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1.
To investigate the variability of primary production of boreal forest ecosystems under the current climatic changes, we compared
the dynamics of annual increments and productivity of the main components of plant community (trees, shrubs, mosses) at three
sites in the north of Siberia (Russia). Annual radial growth of trees and shrubs was mostly defined by summer temperature
regime (positive correlation), but climatic response of woody plants was species specific and depends on local conditions.
Dynamics of annual increments of mosses were opposite to tree growth. The difference in climatic response of the different
vegetation components of the forest ecosystems indicates that these components seem to be adapted to use climatic conditions
during the short and severe northern summer, and decreasing in annual production of one component is usually combined with
the increase of other component productivity. Average productivity in the northern forest ecosystems varies from 0.05 to 0.14 t ha−1 year−1 for trees, from 0.05 to 0.18 t ha−1 year−1 for shrubs and from 0.54 to 0.66 t ha−1 year−1 for mosses. Higher values of tree productivity combined with lower annual moss productivity were found in sites in northern
taiga in comparison with forest-tundra. Different tendencies in the productivity of the dominant species from each vegetation
level (trees, shrubs, mosses) were indicated for the last 10 years studied (1990–1999): while productivity of mosses is increasing,
productivity of trees is decreasing, but there is no obvious trend in the productivity of shrubs. Our results show that in
the long term, the main contribution to changes in annual biomass productivity in forest-tundra and northern taiga ecosystems
under the predicted climatic changes will be determined by living ground cover. 相似文献
2.
Robert G. Björk Leif Klemedtsson Ulf Molau Jan Harndorf Anja Ödman Reiner Giesler 《Plant and Soil》2007,294(1-2):247-261
The spatial distribution of organic soil nitrogen (N) in alpine tundra was studied along a natural environmental gradient,
covering five plant communities, at the Latnjajaure Field Station, northern Swedish Lapland. The five communities (mesic meadow,
meadow snowbed, dry heath, mesic heath, and heath snowbed) are the dominant types in this region and are differentiated by
soil pH. Net N mineralization, net ammonification, and net nitrification were measured using 40-day laboratory incubations
based on extractable NH4+ and NO3−. Nitrification enzyme activity (NEA), denitrification enzyme activity (DEA), amino acid concentrations, and microbial respiration
were measured for soils from each plant community. The results show that net N mineralization rates were more than three times
higher in the meadow ecosystems (mesic meadow 0.7 μg N g−1 OM day−1 and meadow snowbed 0.6 μg N g−1 OM day−1) than the heath ecosystems (dry heath 0.2 μg N g−1 OM day−1, mesic heath 0.1 μg N g−1 OM day−1 and heath snowbed 0.2 μg N g−1 OM day−1). The net N mineralization rates were negatively correlated to organic soil C/N ratio (r = −0.652, P < 0.001) and positively correlated to soil pH (r = 0.701, P < 0.001). Net nitrification, inorganic N concentrations, and NEA rates also differed between plant communities; the values
for the mesic meadow were at least four times higher than the other plant communities, and the snowbeds formed an intermediate
group. Moreover, the results show a different pattern of distribution for individual amino acids across the plant communities,
with snowbeds tending to have the highest amino acid N concentrations. The differences between plant communities along this
natural gradient also illustrate variations between the dominant mycorrhizal associations in facilitating N capture by the
characteristic functional groups of plants.
Responsible Editor: Bernard Nicolardot 相似文献
3.
Five microhabitat types with varying degrees of bird influence were examined. Soils were collected from open polygons, under
mosses and bird nests on a nunatak with breeding snow petrels (Pagodroma nivea) and from open polygons and under mosses on a non-bird nunatak. Nutrient levels (total N and P, nitrate, nitrite and ammonia),
moisture levels and δ
15N values were determined and the organic processes of nitrogen fixation (acetylene reduction) and soil respiration (CO2 flux) were examined. Nests represented the most favourable microhabitat type for soil respiration having the highest nutrient
levels and most favourable temperature and moisture regimes. The soils under mosses were also favourable and appear to act
as a nutrient sink for nutrients originating from the nests. The open polygons were the least favourable for biological activity.
There was little nitrogen fixation in any of the soils except for the soils under mosses from the non-bird nunatak. Fixation
is possibly limited in favourable microhabitat types on the bird nunatak by high nitrogen levels. These results were confirmed
by the δ
15N results, which had high values typical of a seabird signal in the soils from the bird nunatak and values near zero, typical
of soils containing fixed nitrogen, on the non-bird nunatak.
Received: 3 March 1997 / Accepted: 30 March 1998 相似文献
4.
Woody encroachment into herbaceous ecosystems is emerging as an important ecological response to global change. A primary
concern is alterations in C and N cycling and associated variations across a variety of ecosystems. We quantified seasonal
variation in litterfall and litter N concentration in Morella cerifera shrub thickets to assess changes in litterfall and associated N input after shrub expansion on an Atlantic coast barrier
island. We also used the natural abundance of 15N to estimate the proportion of litterfall N originating from symbiotic N fixation. Litterfall for shrub thickets ranged from
8,991 ± 247 to 3,810 ± 399 kg ha−1 year−1 and generally declined with increasing thicket age. Litterfall in three of the four thickets exceeded previous estimates
of aboveground annual net primary production in adjacent grasslands by 300–400%. Leaf N concentration was also higher after
shrub expansion and, coupled with low N resorption efficiency and high litterfall, resulted in a return of as much as 169 kg N ha−1 year−1 to the soil. We estimated that ∼70% of N returned to the soil was from symbiotic N fixation resulting in an ecosystem input
of between 37 and 118 kg ha−1 year−1 of atmospheric N depending on site. Considering the extensive cover of shrub thickets on Virginia barrier islands, N fixation
by shrubs is likely the largest single source of N to the system. The shift from grassland to shrub thicket on barrier islands
results in a substantial increase in litterfall and foliar N concentration that will likely have a major impact on the size
and cycling of ecosystem C and N pools. Increasing C and N availability in these nutrient-poor soils is likely to permanently
reduce cover of native grasses and alter community structure by favoring species with greater N requirements. 相似文献
5.
Foliar δ15N, %N and %P in the dominant woody and herbaceous species across nutrient gradients in New Zealand restiad (family Restionaceae)
raised bogs revealed marked differences in plant δ15N correlations with P. The two heath shrubs, Leptospermum scoparium (Myrtaceae) and Dracophyllum scoparium (Epacridaceae), showed considerable isotopic variation (−2.03 to −15.55‰, and −0.39 to −12.06‰, respectively) across the
bogs, with foliar δ15N strongly and positively correlated with P concentrations in foliage and peat, and negatively correlated with foliar N:P
ratios. For L. scoparium, the isotopic gradient was not linked to ectomycorrhizal (ECM) fractionation as ECMs occurred only on higher nutrient marginal
peats where 15N depletion was least. In strong contrast, restiad species (Empodisma minus Sporadanthus ferrugineus, S. traversii) showed little isotopic variation across the same nutrient gradients. Empodisma minus and S. traversii had δ15N levels consistently around 0‰ (means of −0.12‰ and +0.15‰ respectively), and S. ferrugineus, which co-habited with E. minus, was more depleted (mean −4.97‰). The isotopic differences between heath shrubs and restiads were similar in floristically
dissimilar bogs and may be linked to contrasting nutrient demands, acquisition mechanisms, and root morphology. Leptospermum scoparium shrubs on low nutrient peats were stunted, with low tissue P concentrations, and high N:P ratios, suggesting they were P-limited,
which was probably exacerbated by markedly reduced mycorrhizal colonisations. The coupling of δ15N depletion and %P in heath shrubs suggests that N fractionation is promoted by P limitation. In contrast, the constancy in
δ15N of the restiad species through the N and P gradients suggests that these are not suffering from P limitation. 相似文献
6.
Klaus S. Larsen Anders Michelsen Sven Jonasson Claus Beier Paul Grogan 《Ecosystems》2012,15(6):927-939
Nitrogen (N) is a critical resource for plant growth in tundra ecosystems, and species differences in the timing of N uptake may be an important feature regulating community composition and ecosystem productivity. We added 15N-labelled glycine to a subarctic heath tundra dominated by dwarf shrubs, mosses and graminoids in fall, and investigated its partitioning among ecosystem components at several time points (October, November, April, May, June) through to the following spring/early summer. Soil microbes had acquired 65?±?7% of the 15N tracer by October, but this pool decreased through winter to 37?±?7% by April indicating significant microbial N turnover prior to spring thaw. Only the evergreen dwarf shrubs showed active 15N acquisition before early May indicating that they had the highest potential of all functional groups for acquiring nutrients that became available in early spring. The faster-growing deciduous shrubs did not resume 15N acquisition until after early May indicating that they relied more on nitrogen made available later during the spring/early summer. The graminoids and mosses had no significant increases in 15N tracer recovery or tissue 15N tracer concentrations after the first harvest in October. However, the graminoids had the highest root 15N tracer concentrations of all functional groups in October indicating that they primarily relied on N made available during summer and fall. Our results suggest a temporal differentiation among plant functional groups in the post-winter resumption of N uptake with evergreen dwarf shrubs having the highest potential for early N uptake, followed by deciduous dwarf shrubs and graminoids. 相似文献
7.
Nitrogen fixation in mixed <Emphasis Type="Italic">Hylocomium splendens</Emphasis> moss communities 总被引:1,自引:1,他引:0
The pleurocarpus feather moss, Hylocomium splendens, is one of two co-dominant moss species in boreal forest ecosystems and one of the most common mosses on earth, yet little
is known regarding its capacity to host cyanobacterial associates and thus contribute total ecosystem N. In these studies,
we evaluated the N-fixation potential of the H. splendens–cyanobacteria association and contrasted the N-fixation activity with that of the putative N-fixing moss–cyanobacteria association
of Pleurozium schreberi. Studies were conducted to: quantify N-fixation in H. splendens and P. schreberi in sites ranging from southern to northern Fennoscandia; assess N and P availability as drivers of N-fixation rates; contrast
season-long N-fixation rates for both mosses; and characterize the cyanobacteria that colonize shoots of H. splendens. Nitrogen-fixation rates were generally low at southern latitudes and higher at northern latitudes (64–69°N) potentially
related to anthropogenic N deposition across this gradient. Nitrogen fixation in H. splendens appeared to be less sensitive to N deposition than P. schreberi. The season-long assessment of N-fixation rates at a mixed feather moss site in northern Sweden showed that H. splendens fixed a substantial quantity of N, but about 50% less total N compared to the contribution from P. schreberi. In total, both species provided 1.6 kg fixed N ha−1 year−1. Interestingly, H. splendens demonstrated somewhat higher N-fixation rates at high fertility sites compared to P. schreberi. Nostoc spp. and Stigonema spp. were the primary cyanobacteria found to colonize H. splendens and P. schreberi. These results suggest that H. splendens with associated Nostoc or Stigonema communities contributes a significant quantity of N to boreal forest ecosystems, but the contribution is subordinate to that
of P. schreberi at northern latitudes. Epiphytic cyanobacteria are likely a key factor determining the co-dominant presence of these two
feather mosses across the boreal biome. 相似文献
8.
Site-dependent N uptake from N-form mixtures by arctic plants,soil microbes and ectomycorrhizal fungi 总被引:1,自引:0,他引:1
Soil microbes constitute an important control on nitrogen (N) turnover and retention in arctic ecosystems where N availability
is the main constraint on primary production. Ectomycorrhizal (ECM) symbioses may facilitate plant competition for the specific
N pools available in various arctic ecosystems. We report here our study on the N uptake patterns of coexisting plants and
microbes at two tundra sites with contrasting dominance of the circumpolar ECM shrub Betula nana. We added equimolar mixtures of glycine-N, NH4+–N and NO3−–N, with one N form labelled with 15N at a time, and in the case of glycine, also labelled with 13C, either directly to the soil or to ECM fungal ingrowth bags. After 2 days, the vegetation contained 5.6, 7.7 and 9.1% (heath
tundra) and 7.1, 14.3 and 12.5% (shrub tundra) of the glycine-, NH4+- and NO3−–15N, respectively, recovered in the plant–soil system, and the major part of 15N in the soil was immobilized by microbes (chloroform fumigation-extraction). In the subsequent 24 days, microbial N turnover
transferred about half of the immobilized 15N to the non-extractable soil organic N pool, demonstrating that soil microbes played a major role in N turnover and retention
in both tundra types. The ECM mycelial communities at the two tundras differed in N-form preferences, with a higher contribution
of glycine to total N uptake at the heath tundra; however, the ECM mycelial communities at both sites strongly discriminated
against NO3−. Betula nana did not directly reflect ECM mycelial N uptake, and we conclude that N uptake by ECM plants is modulated by the N uptake
patterns of both fungal and plant components of the symbiosis and by competitive interactions in the soil. Our field study
furthermore showed that intact free amino acids are potentially important N sources for arctic ECM fungi and plants as well
as for soil microorganisms.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
9.
In this study we show that the natural abundance of the nitrogen isotope 15, δ15N, of plants in heath tundra and at the tundra-forest ecocline is closely correlated with the presence and type of mycorrhizal
association in the plant roots. A total of 56 vascular plant species, 7 moss species, 2 lichens and 6 species of fungi from
four heath and forest tundra sites in Greenland, Siberia and Sweden were analysed for δ15N and N concentration. Roots of vascular plants were examined for mycorrhizal colonization, and the soil organic matter was
analysed for δ15N, N concentration and soil inorganic, dissolved organic and microbial N. No arbuscular mycorrhizal (AM) colonizations were
found although potential host plants were present in all sites. The dominant species were either ectomycorrhizal (ECM) or
ericoid mycorrhizal (ERI). The δ15N of ECM or ERI plants was 3.5–7.7‰ lower than that of non-mycorrhizal (NON) species in three of the four sites. This corresponds
to the results in our earlier study of mycorrhiza and plant δ15N which was limited to one heath and one fellfield in N Sweden. Hence, our data suggest that the δ15N pattern: NON/AM plants > ECM plants ≥ ERI plants is a general phenomenon in ecosystems with nutrient-deficient organogenic
soils. In the fourth site, a␣birch forest with a lush herb/shrub understorey, the differences between functional groups were
considerably smaller, and only the ERI species differed (by 1.1‰) from the NON species. Plants of all functional groups from
this site had nearly twice the leaf N concentration as that found in the same species at the other three sites. It is likely
that low inorganic N availability is a prerequisite for strong δ15N separation among functional groups. Both ECM roots and fruitbodies were 15N enriched compared to leaves which suggests that the difference in δ15N between plants with different kinds of mycorrhiza could be due to isotopic fractionation at the␣fungal-plant interface.
However, differences in δ15N between soil N forms absorbed by the plants could also contribute to the wide differences in plant δ15N found in most heath and forest tundra ecosystems. We hypothesize that during microbial immobilization of soil ammonium the
microbial N pool could become 15N-depleted and the remaining, plant-available soil ammonium 15N-enriched. The latter could be a main source of N for NON/AM plants which usually have high δ15N. In contrast, amino acids and other soil organic N compounds presumably are 15N-depleted, similar to plant litter, and ECM and ERI plants with high uptake of these N forms hence have low leaf δ15N. Further indications come from the δ15N of mosses and lichens which was similar to that of ECM plants. Tundra cryptogams (and ECM and ERI plants) have previously
been shown to have higher uptake of amino acid than ammonium N; their low δ15N might therefore reflect the δ15N of free amino acids in the soil. The concentration of dissolved organic N was 3–16 times higher than that of inorganic N
in the sites. Organic nitrogen could be an important N source for ECM and, in particular, ERI plants in heath and forest tundra
ecosystems with low release rate of inorganic N from the soil organic matter.
Received: 8 June 1997 / Accepted: 28 February 1998 相似文献
10.
Summary Tillage has been shown to affect the uptake of phosphorus (P) and yield of soybeans, [Glycine max (L.) Merr.], but there is little information concerning the effects of P fertilization on nitrogen (N2) fixation in soybeans under no-tillage. Two field experiments were conducted in 1980 and 1981 to determine the effects of
soil P on N2 fixation under no-tillage and to study the interaction of P fertilization and tillage of N2 fixation, nutrient uptake, and yield of soybeans. In Exp. I, P was applied in 1977 at five rates up to 384 kg P ha−1 and the effects of residual soil P were evaluated in 1980 and 1981 under no-tillage management. Nitrogen fixation rates,
as measured by acetylene reduction assay, were significantly affected by soil P in Exp. I, but the assay proved to be a poor
technique for estimating total plant N in these tests. Acetylene reduction rates and plant P increased rapidly as soil P increased
from 2 to 20 mg kg−1, with little additional increase above 20 mg P kg−1.
In Exp. II, rates (0, 32, 64, and 128 kg P ha−1) and time (fall, spring and fall plus spring) of P application were compared under conventional tillage and no tillage. However,
plant P increased with increasing levels of applied P. Applied P had no affect on acetylene reduction rates but rates were
greater for no-tillage than conventional tillage at the V9 and R5 stages of growth in 1981. Plant uptake of P was more efficient
under no-tillage than under conventional tillage in 1980 and 1981. Application of 64 kg P ha−1 under no-tillage resulted in equivalent plant P levels as the 128 kg P ha−1 applied under conventional tillage. 相似文献
11.
Recent studies have revealed that nitrogen fixation by cyanobacteria living in association with feather mosses is a major input of nitrogen to boreal forests. We characterized the community composition and diversity of cyanobacterial nifH phylotypes associated with each of two feather moss species (Pleurozium schreberi and Hylocomium splendens) on each of 30 lake islands varying in ecosystem properties in northern Sweden. Nitrogen fixation was measured using acetylene reduction, and nifH sequences were amplified using general and cyanobacterial selective primers, separated and analyzed using density gradient gel electrophoresis (DGGE) or cloning, and further sequenced for phylogenetic analyses. Analyses of DGGE fingerprinting patterns revealed two host-specific clusters (one for each moss species), and sequence analysis showed five clusters of nifH phylotypes originating from heterocystous cyanobacteria. For H. splendens only, N(2) fixation was related to both nifH composition and diversity among islands. We demonstrated that the cyanobacterial communities associated with feather mosses show a high degree of host specificity. However, phylotype composition and diversity, and nitrogen fixation, did not differ among groups of islands that varied greatly in their availability of resources. These results suggest that moss species identity, but not extrinsic environmental conditions, serves as the primary determinant of nitrogen-fixing cyanobacterial communities that inhabit mosses. 相似文献
12.
Exotic grass invasion promotes fire which drives the conversion of native woodlands to exotic grasslands in the seasonally
dry submontane forests of the island of Hawai'i. We compared potential rates of N fixation in an unburned forest site and
a converted grassland site using the acetylene reduction assay. In addition to measuring rates of N fixation on separate and
mixed substrates in each site, we tested the effect of abiotic factors on rates of N fixation of specific substrates. We hypothesized
that rates of N fixation would be higher in the converted grassland site. N fixation estimates were 4.9 kg N ha−1 year−1 for the unburned forest, and 0.10 kg N ha−1 year−1 for the grassland site, so our hypothesis was rejected. The N fixation in the unburned forest occurs mostly on the leaf litter
of native woody species. These substrates are absent from the grassland site, except for wood debris which was not consumed
during the fires. No nitrogenase activity was detected in the rhizosphere and litter of grasses, the rhizospheres of shrubs
or in soil. Although wood debris is not a significant contributor to the N fixed in the unburned forest, it contributes the
majority of N fixed in the grassland. The response of nitrogenase activity to varying conditions of moisture and temperature
suggests that microclimatic differences between sites do not control differences in N fixation activity; rather, these differences
are due to the abundance of N-fixing substrates. The substantial decrease in N fixation activity after the conversion from
woodland to grassland implies that ecosystem-level rates of N accretion are decreased by fire in these sites so much that
the N lost during volatilization due to fire is not replenished over the long term by N fixation.
Received: 10 January 1997 / Accepted: 7 August 1997 相似文献
13.
Lorna E. Street Mark H. Garnett Jens‐Arne Subke Robert Baxter Joshua F. Dean Philip A. Wookey 《Global Change Biology》2020,26(8):4559-4571
Carbon cycle feedbacks from permafrost ecosystems are expected to accelerate global climate change. Shifts in vegetation productivity and composition in permafrost regions could influence soil organic carbon (SOC) turnover rates via rhizosphere (root zone) priming effects (RPEs), but these processes are not currently accounted for in model predictions. We use a radiocarbon (bomb‐14C) approach to test for RPEs in two Arctic tall shrubs, alder (Alnus viridis (Chaix) DC.) and birch (Betula glandulosa Michx.), and in ericaceous heath tundra vegetation. We compare surface CO2 efflux rates and 14C content between intact vegetation and plots in which below‐ground allocation of recent photosynthate was prevented by trenching and removal of above‐ground biomass. We show, for the first time, that recent photosynthate drives mineralization of older (>50 years old) SOC under birch shrubs and ericaceous heath tundra. By contrast, we find no evidence of RPEs in soils under alder. This is the first direct evidence from permafrost systems that vegetation influences SOC turnover through below‐ground C allocation. The vulnerability of SOC to decomposition in permafrost systems may therefore be directly linked to vegetation change, such that expansion of birch shrubs across the Arctic could increase decomposition of older SOC. Our results suggest that carbon cycle models that do not include RPEs risk underestimating the carbon cycle feedbacks associated with changing conditions in tundra regions. 相似文献
14.
Nitrogen (N2) fixation is a major source of available N in ecosystems that receive low amounts of atmospheric N deposition. In boreal forest and subarctic tundra, the feather moss Hylocomium splendens is colonized by N2 fixing cyanobacteria that could contribute fundamentally to increase the N pool in these ecosystems. However, N2 fixation in mosses is inhibited by N input. Although this has been shown previously, the ability of N2 fixation to grow less sensitive towards repeated, increased N inputs remains unknown. Here, we tested if N2 fixation in H. splendens can recover from increased N input depending on the N load (0, 5, 20, 80, 320 kg N ha-1 yr-1) after a period of N deprivation, and if sensitivity towards increased N input can decrease after repeated N additions. Nitrogen fixation in the moss was inhibited by the highest N addition, but was promoted by adding 5 kg N ha-1 yr-1, and increased in all treatments during a short period of N deprivation. The sensitivity of N2 fixation towards repeated N additions seem to decrease in the 20 and 80 kg N additions, but increased in the highest N addition (320 kg N ha-1 yr-1). Recovery of N in leachate samples increased with increasing N loads, suggesting low retention capabilities of mosses if N input is above 5 kg N ha-1 yr-1. Our results demonstrate that the sensitivity towards repeated N additions is likely to decrease if N input does not exceed a certain threshold. 相似文献
15.
Various types of sub-aerially exposed microbial mats, including emergent mats, beach sand, beach rock and Kopara mats, are
widespread on the 78 km (25 km2) of rim surrounding the Tikehau atoll lagoon. These mats form laminated accretions or diffuse microbial communities growing
under high insolation and temperatures, and are therefore subject to desiccation. Both heterocystous and non-heterocystous
cyanobacteria occur in these mats. Using acetylene reduction techniques, nitrogenase activity was observed at all sites over
a period of 5 years and was 3–17 times higher during daylight than at night in all communities except for beach rock. 15N2 measurements indicated a molar ratio of acetylene reduction to N2 fixed of 1.6 for all exposed communities. Estimated N2 fixation ranged from 1.44 to 8.0 mg N m−2 day−1 in these exposed communities (mean of 4.66 mg N m−2 day−1) with beachrock showing the highest rates. For the whole reef rim, daily N2 fixation amounted to 98.42 kg N day−1 which represents 28% of the rate of fixation in the entire lagoon (area 400 km2). 相似文献
16.
John H. Markham 《Oecologia》2009,161(2):353-359
Traditionally it has been thought that most boreal forest communities lack a significant input of biologically fixed nitrogen.
Recent discoveries of nitrogen fixation by cyanobacteria associated with mosses have resulted in a re-evaluation of this view.
While it is recognized that rates of nitrogen fixation in mosses can be highly variable, there is little understanding as
to why this occurs. I monitored nitrogen fixation, using acetylene reduction, in wet lowland and dry upland boreal forest
communities, in central Canada, over a growing season. At the peak of nitrogen fixation in mid summer, Sphagnum capillifolium had an 11 times higher rate of fixation than Pleurozium schreberi. Variation in canopy openness and precipitation had no effect on rates of fixation over the growing season. In P. schreberi fixation rates did not vary between sites. Temperature had a positive effect on fixation rates in both S. capillifolium and P. schreberi, but the effect was 4 times more pronounced in S. capillifolium. Seasonal rates of nitrogen fixation were estimated at 193 mg N m−2 for S. capillifolium and 23 mg N m−2 for P. schreberi. With moderate increases in climate warming, predicted increases in nitrogen fixation in S. capillifolium are sufficient to raise its decomposition rate. Increased temperatures may therefore act synergistically to change boreal
systems from a sink to a source of carbon. 相似文献
17.
N fixation in feather moss carpets is maximized in late secondary successional boreal forests; however, there is limited understanding
of the ecosystem factors that drive cyanobacterial N fixation in feather mosses with successional stage. We conducted a reciprocal
transplant experiment to assess factors in both early and late succession that control N fixation in feather moss carpets
dominated by Pleurozium schreberi. In 2003, intact microplots of moss carpets (30 cm × 30 cm × 10–20 cm deep) were excavated from three early secondary successional
(41–101 years since last fire) forest sites and either replanted within the same stand or transplanted into one of three late
successional (241–356 years since last fire) forest sites and the transverse was done for late successional layers of moss.
Moss plots were monitored for changes in N-fixation rates by acetylene reduction (June 2003–September 2005) and changes in
the presence of cyanobacteria on moss shoots by microscopy (2004). Forest nutrient status was measured using ionic resin capsules
buried in the humus layer. Late successional forests exhibit high rates of N fixation and consistently high numbers of cyanobacteria
on moss shoots, but low levels of available N. Conversely, early successional forests have higher N availability and have
low rates of N fixation and limited presence of cyanobacteria on moss shoots. Transplantation of moss carpets resulted in
a significant shift in presence and activity of cyanobacteria 1 year after initiation of the experiment responding to N fertility
differences in early versus late successional forests. 相似文献
18.
Van Geest GJ Hessen DO Spierenburg P Dahl-Hansen GA Christensen G Faerovig PJ Brehm M Loonen MJ Van Donk E 《Oecologia》2007,153(3):653-662
A dramatic increase in the breeding population of geese has occurred over the past few decades at Svalbard. This may strongly
impact the fragile ecosystems of the Arctic tundra because many of the ultra-oligotrophic freshwater systems experience enrichment
from goose feces. We surveyed 21 shallow tundra ponds along a gradient of nutrient enrichment based on exposure to geese.
Concentrations of total phosphorus (P) and dissolved inorganic nitrogen (DIN) in the tundra ponds ranged from 2–76 to 2–23 μg l−1 respectively, yet there was no significant increase in phytoplankton biomass (measured as chlorophyll a; range: 0.6–7.3 μg l−1) along the nutrient gradient. This lack of response may be the result of the trophic structure of these ecosystems, which
consists of only a two-trophic level food chain with high biomasses of the efficient zooplankton grazer Daphnia in the absence of fish and scarcity of invertebrate predators. Our results indicate that this may cause a highly efficient
grazing control of phytoplankton in all ponds, supported by the fact that large fractions of the nutrient pools were bound
in zooplankton biomass. The median percentage of Daphnia–N and Daphnia–P content to particulate (sestonic) N and P was 338 and 3009%, respectively, which is extremely high compared to temperate
lakes. Our data suggest that Daphnia in shallow arctic ponds is heavily subsidized by major inputs of energy from other food sources (bacteria, benthic biofilm),
which may be crucial to the persistence of strong top–down control of pelagic algae by Daphnia. 相似文献
19.
Biological nitrogen (N2) fixation performed by diazotrophs (N2 fixing bacteria) is thought to be one of the main sources of plant available N in pristine ecosystems like arctic tundra. However, direct evidence of a transfer of fixed N2 to non-diazotroph associated plants is lacking to date. Here, we present results from an in situ 15N–N2 labelling study in the High Arctic. Three dominant vegetation types (organic crust composed of free-living cyanobacteria, mosses, cotton grass) were subjected to acetylene reduction assays (ARA) performed regularly throughout the growing season, as well as 15N–N2 incubations. The 15N-label was followed into the dominant N2 fixer associations, soil, soil microbial biomass and non-diazotroph associated plants three days and three weeks after labelling. Mosses contributed most to habitat N2 fixation throughout the measuring campaigns, and N2 fixation activity was highest at the beginning of the growing season in all plots. Fixed 15N–N2 became quickly (within 3 days) available to non-diazotroph associated plants in all investigated vegetation types, proving that N2 fixation is an actual source of available N in pristine ecosystems. 相似文献
20.