Periphyton nitrogenase activity as an indicator of wetland eutrophication: spatial patterns and response to phosphorus dosing in a northern Everglades ecosystem

The use of periphyton nitrogenase activity (biological N₂ fixation) as an indicator of wetland P impact was assessed using patterns of nutrient content (C, N, P, Ca, Mg, K, Fe, and Mn) and acetylene reduction (AR) in floating cyanobacterial periphyton mat (metaphyton) communities of a P-enriched portion of the Florida Everglades, USA (Water Conservation Area-2A, WCA-2A). Spatial patterns of nutrients indicate the enrichment of floating mat periphyton N, P, Fe, and K, and the reduction of Mn and TN:TP in enriched marsh areas. In highly enriched areas, floating mat periphyton AR was approximately threefold greater than that in less enriched, interior marsh zones. Multiple regression models indicated AR dependence on P in eutrophic WCA-2A areas while the AR of more interior marsh periphyton mats was more closely related to tissue levels of Ca and Fe. Nitrogenase activity of floating mat periphyton from P-loaded mesocosms revealed a significant enhancement of N₂ fixation in samples receiving approximately 2–3 mg P m⁻² of cumulative P dosing or with biomass TP content of 100–300 mg kg⁻¹. At P contents above the optimum, mat periphyton AR was suppressed possibly as a result of changes in species composition or increased levels of NH₄⁺. After 3 years of dosing, consistently high AR occurred only at low rates of P enrichment (0.4–0.8 g P m⁻² yr⁻¹), and the patterns appeared to be seasonal. These findings agree with the hypothesis that P availability is a key determinant of nitrogenase activity in aquatic systems, and thus, may support the use of periphyton nitrogenase to indicate P impacts in P-limited systems. These results also demonstrate the potential existence of a P threshhold for biogeochemical alteration of periphyton mat function in the Everglades, and that cumulative loading of limiting nutrients (i.e., P), rather than instantaneous concentrations, should be considered when evaluating nutrient criteria.     

N2 fixation and nitrogen allocation to above and below ground plant parts in red clover-grasslands

Leys, used for grazing or production of forage to be conserved as silage or hay, are very important crops in northern areas. In order to measure the N₂ fixation in leys of varying ages and during different parts of the season, detailed measurements were taken of yield, N₂ fixation and the amounts of N remaining in the field after harvesting red clover (Trifolium pratense L.)-grass leys at a site in northern Sweden, where they are generally harvested twice per growing season. Entire plants, including stubble and roots, were sampled at the time of first and second harvest and, in addition, at the end of the growing season in three neighbouring fields, carrying a first, a second and a third year ley, respectively. N₂ fixation was measured by both ¹⁵N isotope dilution (ID) and ¹⁵N natural abundance (NA) methods. The proportion of clover dry matter (DM) in the stands increased from the first to the second harvest, but the grasses dominated throughout the entire season, especially below ground. The N concentrations, in both herbage and whole plants, were about twice as high in the clover as in the grasses. Seasonal variations in N concentrations were minor, and total N contents followed the same trends as DM. The clover acquired nearly all of its N from N₂ fixation: the proportion of N in clover herbage derived from N₂ fixation was often >0.8 throughout the season. The variations in the amounts of N₂ fixed during the course of the season corresponded well to the seasonal changes in clover biomass. Amounts of fixed N₂ allocated to clover herbage during the whole season were in the range 4 to 6 g N m⁻² in this unusually rainy year. Calculations of daily N allocation rates to herbage showed that N uptake rates were similar, and high, in grasses during May–June and July–August, while N₂ fixation rates in clover were about 10-fold as high in July–August as in May–June, reflecting the need for N in clover growth. The proportion of N remaining in clover stubble and roots after the first and second harvests was about 60 and 25%, respectively, while about 60% of the N in grasses remained in stubble and roots after both harvests. The considerable amounts of biomass and N that were left in field after harvesting red clover-grass leys are important for re-growth of the plants and provide substantial N fertilization for the next crop in the crop rotation.     

Seasonal variation in nitrogen fixation and effects of climate change in a subarctic heath

Background and aims

Nitrogen fixation associated with cryptogams is potentially very important in arctic and subarctic terrestrial ecosystems, as it is a source of new nitrogen (N) into these highly N limited systems. Moss-, lichen- and legume-associated N₂ fixation was studied with high frequency (every second week) during spring, summer, autumn and early winter to uncover the seasonal variation in input of atmospheric N₂ to a subarctic heath with an altered climate.

Methods

We estimated N₂ fixation from ethylene production by acetylene reduction assay in situ in a field experiment with the treatments: long- vs. short-term summer warming using plastic tents and litter addition (simulating expansion of the birch forest).

Results

N₂ fixation activity was measured from late April to mid November and 33 % of all N₂ was fixed outside the vascular plant growing season (Jun–Aug). This substantial amount underlines the importance of N₂ fixation in the cold period. Warming increased N₂ fixation two- to fivefold during late spring. However, long-term summer warming tended to decrease N₂ fixation outside the treatment (tents present) period. Litter alone did not alter N₂ fixation but in combination with warming N₂ fixation increased, probably because N₂ fixation became phosphorus limited under higher temperatures, which was alleviated by the P supply from the litter.

Conclusion

In subarctic heath, the current N₂ fixation period extends far beyond the vascular plant growing season. Climate warming and indirect effects such as vegetation changes affect the process of N₂ fixation in different directions and thereby complicate predictions of future N cycling.     

Effect of nitrogen fertilization and cropping system of the reference crop on estimation of N2 fixation by vetch using15N methodology

This study was conducted to examine the effects of varying N rates and cropping systems (mixedversus pure stand) on the suitability of oats (Avena sativa L.) for estimating N₂ fixed in sequentially harvested vetch (Vicia sativa L.) over two growing seasons (1984–85 and 1985–86). The N rates were, 20 and 100 kg N ha^–1 in 1984–85 and 15 and 60 kg N ha^–1 in 1985–86. In the 1984–85 season, vetch at maturity derived 76 and 63% N from fixation at the high and low N rates respectively. The corresponding values for the second season were 66 and 42%. Except in the 1985–86 season when some significantly higher values of % N₂ fixed were estimated by using the reference crop grown at the higher (A-value approach) than at the lower N rate (isotope-dilution approach), both approaches resulted in similar measurements of N₂ fixed. In the 1984–85 season, similar values of N₂ fixed were obtained using either the pure or mixed stand oats reference crops. Although in the 1985–86 season, the mixed reference crop occasionally estimated lower % N₂ fixed than pure oats, total N₂ fixed estimates were always similar (P<0.05). Thus, in general, N fertilization and cropping system of the reference crop did not significantly influence estimates of N₂ fixation.     

Effects of hydrologic and water quality drivers on periphyton dynamics in the southern Everglades

Everglades periphyton mats are tightly-coupled autotrophic (algae and cyanobacteria) and heterotrophic (eubacteria, fungi and microinvertebrates) microbial assemblages. We investigated the effect of water column total phosphorus and nitrogen concentrations, water depth and hydroperiod on periphyton of net production, respiration, nutrient content, and biomass. Our study sites were located along four transects that extended southward with freshwater sheetflow through sawgrass-dominated marsh. The water source for two of the transects were canal-driven and anchored at canal inputs. The two other transects were rain-driven (ombrotrophic) and began in sawgrass-dominated marsh. Periphyton dynamics were examined for upstream and downstream effects within and across the four transects. Although all study sites were characterized as short hydroperiod and phosphorus-limited oligotrophic, they represent gradients of hydrologic regime, water source and water quality of the southern Everglades. Average periphyton net production of 1.08 mg C AFDW⁻¹ h⁻¹ and periphyton whole system respiration of 0.38 mg C AFDW⁻¹ h⁻¹ rates were net autotrophic. Biomass was generally highest at ombrotrophic sites and sites downstream of canal inputs. Mean biomass over all our study sites was high, 1517.30 g AFDW m⁻². Periphyton was phosphorus-limited. Average periphyton total phosphorus content was 137.15 μg P g⁻¹ and average periphyton total N:P ratio was 192:1. Periphyton N:P was a sensitive indicator of water source. Even at extremely low mean water total phosphorus concentrations ( ≤ 0.21 μmol l⁻¹), we found canal source effects on periphyton dynamics at sites adjacent to canal inputs, but not downstream of inflows. These canal source effects were most pronounced at the onset of wet season with initial rewetting. Spatial and temporal variability in periphyton dynamics could not solely be ascribed to water quality, but was often associated with both hydrology and water source.     

Ecosystem nitrogen fixation throughout the snow‐free period in subarctic tundra: effects of willow and birch litter addition and warming

Nitrogen (N) fixation in moss‐associated cyanobacteria is one of the main sources of available N for N‐limited ecosystems such as subarctic tundra. Yet, N₂ fixation in mosses is strongly influenced by soil moisture and temperature. Thus, temporal scaling up of low‐frequency in situ measurements to several weeks, months or even the entire growing season without taking into account changes in abiotic conditions cannot capture the variation in moss‐associated N₂ fixation. We therefore aimed to estimate moss‐associated N₂ fixation throughout the snow‐free period in subarctic tundra in field experiments simulating climate change: willow (Salix myrsinifolia) and birch (Betula pubescens spp. tortuosa) litter addition, and warming. To achieve this, we established relationships between measured in situ N₂ fixation rates and soil moisture and soil temperature and used high‐resolution measurements of soil moisture and soil temperature (hourly from May to October) to model N₂ fixation. The modelled N₂ fixation rates were highest in the warmed (2.8 ± 0.3 kg N ha^?1) and birch litter addition plots (2.8 ± 0.2 kg N ha^?1), and lowest in the plots receiving willow litter (1.6 ± 0.2 kg N ha^?1). The control plots had intermediate rates (2.2 ± 0.2 kg N ha^?1). Further, N₂ fixation was highest during the summer in the warmed plots, but was lowest in the litter addition plots during the same period. The temperature and moisture dependence of N₂ fixation was different between the climate change treatments, indicating a shift in the N₂ fixer community. Our findings, using a combined empirical and modelling approach, suggest that a longer snow‐free period and increased temperatures in a future climate will likely lead to higher N₂ fixation rates in mosses. Yet, the consequences of increased litter fall on moss‐associated N₂ fixation due to shrub expansion in the Arctic will depend on the shrub species’ litter traits.     

Hydrological and nutrient budgets of freshwater and estuarine wetlands of Taylor Slough in Southern Everglades, Florida (U.S.A.)

Hydrological restoration of the Southern Everglades will result in increased freshwater flow to the freshwater and estuarine wetlands bordering Florida Bay. We evaluated the contribution of surface freshwater runoff versus atmospheric deposition and ground water on the water and nutrient budgets of these wetlands. These estimates were used to assess the importance of hydrologic inputs and losses relative to sediment burial, denitrification, and nitrogen fixation. We calculated seasonal inputs and outputs of water, total phosphorus (TP) and total nitrogen (TN) from surface water, precipitation, and evapotranspiration in the Taylor Slough/C-111 basin wetlands for 1.5 years. Atmospheric deposition was the dominant source of water and TP for these oligotrophic, phosphorus-limited wetlands. Surface water was the major TN source of during the wet season, but on an annual basis was equal to the atmospheric TN deposition. We calculated a net annual import of 31.4 mg m^–2 yr^–1 P and 694 mg m^–2 yr^–1N into the wetland from hydrologic sources. Hydrologic import of P was within range of estimates of sediment P burial (33–70 mg m^–2 yr^–1 P), while sediment burial of N (1890–4027 mg m^–2 yr^–1 N) greatly exceeded estimated hydrologic N import. High nitrogen fixation rates or an underestimation of groundwater N flux may explain the discrepancy between estimates of hydrologic N import and sediment N burial rates.     

Symbiotic N2 fixation of Alnus incana ssp. rugosa in shrub wetlands of the Adirondack Mountains, New York, USA

Surface waters in forested watersheds in the Adirondack Mountains and northern New York State are susceptible to nitrogen (N) saturation. Atmospheric deposition of N to watersheds in this region has been measured but the extent of internal N inputs from symbiotic N₂ fixation in alder-dominated wetlands is not known. We estimated N₂ fixation by speckled alder in these wetlands by the ¹⁵N natural abundance method and by acetylene reduction using a flow-through system. Foliar N derived from fixation (%N_dfa) was estimated for five wetlands. The '¹⁵N of speckled alder foliage from four of the five sites did not differ significantly (PА.05) from that of nodulated speckled alders grown in N-free water culture (-1.2ǂ.1‰). Estimates from the ¹⁵N natural abundance method indicated that alders at these sites derive 85-100% of their foliar N from N₂ fixation. At one of the sites, we also measured biomass and N content and estimated that the alder foliage contained 43 kg N ha^-1 of fixed N in 1997. This estimate was based on a foliar N content of 55.4lj kg N ha^-1 (mean-SE), 86dž%N_dfa, and an assumption that 10% of foliar N was derived from reserves in woody tissues. At this site, we further estimated via acetylene reduction that 37ᆞ kg N ha^-1 was fixed by speckled alders in 1998. This estimate used the theoretical 4:1 C₂H₂ reduction to N₂ fixation ratio and assumed no night-time fixation late in the season. Nitrogen inputs in wet and dry deposition at this site are approximately 8 kg N ha^-1 year^-1. We conclude that speckled alder in wetlands of northern New York State relies heavily on N₂ fixation to meet N demands, and symbiotic N₂ fixation in speckled alders adds substantial amounts of N to alder-dominated wetlands in the Adirondack Mountains. These additions may be important for watershed N budgets, where alder-dominated wetlands occupy a large proportion of watershed area.     

Fluxes of nitrous oxide, methane and carbon dioxide during freezing–thawing cycles in an Inner Mongolian steppe

Combined measurements of nitrification activity and N₂O emissions were performed in a lowland and a montane tropical rainforest ecosystem in NE-Australia over a 18 months period from October 2001 until May 2003. At both sites gross nitrification rates, measured by the BaPS technique, showed a strong seasonal pattern with significantly higher rates of gross nitrification during wet season conditions. Nitrification rates at the montane site (1.48?±?0.24–18.75?±?2.38 mg N kg^?1 day^?1) were found to be significantly higher than at the lowland site (1.65?±?0.21–4.54?±?0.27 mg N kg^?1 day^?1). The relationship between soil moisture and gross nitrification rates could be described best by O’Neill functions having a soil moisture optimum of nitrification at app. 65% WFPS. At the lowland site, for which continuous measurements of N₂O emissions were available, nitrification was positively correlated with N₂O emission. Nitrification contributed significantly to N₂O formation during dry season (app.85%) but less (app. 30%) during wet season conditions. In average 0.19‰ of the N metabolized by nitrification was released as N₂O. The N₂O fraction loss for nitrification was positively correlated with changes in soil moisture and varied slightly between 0.15 and 0.22‰. Our results demonstrate that combined N₂O emission and microbial N turnover studies covering prolonged observation periods are needed to clarify and quantify the role of the microbial processes nitrification and denitrification for annual N₂O emissions from soils of terrestrial ecosystems.     

Catabolic diversity of periphyton and detritus microbial communities in a subtropical wetland

The catabolic diversity of wetland microbial communities may be a sensitive indicator of nutrient loading or changes in environmental conditions. The objectives of this study were to assess the response of periphyton and microbial communities in water conservation area-2a (WCA-2a) of the Everglades to additions of C-substrates and inorganic nutrients. Carbon dioxide and CH₄ production rates were measured using 14 days incubation for periphyton, which typifies oligotrophic areas, and detritus, which is prevalent at P-impacted areas of WCA-2a. The wetland was characterized by decreasing P levels from peripheral to interior, oligotrophic areas. Microbial biomass and N mineralization rates were higher for oligotrophic periphyton than detritus. Methane production rates were also higher for unamended periphyton (80 mg CH₄-C kg⁻¹ d⁻¹) than detritus (22 mg CH₄-C kg⁻¹ d⁻¹), even though the organic matter content was higher for detritus (80%) than periphyton (69%). Carbon dioxide production for unamended periphyton (222 mg CO₂-C kg⁻¹ d⁻¹) was significantly greater than unamended detritus (84 mg CO₂-C kg⁻¹ d⁻¹). The response of the heterotrophic microbial community to added C-substrates was related to the nutrient status of the wetland, as substrate-induced respiration (SIR) was higher for detritus than periphyton. Amides and polysaccharides stimulated SIR more than other C-substrates, and methanogenesis was greater contributor to SIR for periphyton than detritus. Inorganic P addition stimulated CO₂ and CH₄ production for periphyton but not detritus, indicating a P limitation in the interior areas of WCA-2a. Continued nutrient loading into oligotrophic areas of WCA-2a or enhanced internal nutrient cycling may stimulate organic matter decomposition and further contribute to undesirable changes to the Everglades ecosystem caused by nutrient enrichment.     

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.     

Diazotrophy in Alluvial Meadows of Subarctic River Systems

There is currently limited understanding of the contribution of biological N₂ fixation (diazotrophy) to the N budget of large river systems. This natural source of N in boreal river systems may partially explain the sustained productivity of river floodplains in Northern Europe where winter fodder was harvested for centuries without fertilizer amendments. In much of the world, anthropogenic pollution and river regulation have nearly eliminated opportunities to study natural processes that shaped early nutrient dynamics of large river systems; however, pristine conditions in northern Fennoscandia allow for the retrospective evaluation of key biochemical processes of historical significance. We investigated biological N₂ fixation (diazotrophy) as a potential source of nitrogen fertility at 71 independent floodplain sites along 10 rivers and conducted seasonal and intensive analyses at a subset of these sites. Biological N₂ fixation occurred in all floodplains, averaged 24.5 kg N ha⁻¹ yr⁻¹ and was down regulated from over 60 kg N ha⁻¹ yr⁻¹ to 0 kg N ha⁻¹ yr⁻¹ by river N pollution. A diversity of N₂-fixing cyanobacteria was found to colonize surface detritus in the floodplains. The data provide evidence for N₂ fixation to be a fundamental source of new N that may have sustained fertility at alluvial sites along subarctic rivers. Such data may have implications for the interpretation of ancient agricultural development and the design of contemporary low-input agroecosystems.     

Periphyton chemistry and nitrogenase activity in a northern Everglades ecosystem

Cyanobacterial peri­phyton communities are a dominant feature of oligotrophic Everglades marshes, however, little is known regarding the biogeochemical aspects of this ecosystem component. This study was undertaken to investigate the potential for N₂ fixation in the peri­phyton communities of a hydrologically-controlled portion of the northern Everglades marsh (Water Conservation Area 2A, WCA-2A). The objectives of this research were to characterize the temporal patterns of nutrient composition and N₂ fixation of the natural WCA-2A peri­phyton communities and to compare fixation rates of peri­phyton with those of other ecosystem components in both natural and nutrient-impacted WCA-2A areas. In general, N₂ fixation (measured by the acetylene reduction (AR) method) of natural WCA-2A peri­phyton was enhanced under light conditions showing a nitrogenase pattern characteristic of autotrophic cyanobacteria. Winter (November–March) rates of AR expressed per gram organic carbon (gOC) ranged from 147–240 nmol C₂H₂ g OC^–1 h^–1, while summer rates were elevated with an observed peak of 1148 nmol C₂H₂ g OC^–1 h^–1 in July 1998. This translates into an estimated yearly contribution of approximately 10 g N m^–2 to an unimpacted WCA-2A slough ecosystem. Nitrogenase activity did not correlate seasonally with nutrients (Ca, Mg, Fe, N, P, Mn), but closely followed measured N stable isotopic ratios (¹⁵N) in floating peri­phyton. In oligotrophic marsh areas, AR (on a weight basis) decreased in the order floating peri­phyton > benthic peri­phyton floc > soil > water > detrital plant biomass, while highest AR rates were observed for detrital biomass in areas impacted by agricultural discharges.     

Investigating patterns of symbiotic nitrogen fixation during vegetation change from grassland to woodland using fine scale δ15N measurements

Biological nitrogen fixation (BNF) in woody plants is often investigated using foliar measurements of δ¹⁵N and is of particular interest in ecosystems experiencing increases in BNF due to woody plant encroachment. We sampled δ¹⁵N along the entire N uptake pathway including soil solution, xylem sap and foliage to (1) test assumptions inherent to the use of foliar δ¹⁵N as a proxy for BNF; (2) determine whether seasonal divergences occur between δ¹⁵N_{xylem sap} and δ¹⁵N_{soil inorganic N} that could be used to infer variation in BNF; and (3) assess patterns of δ¹⁵N with tree age as indicators of shifting BNF or N cycling. Measurements of woody N‐fixing Prosopis glandulosa and paired reference non‐fixing Zanthoxylum fagara at three seasonal time points showed that δ¹⁵N_{soil inorganic N} varied temporally and spatially between species. Fractionation between xylem and foliar δ¹⁵N was consistently opposite in direction between species and varied on average by 2.4‰. Accounting for these sources of variation caused percent nitrogen derived from fixation values for Prosopis to vary by up to ～70%. Soil–xylem δ¹⁵N separation varied temporally and increased with Prosopis age, suggesting seasonal variation in N cycling and BNF and potential long‐term increases in BNF not apparent through foliar sampling alone.     

System controls on the aqueous distribution of mercury in the northern Florida Everglades

The forms and partitioning of aqueous mercury species in the canals and marshes of the Northern Florida Everglades exhibit strong spatial and temporal variability. In canals feeding Water Conservation Area (WCA) 2A, unfiltered total Hg (HgT_U) is less than 3 ng L^-1 and relatively constant. In contrast, methyl mercury (MeHg) exhibited a strong seasonal pattern, with highest levels entering WCA-2A marshes during July. Stagnation and reduced flows also lead to particle enrichment of MeHg. In the marshes of WCA-2A, 2B and 3A, HgT_U is usually <5 ng L^-1 with no consistent north–south patterns. However, for individual dates, aqueous unfiltered MeHg (MeHg_U) levels increase from north to south with generally lowest levels in the eutrophied regions of northern WCA-2A. A strong relationship between filtered Hg species and dissolved organic carbon (DOC), evident for rivers draining wetlands in Wisconsin, was not apparent in the Everglades, suggesting either differences in the binding sites of DOC between the two regions, or non-organic Hg complexation in the Everglades.     

Nitrogen fixation varies spatially and seasonally in linked stream-lake ecosystems

We performed surveys of nitrogen (N₂)-fixation in three oligotrophic lake-stream systems in the Sawtooth Mountains of central Idaho to address two questions: (1) Which habitat types within linked lake-stream systems (lake pelagic, lake benthic, and stream) exhibit the highest rates of N₂ fixation?, and (2) How does N₂ fixation compare to the hydrologic flux of nitrogen? A seasonal survey showed that N₂ fixation in a single lake and its outlet stream peaked in late summer, when hydrologic N fluxes were lowest. Benthic lake N₂-fixation rates by epiphytes were highest at mid-lake depths, where their percent cover was highest, while rates by epipelon were greatest at shallow lake depths. Pelagic N₂ fixation was below detection. Stream N₂-fixation rates were greatest on rock substrates and in the lake outlet stream. These patterns were supported by a baseflow survey (late July) in three lake-stream ecosystems which confirmed that N₂-fixation rates peaked in the lake benthos at shallow depths and on rock substrates in outlet streams. Scaling N₂-fixation rates to whole lake and stream areas revealed that N₂ fixation could exceed the nitrate, and sometimes the total dissolved nitrogen flux during baseflow in lakes and outlet streams. Despite low rates, total N₂-fixation contributions (kg/day) from lakes were greater because they had far larger surface areas than the stream environments. Fixed nitrogen contributions from stream outlets were also relatively high because of high N₂-fixation rates and despite low surface areas. This study suggests that N₂ fixation could be a seasonally important nitrogen source to nutrient deficient subalpine lake-stream ecosystems. In addition, the frequency and location of lakes could control N₂-fixation contributions to watersheds by providing a large area for within-lake N₂ fixation, and creating conditions favorable for N₂ fixation in outlet streams.     

The role of N<Subscript>2</Subscript> fixation in alleviating N limitation in wetland metaphyton: enzymatic,isotopic, and elemental evidence

The role of nitrogen (N₂) fixation in balancing N supply to wetland metaphyton was assessed by comparing primary production with enzymatic, isotopic, and elemental correlates. Primary production, N₂ fixation (acetylene reduction, AR), phosphatase activity, C:N:P ratio, and N isotopic composition of metaphyton were measured along a nutrient gradient in a freshwater marsh during May through September 2004. N₂ fixation and phosphatase activity in metaphyton were negatively correlated with inorganic N and P concentrations, respectively. Although metaphyton N₂ fixation demonstrated a clear spatial pattern along the nutrient gradient, N₂ fixation rates varied monthly and decreased sharply in September. However, the percent contribution of N₂ fixation to N uptake by metaphyton consistently decreased throughout the summer. Furthermore, the decreased contribution of N₂ fixation to N uptake corresponded with an increase in metaphyton N content during the growing season. Nitrogen isotopic data suggested the sustained importance of an atmospheric N₂ source through September at the most downstream (nutrient poor) site even though the percent contribution of N₂ fixition to N uptake was lowest in that month. This suggests that metaphyton were efficiently accumulating and recycling fixed N₂ in support of primary production. Over the course of the summer, metaphyton primary production showed a weak inverse correlation with metaphyton phosphatase activity (r ² = 0.58). The largest residuals in this regression corresponded to the largest vartiation in metaphyton N content. When metaphyton primary production was normalized to metaphyton N content, production rates for the entire growing season were more strongly inversely correlated with metaphyton phosphatase activity (r ² = 0.78). Results of the study suggest that N₂ fixation in N poor areas may adequately supplement community N requirements in metaphyton, thereby causing limitation by other elemental resources such as P.     

Significant nonsymbiotic nitrogen fixation in Patagonian ombrotrophic bogs

Nitrogen (N) nutrition in pristine peatlands relies on the natural input of inorganic N through atmospheric deposition or biological dinitrogen (N₂) fixation. However, N₂ fixation and its significance for N cycling, plant productivity, and peat buildup are mostly associated with the presence of Sphagnum mosses. Here, we report high nonsymbiotic N₂‐fixation rates in two pristine Patagonian bogs with diversified vegetation and natural N deposition. Nonsymbiotic N₂ fixation was measured in samples from 0 to 10, 10 to 20, and 40 to 50 cm depth using the ¹⁵N₂ assay as well as the acetylene reduction assay (ARA). The ARA considerably underestimated N₂ fixation and can thus not be recommended for peatland studies. Based on the ¹⁵N₂ assay, high nonsymbiotic N₂‐fixation rates of 0.3–1.4 μmol N₂ g^?1 day^?1 were found down to 50 cm under micro‐oxic conditions (2 vol.%) in samples from plots covered by Sphagnum magellanicum or by vascular cushion plants, latter characterized by dense and deep aerenchyma roots. Peat N concentrations point to greater potential of nonsymbiotic N₂ fixation under cushion plants, likely because of the availability of easily decomposable organic compounds and oxic conditions in the rhizosphere. In the Sphagnum plots, high N₂ fixation below 10 cm depth rather reflects the potential during dry periods or low water level when oxygen penetrates the top peat layer and triggers peat mineralization. Natural abundance of the ¹⁵N isotope of live Sphagnum (5.6 δ‰) from 0 to 10 cm points to solely N uptake from atmospheric deposition and nonsymbiotic N₂ fixation. A mean ¹⁵N signature of ?0.7 δ‰ of peat from the cushion plant plots indicates additional N supply from N mineralization. Our findings suggest that nonsymbiotic N₂ fixation overcomes N deficiency in different vegetation communities and has great significance for N cycling and peat accumulation in pristine peatlands.     

The Role of Nitrogen Fixation in Cyanobacterial Bloom Toxicity in a Temperate,Eutrophic Lake

Toxic cyanobacterial blooms threaten freshwaters worldwide but have proven difficult to predict because the mechanisms of bloom formation and toxin production are unknown, especially on weekly time scales. Water quality management continues to focus on aggregated metrics, such as chlorophyll and total nutrients, which may not be sufficient to explain complex community changes and functions such as toxin production. For example, nitrogen (N) speciation and cycling play an important role, on daily time scales, in shaping cyanobacterial communities because declining N has been shown to select for N fixers. In addition, subsequent N pulses from N₂ fixation may stimulate and sustain toxic cyanobacterial growth. Herein, we describe how rapid early summer declines in N followed by bursts of N fixation have shaped cyanobacterial communities in a eutrophic lake (Lake Mendota, Wisconsin, USA), possibly driving toxic Microcystis blooms throughout the growing season. On weekly time scales in 2010 and 2011, we monitored the cyanobacterial community in a eutrophic lake using the phycocyanin intergenic spacer (PC-IGS) region to determine population dynamics. In parallel, we measured microcystin concentrations, N₂ fixation rates, and potential environmental drivers that contribute to structuring the community. In both years, cyanobacterial community change was strongly correlated with dissolved inorganic nitrogen (DIN) concentrations, and Aphanizomenon and Microcystis alternated dominance throughout the pre-toxic, toxic, and post-toxic phases of the lake. Microcystin concentrations increased a few days after the first significant N₂ fixation rates were observed. Then, following large early summer N₂ fixation events, Microcystis increased and became most abundant. Maximum microcystin concentrations coincided with Microcystis dominance. In both years, DIN concentrations dropped again in late summer, and N₂ fixation rates and Aphanizomenon abundance increased before the lake mixed in the fall. Estimated N inputs from N₂ fixation were large enough to supplement, or even support, the toxic Microcystis blooms.     

The effects of snowpack properties and plant strategies on litter decomposition during winter in subalpine meadows

Background and aims

Approximately 50 % of belowground organic carbon is present in the northern permafrost region and due to changes in climate there are concerns that this carbon will be rapidly released to the atmosphere. The release of carbon in arctic soils is thought to be intimately linked to the N cycle through the N cycle’s influence on microbial activity. The majority of new N input into arctic systems occurs through N₂-fixation; therefore, N₂-fixation may be the key driver of greenhouse gases from these ecosystems.

Methods

At Alexandra Fjord lowland, Ellesmere Island, Canada concurrent measurements of N₂-fixation, N mineralization and nitrification rates, dissolved organic soil N (DON) and C, inorganic soil N and surface greenhouse gas fluxes (CO₂, N₂O and CH₄) were taken in two ecosystem types (Wet Sedge Meadow and Dryas Heath) over the 2009 growing season (June-August). Using Structural Equation Modelling we evaluated the hypothesis that CO₂, CH₄ and N₂O flux are linked to N₂-fixation via the N cycle.

Results

The soil N cycle was linked to CO₂ flux in the Dryas Heath ecosystem via DON concentrations, but there was no link between the soil N cycle and CO₂ flux in the Wet Sedge Meadow. Methane flux was also not linked to the soil N cycle, nor surface soil temperature or moisture in either ecosystem. The soil N cycle was closely linked to N₂O emissions but via nitrification in the Wet Sedge Meadow and inorganic N in the Dryas Heath, indicating the important role of nitrification in net N₂O flux from arctic ecosystems.

Conclusions

Our results should be interpreted with caution given the high variability in both the rates of the N cycling processes and greenhouse gas flux found in both ecosystems over the growing season. However, while N₂-fixation and other N cycling processes may play a more limited role in instantaneous CO₂ emissions, these processes clearly play an important role in controlling N₂O emissions.