作为补充手段联合其它生态恢复措施共同推动石漠化地区的生态恢复与重建。     

Effects of elevated CO2 and vascular plants on evapotranspiration in bog vegetation

We determined evapotranspiration in three experiments designed to study the effects of elevated CO₂ and increased N deposition on ombrotrophic bog vegetation. Two experiments used peat monoliths with intact bog vegetation in containers, with one experiment outdoors and the other in a greenhouse. A third experiment involved monocultures and mixtures of Sphagnum magellanicum and Eriophorum angustifolium in containers in the same greenhouse. To determine water use of the bog vegetation in July–August for each experiment and each year we measured water inputs and outputs from the containers. We studied the effects of elevated CO₂ and N supply on evapotranspiration in relation to vascular plant biomass and exposure of the moss surface (measured as height of the moss surface relative to the container edge). Elevated CO₂ reduced water use of the bog vegetation in all three experiments, but the CO₂ effect on evapotranspiration interacted with vascular plant biomass and exposure of the moss surface. Evapotranspiration in the outdoor experiment was largely determined by evaporation from the Sphagnum moss surface (as affected by exposure to wind) and less so by vascular plant transpiration. Nevertheless, elevated CO₂ significantly reduced evapotranspiration by 9–10% in the outdoor experiment. Vascular plants reduced evapotranspiration in the outdoor experiment, but increased water use in the greenhouse experiments. The relation between vascular plant abundance and evapotranspiration appears to depend on wind conditions; suggesting that vascular plants reduce water losses mainly by reducing wind speed at the moss surface. Sphagnum growth is very sensitive to changes in water level; low water availability can have deleterious effects. As a consequence, reduced evapotranspiration in summer, whether caused by elevated CO₂ or by small increases in vascular plant cover, is expected to favour Sphagnum growth in ombrotrophic bog vegetation.     

Protozoan Colonization Rates and Trophic Status of Some Freshwater Wetland Lakes

Primary productivity, chlorophyll a, phosphorus and nitrogen nutrients, and other chemical and physical parameters were measured in 13 wetland lakes in northern lower Michigan. These lakes included several examples located in each of the four major wetland types—bogs, fens, marshes, and swamps. Of the four types, the brown-colored waters of the acid bog lakes generally had the highest levels of primary productivity, chlorophyll a, phosphorus, and nitrogen. Primary productivity correlated positively with water color, total-N, and NH₃-N (α≤ 0.05). By these measures, waters of the bogs were the most eutrophic of the four types of wetland lakes. These findings would seem to contradict the generally-held concept that “dystrophic” bog lakes are extremely oligotrophic. Protozoan colonization of artificial substrate islands was monitored at each wetland site. The correlation between protozoan colonization rates (G values in the MacArthur-Wilson noninteractive model) and primary productivity, measured by 24-h light and dark bottle incubations, was significant at the 95% confidence level (r= 0.850, P= 0.001) and with water color at the 90% confidence level (r= 0.599, P= 0.084). It was concluded that protozoan colonization rate was an excellent indicator of the trophic status of wetland lakes.