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1.
P. W. Inglett E. M. D’Angelo K. R. Reddy P. V. McCormick S. E. Hagerthey 《Wetlands Ecology and Management》2009,17(2):131-144
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. 相似文献
2.
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. 相似文献
3.
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. 相似文献4.
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. 相似文献
5.
Effects of hydrologic and water quality drivers on periphyton dynamics in the southern Everglades 总被引:2,自引:2,他引:0
David M. Iwaniec Daniel L. Childers Damon Rondeau Christopher J. Madden Colin Saunders 《Hydrobiologia》2006,569(1):223-235
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. 相似文献
6.
Ecosystem nitrogen fixation throughout the snow‐free period in subarctic tundra: effects of willow and birch litter addition and warming
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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. 相似文献
7.
Hydrological and nutrient budgets of freshwater and estuarine wetlands of Taylor Slough in Southern Everglades, Florida (U.S.A.) 总被引:4,自引:0,他引:4
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. 相似文献
8.
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 (%Ndfa) 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%Ndfa, 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. 相似文献
9.
J. Holst C. Liu Z. Yao N. Brüggemann X. Zheng M. Giese K. Butterbach-Bahl 《Plant and Soil》2008,309(1-2):105-117
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. 相似文献
10.
Catabolic diversity of periphyton and detritus microbial communities in a subtropical wetland 总被引:1,自引:0,他引:1
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. 相似文献
11.
《Environmental and Experimental Botany》2006,55(1-2):97-109
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. 相似文献
12.
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. 相似文献
13.
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. 相似文献
14.
Investigating patterns of symbiotic nitrogen fixation during vegetation change from grassland to woodland using fine scale δ15N measurements
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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 δ15Nxylem sap and δ15Nsoil 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 δ15Nsoil 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. 相似文献
15.
System controls on the aqueous distribution of mercury in the northern Florida Everglades 总被引:2,自引:0,他引:2
James P. Hurley David P. Krabbenhoft Lisa B. Cleckner Mark L. Olson George R. Aiken Peter S. Rawlik Jr. 《Biogeochemistry》1998,40(2-3):293-311
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 (HgTU) 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, HgTU is usually <5 ng L-1 with no consistent north–south patterns. However, for individual dates, aqueous unfiltered MeHg (MeHgU) 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. 相似文献
16.
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. 相似文献
17.
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. 相似文献
18.
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. 相似文献
19.
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. 相似文献
20.
Patrick Saccone Samuel Morin Florence Baptist Jean-Marc Bonneville Marie-Pascale Colace Florent Domine Mathieu Faure Roberto Geremia Jonathan Lochet Franck Poly Sandra Lavorel Jean-Christophe Clément 《Plant and Soil》2013,362(1-2):215-229