排序方式: 共有53条查询结果,搜索用时 15 毫秒
31.
Nicole S. Nowinski Susan E. Trumbore Edward A. G. Schuur Michelle C. Mack Gaius R. Shaver 《Ecosystems》2008,11(1):16-25
Nutrient availability in the arctic is expected to increase in the next century due to accelerated decomposition associated
with warming and, to a lesser extent, increased nitrogen deposition. To explore how changes in nutrient availability affect
ecosystem carbon (C) cycling, we used radiocarbon to quantify changes in belowground C dynamics associated with long-term
fertilization of graminoid-dominated tussock tundra at Toolik Lake, Alaska. Since 1981, yearly fertilization with nitrogen
(N) and phosphorus (P) has resulted in a shift to shrub-dominated vegetation. These combined changes have altered the quantity
and quality of litter inputs, the vertical distribution and dynamics of fine roots, and the decomposition rate of soil organic
C. The loss of C from the deep organic and mineral soil has more than offset the C accumulation in the litter and upper organic
soil horizons. In the litter and upper organic horizons, radiocarbon measurements show that increased inputs resulted in overall
C accumulation, despite being offset by increased decomposition in some soil pools. To reconcile radiocarbon observations
in the deeper organic and mineral soil layers, where most of the ecosystem C loss occurred, both a decrease in input of new
root material and a dramatic increase of decomposition rates in centuries-old soil C pools were required. Therefore, with
future increases in nutrient availability, we may expect substantial losses of C which took centuries to accumulate. 相似文献
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Are tundra ecosystems currently a carbon source or sink? What is the future trajectory of tundra carbon fluxes in response to climate change? These questions are of global importance because of the vast quantities of organic carbon stored in permafrost soils. In this meta‐analysis, we compile 40 years of CO2 flux observations from 54 studies spanning 32 sites across northern high latitudes. Using time‐series analysis, we investigated if seasonal or annual CO2 fluxes have changed over time, and whether spatial differences in mean annual temperature could help explain temporal changes in CO2 flux. Growing season net CO2 uptake has definitely increased since the 1990s; the data also suggest (albeit less definitively) an increase in winter CO2 emissions, especially in the last decade. In spite of the uncertainty in the winter trend, we estimate that tundra sites were annual CO2 sources from the mid‐1980s until the 2000s, and data from the last 7 years show that tundra continue to emit CO2 annually. CO2 emissions exceed CO2 uptake across the range of temperatures that occur in the tundra biome. Taken together, these data suggest that despite increases in growing season uptake, tundra ecosystems are currently CO2 sources on an annual basis. 相似文献
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Christina Schdel Edward A. G. Schuur Rosvel Bracho Bo Elberling Christian Knoblauch Hanna Lee Yiqi Luo Gaius R. Shaver Merritt R. Turetsky 《Global Change Biology》2014,20(2):641-652
High‐latitude ecosystems store approximately 1700 Pg of soil carbon (C), which is twice as much C as is currently contained in the atmosphere. Permafrost thaw and subsequent microbial decomposition of permafrost organic matter could add large amounts of C to the atmosphere, thereby influencing the global C cycle. The rates at which C is being released from the permafrost zone at different soil depths and across different physiographic regions are poorly understood but crucial in understanding future changes in permafrost C storage with climate change. We assessed the inherent decomposability of C from the permafrost zone by assembling a database of long‐term (>1 year) aerobic soil incubations from 121 individual samples from 23 high‐latitude ecosystems located across the northern circumpolar permafrost zone. Using a three‐pool (i.e., fast, slow and passive) decomposition model, we estimated pool sizes for C fractions with different turnover times and their inherent decomposition rates using a reference temperature of 5 °C. Fast cycling C accounted for less than 5% of all C in both organic and mineral soils whereas the pool size of slow cycling C increased with C : N. Turnover time at 5 °C of fast cycling C typically was below 1 year, between 5 and 15 years for slow turning over C, and more than 500 years for passive C. We project that between 20 and 90% of the organic C could potentially be mineralized to CO2 within 50 incubation years at a constant temperature of 5 °C, with vulnerability to loss increasing in soils with higher C : N. These results demonstrate the variation in the vulnerability of C stored in permafrost soils based on inherent differences in organic matter decomposability, and point toward C : N as an index of decomposability that has the potential to be used to scale permafrost C loss across landscapes. 相似文献
35.
Plant Species Composition and Productivity following Permafrost Thaw and Thermokarst in Alaskan Tundra 总被引:1,自引:0,他引:1
Edward A. G. Schuur Kathryn G. Crummer Jason G. Vogel Michelle C. Mack 《Ecosystems》2007,10(2):280-292
Climate warming is expected to have a large impact on plant species composition and productivity in northern latitude ecosystems.
Warming can affect vegetation communities directly through temperature effects on plant growth and indirectly through alteration
of soil nutrient availability. In addition, warming can cause permafrost to thaw and thermokarst (ground subsidence) to develop,
which can alter the structure of the ecosystem by altering hydrological patterns within a site. These multiple direct and
indirect effects of permafrost thawing are difficult to simulate in experimental approaches that often manipulate only one
or two factors. Here, we used a natural gradient approach with three sites to represent stages in the process of permafrost
thawing and thermokarst. We found that vascular plant biomass shifted from graminoid-dominated tundra in the least disturbed
site to shrub-dominated tundra at the oldest, most subsided site, whereas the intermediate site was co-dominated by both plant
functional groups. Vascular plant productivity patterns followed the changes in biomass, whereas nonvascular moss productivity
was especially important in the oldest, most subsided site. The coefficient of variation for soil moisture was higher in the
oldest, most subsided site suggesting that in addition to more wet microsites, there were other microsites that were drier.
Across all sites, graminoids preferred the cold, dry microsites whereas the moss and shrubs were associated with the warm,
moist microsites. Total nitrogen contained in green plant biomass differed across sites, suggesting that there were increases
in soil nitrogen availability where permafrost had thawed. 相似文献
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Relationships among precipitation regime, nutrient availability, and carbon turnover in tropical rain forests 总被引:1,自引:0,他引:1
The effect of high precipitation regime in tropical forests is poorly known despite indications of its potentially negative effects on nutrient availability and carbon (C) cycling. Our goal was to determine if there was an effect of high rainfall on nitrogen (N) and phosphorous (P) availability and indexes of C cycling in lowland tropical rain forests exposed to a broad range of mean annual precipitation (MAP). We predicted that C turnover time would increase with MAP while the availability of N and P would decrease. We studied seven Neotropical lowland forests covering a MAP range between 2,700 and 9,500 mm. We used radiocarbon (?14C) from the atmosphere and respired from soil organic matter to estimate residence time of C in plants and soils. We also used C, N, and P concentrations and the stable isotope ratio of N (δ15N) in live and dead plant tissues and in soils as proxies for nutrient availability. Negative δ15N values indicated that the wettest forests had N cycles that did not exhibit isotope-fractionating losses and were potentially N-limited. Element ratios (N:P and C:P) in senescent leaves, litter, and live roots showed that P resorption increased considerably with MAP, which points towards increasing P-limitation under high MAP regimes. Soil C content increased with MAP but C turnover time only showed a weak relationship with MAP, probably due to variations in soil parent material and age along the MAP gradient. In contrast, comparing C turnover directly to nutrient availability showed strong relationships between C turnover time, N availability (δ15N), and P availability (N:P) in senescent leaves and litter. Thus, an effect of MAP on carbon cycling appeared to be indirectly mediated by nutrient availability. Our results suggest that soil nutrient availability plays a central role in the dynamic of C cycling in tropical rain forests. 相似文献
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Sara S. Roscioni Harm Maarsingh Carolina R.S. Elzinga Janke Schuur Mark Menzen Andrew J. Halayko Herman Meurs Martina Schmidt 《Journal of cellular and molecular medicine》2011,15(7):1551-1563
Dysfunctional regulation of airway smooth muscle tone is a feature of obstructive airway diseases such as asthma and chronic obstructive pulmonary disease. Airway smooth muscle contraction is directly associated with changes in the phosphorylation of myosin light chain (MLC), which is increased by Rho and decreased by Rac. Although cyclic adenosine monophosphate (cAMP)‐elevating agents are believed to relieve bronchoconstriction mainly via activation of protein kinase A (PKA), here we addressed the role of the novel cAMP‐mediated exchange protein Epac in the regulation of airway smooth muscle tone. Isometric tension measurements showed that specific activation of Epac led to relaxation of guinea pig tracheal preparations pre‐contracted with methacholine, independently of PKA. In airway smooth muscle cells, Epac activation reduced methacholine‐induced MLC phosphorylation. Moreover, when Epac was stimulated, we observed a decreased methacholine‐induced RhoA activation, measured by both stress fibre formation and pull‐down assay whereas the same Epac activation prevented methacholine‐induced Rac1 inhibition measured by pull‐down assay. Epac‐driven inhibition of both methacholine‐induced muscle contraction by Toxin B‐1470, and MLC phosphorylation by the Rac1‐inhibitor NSC23766, were significantly attenuated, confirming the importance of Rac1 in Epac‐mediated relaxation. Importantly, human airway smooth muscle tissue also expresses Epac, and Epac activation both relaxed pre‐contracted human tracheal preparations and decreased MLC phosphorylation. Collectively, we show that activation of Epac relaxes airway smooth muscle by decreasing MLC phosphorylation by skewing the balance of RhoA/Rac1 activation towards Rac1. Therefore, activation of Epac may have therapeutical potential in the treatment of obstructive airway diseases. 相似文献
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Erik A. Hobbie Andrew P. Ouimette Edward A. G. Schuur Daniel Kierstead James M. Trappe Katriina Bendiksen Esteri Ohenoja 《Biogeochemistry》2013,114(1-3):381-389
Organic nitrogen use by mycorrhizal fungi and associated plants could fuel productivity in nitrogen-limited systems. To test whether fungi assimilated soil-derived organic nitrogen, we compared the 14C signal (expressed as Δ14C) from 1950s to 1960s thermonuclear testing in protein and structural carbon of ectomycorrhizal fungi. As expected, structural carbon had Δ14C similar to recent photosynthesis; however, protein Δ14C was either higher or lower than structural carbon depending on the fungal taxa. This suggests that some protein carbon derived from uptake of organic nitrogen with different Δ14C signals. Specimens from two taxa (Lactarius and Russula) adapted to taking up soluble nutrients had protein higher than structural carbon in Δ14C, indicating uptake of young, post-bomb organic nitrogen, whereas two taxa (Cortinarius and Leccinum) adapted to using insoluble, complex organic nutrients had protein lower than structural carbon in Δ14C, indicating uptake of old, pre-bomb organic nitrogen. Tuber, a genus common in mineral soil, was also consistently lower in Δ14C for protein than for structural carbon, with an estimated 10 % of protein carbon originating from old, deep organic nitrogen for this taxon. Our results indicate that radiocarbon can provide evidence of organic nitrogen use in ectomycorrhizal fungi and reflects the exploration depth of different taxa. 相似文献
39.
Claire C. Treat Susan M. Natali Jessica Ernakovich Colleen M. Iversen Massimo Lupascu Anthony David McGuire Richard J. Norby Taniya Roy Chowdhury Andreas Richter Hana Šantrůčková Christina Schädel Edward A. G. Schuur Victoria L. Sloan Merritt R. Turetsky Mark P. Waldrop 《Global Change Biology》2015,21(7):2787-2803
Permafrost thaw can alter the soil environment through changes in soil moisture, frequently resulting in soil saturation, a shift to anaerobic decomposition, and changes in the plant community. These changes, along with thawing of previously frozen organic material, can alter the form and magnitude of greenhouse gas production from permafrost ecosystems. We synthesized existing methane (CH4) and carbon dioxide (CO2) production measurements from anaerobic incubations of boreal and tundra soils from the geographic permafrost region to evaluate large‐scale controls of anaerobic CO2 and CH4 production and compare the relative importance of landscape‐level factors (e.g., vegetation type and landscape position), soil properties (e.g., pH, depth, and soil type), and soil environmental conditions (e.g., temperature and relative water table position). We found fivefold higher maximum CH4 production per gram soil carbon from organic soils than mineral soils. Maximum CH4 production from soils in the active layer (ground that thaws and refreezes annually) was nearly four times that of permafrost per gram soil carbon, and CH4 production per gram soil carbon was two times greater from sites without permafrost than sites with permafrost. Maximum CH4 and median anaerobic CO2 production decreased with depth, while CO2:CH4 production increased with depth. Maximum CH4 production was highest in soils with herbaceous vegetation and soils that were either consistently or periodically inundated. This synthesis identifies the need to consider biome, landscape position, and vascular/moss vegetation types when modeling CH4 production in permafrost ecosystems and suggests the need for longer‐term anaerobic incubations to fully capture CH4 dynamics. Our results demonstrate that as climate warms in arctic and boreal regions, rates of anaerobic CO2 and CH4 production will increase, not only as a result of increased temperature, but also from shifts in vegetation and increased ground saturation that will accompany permafrost thaw. 相似文献
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