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1.
Forest and other upland soils are important sinks for atmospheric CH 4, consuming 20 to 60 Tg of CH 4 per year. Consumption of atmospheric CH 4 by soil is a microbiological process. However, little is known about the methanotrophic bacterial community in forest soils. We measured vertical profiles of atmospheric CH 4 oxidation rates in a German forest soil and characterized the methanotrophic populations by PCR and denaturing gradient gel electrophoresis (DGGE) with primer sets targeting the pmoA gene, coding for the α subunit of the particulate methane monooxygenase, and the small-subunit rRNA gene (SSU rDNA) of all life. The forest soil was a sink for atmospheric CH 4 in situ and in vitro at all times. In winter, atmospheric CH 4 was oxidized in a well-defined subsurface soil layer (6 to 14 cm deep), whereas in summer, the complete soil core was active (0 cm to 26 cm deep). The content of total extractable DNA was about 10-fold higher in summer than in winter. It decreased with soil depth (0 to 28 cm deep) from about 40 to 1 μg DNA per g (dry weight) of soil. The PCR product concentration of SSU rDNA of all life was constant both in winter and in summer. However, the PCR product concentration of pmoA changed with depth and season. pmoA was detected only in soil layers with active CH 4 oxidation, i.e., 6 to 16 cm deep in winter and throughout the soil core in summer. The same methanotrophic populations were present in winter and summer. Layers with high CH 4 consumption rates also exhibited more bands of pmoA in DGGE, indicating that high CH 4 oxidation activity was positively correlated with the number of methanotrophic populations present. The pmoA sequences derived from excised DGGE bands were only distantly related to those of known methanotrophs, indicating the existence of unknown methanotrophs involved in atmospheric CH 4 consumption. 相似文献
2.
Most studies of greenhouse gas fluxes from forest soils in the coastal rainforest have considered carbon dioxide (CO 2), whereas methane (CH 4) has not received the same attention. Soil hydrology is a key driver of CH 4 dynamics in ecosystems, but the impact on the function and distribution of the underlying microbial communities involved in CH 4 cycling and the resultant net CH 4 exchange is not well understood at this scale. We studied the growing season variations of in situ CH 4 fluxes, microbial gene abundances of methanotrophs (CH 4 oxidizers) and methanogens (CH 4 producers), soil hydrology, and nutrient availability in three typical forest types across a soil moisture gradient. CH 4 displayed a spatial variability changing from a net uptake in the upland soils (3.9–46 µmol CH 4 m ?2 h ?1) to a net emission in the wetter soils (0–90 μmol CH 4 m ?2 h ?1). Seasonal variations of CH 4 fluxes were related to soil hydrology in both upland and wet soils. Thus, in the upland soils, uptake rates increased with the decreasing soil moisture, whereas CH 4 emission was inversely related to the water table depth in the wet soils. Spatial variability of CH 4 exchange was related to the abundance of genes involved in CH 4 oxidation and production, but there was no indication of a temporal link between microbial groups and CH 4 exchange. Our data show that the abundances of genes involved in CH 4 oxidation and production are strongly influenced by soil moisture and each other and grouped by the upland–wetland classification but not forest type. 相似文献
3.
The fate of carbon (C) contained within permafrost in boreal forest environments is an important consideration for the current and future carbon cycle as soils warm in northern latitudes. Currently, little is known about the microbiology or chemistry of permafrost soils that may affect its decomposition once soils thaw. We tested the hypothesis that low microbial abundances and activities in permafrost soils limit decomposition rates compared with active layer soils. We examined active layer and permafrost soils near Fairbanks, AK, the Yukon River, and the Arctic Circle. Soils were incubated in the lab under aerobic and anaerobic conditions. Gas fluxes at ?5 and 5 °C were measured to calculate temperature response quotients ( Q10). The Q10 was lower in permafrost soils (average 2.7) compared with active layer soils (average 7.5). Soil nutrients, leachable dissolved organic C (DOC) quality and quantity, and nuclear magnetic resonance spectroscopy of the soils revealed that the organic matter within permafrost soils is as labile, or even more so, than surface soils. Microbial abundances (fungi, bacteria, and subgroups: methanogens and Basidiomycetes) and exoenzyme activities involved in decomposition were lower in permafrost soils compared with active layer soils, which, together with the chemical data, supports the reduced Q10 values. CH 4 fluxes were correlated with methanogen abundance and the highest CH 4 production came from active layer soils. These results suggest that permafrost soils have high inherent decomposability, but low microbial abundances and activities reduce the temperature sensitivity of C fluxes. Despite these inherent limitations, however, respiration per unit soil C was higher in permafrost soils compared with active layer soils, suggesting that decomposition and heterotrophic respiration may contribute to a positive feedback to warming of this eco region. 相似文献
4.
Microbial oxidation is the only biological sink for atmospheric methane. We assessed seasonal changes in atmospheric methane oxidation and the underlying methanotrophic communities in grassland near Giessen (Germany), along a soil moisture gradient. Soil samples were taken from the surface layer (0–10 cm) of three sites in August 2007, November 2007, February 2008 and May 2008. The sites showed seasonal differences in hydrological parameters. Net uptake rates varied seasonally between 0 and 70 μg CH 4 m −2 h −1. Greatest uptake rates coincided with lowest soil moisture in spring and summer. Over all sites and seasons, the methanotrophic communities were dominated by uncultivated methanotrophs. These formed a monophyletic cluster defined by the RA14, MHP and JR1 clades, referred to as upland soil cluster alphaproteobacteria (USCα)-like group. The copy numbers of pmoA genes ranged between 3.8 × 10 5–1.9 × 10 6 copies g −1 of soil. Temperature was positively correlated with CH 4 uptake rates ( P<0.001), but had no effect on methanotrophic population dynamics. The soil moisture was negatively correlated with CH 4 uptake rates ( P<0.001), but showed a positive correlation with changes in USCα-like diversity ( P<0.001) and pmoA gene abundance ( P<0.05). These were greatest at low net CH 4 uptake rates during winter times and coincided with an overall increase in bacterial 16S rRNA gene abundances ( P<0.05). Taken together, soil moisture had a significant but opposed effect on CH 4 uptake rates and methanotrophic population dynamics, the latter being increasingly stimulated by soil moisture contents >50 vol% and primarily related to members of the MHP clade. 相似文献
5.
Forest soils are an important component of CO 2 and CH 4 fluxes at the global scale, but the magnitude of these fluxes varies greatly in space and time within a landscape. Understanding the spatial and temporal distributions of these fluxes across complex landscapes remains a major challenge for researchers and land managers alike. We investigated the spatiotemporal variability of soil-atmosphere CO 2 and CH 4 fluxes and the relationships of these fluxes to chemical and physical soil properties distributed across a topographically-heterogeneous landscape. Soil CO 2 and CH 4 fluxes were measured along with soil temperature, moisture, bulk density, texture, carbon, sorption capacity, and dissolved organic matter quality over 2 years along hillslope transects spanning valley bottom, transition zone, and upland landscape positions in a temperate forest watershed. Transition zone soil CO 2 efflux was 54–160% higher than low-lying valley bottoms, and 15–54% higher than uplands. Net seasonal CH 4 uptake was 58–150% higher in transition zone soils than in uplands, while valley bottoms were occasionally large net sources (up to 19 nmol CH 4 m ?2 s ?1). Soil CO 2 efflux and net CH 4 uptake were both positively associated with seasonal temperature, and were highest in soils with relatively high carbon and clay content, and relatively low bulk density, moisture, and sorption capacity. We concluded that: (1) transition zone soils act as landscape hotspots for net CH 4 uptake in addition to CO 2 efflux, and (2) that this spatial distribution is more consistent across seasons for net CH 4 uptake than for CO 2 efflux. 相似文献
6.
Wetlands are important sources of methane (CH 4) and sinks of carbon dioxide (CO 2). However, little is known about CH 4 and CO 2 fluxes and dynamics of seasonally flooded tropical forests of South America in relation to local carbon (C) balances and atmospheric exchange. We measured net ecosystem fluxes of CH 4 and CO 2 in the Pantanal over 2014–2017 using tower‐based eddy covariance along with C measurements in soil, biomass and water. Our data indicate that seasonally flooded tropical forests are potentially large sinks for CO 2 but strong sources of CH 4, particularly during inundation when reducing conditions in soils increase CH 4 production and limit CO 2 release. During inundation when soils were anaerobic, the flooded forest emitted 0.11 ± 0.002 g CH 4‐C m ?2 d ?1 and absorbed 1.6 ± 0.2 g CO 2‐C m ?2 d ?1 (mean ± 95% confidence interval for the entire study period). Following the recession of floodwaters, soils rapidly became aerobic and CH 4 emissions decreased significantly (0.002 ± 0.001 g CH 4‐C m ?2 d ?1) but remained a net source, while the net CO 2 flux flipped from being a net sink during anaerobic periods to acting as a source during aerobic periods. CH 4 fluxes were 50 times higher in the wet season; DOC was a minor component in the net ecosystem carbon balance. Daily fluxes of CO 2 and CH 4 were similar in all years for each season, but annual net fluxes varied primarily in relation to flood duration. While the ecosystem was a net C sink on an annual basis (absorbing 218 g C m ?2 (as CH 4‐C + CO 2‐C) in anaerobic phases and emitting 76 g C m ?2in aerobic phases), high CH 4 effluxes during the anaerobic flooded phase and modest CH 4 effluxes during the aerobic phase indicate that seasonally flooded tropical forests can be a net source of radiative forcings on an annual basis, thus acting as an amplifying feedback on global warming. 相似文献
8.
This study was conducted to evaluate the impacts of N fertilizer and landscape position on carbon dioxide (CO 2) and methane (CH 4) fluxes from a US Northern Great Plains landscape seeded to switchgrass ( Panicum virgatum L.). The experimental design included three N levels (low, 0 kg N ha −1; medium, 56 kg N ha −1; and high, 112 kg N ha −1) replicated four times. The experiment was repeated at shoulder and footslope positions. Soil CO 2 and CH 4 fluxes were monitored once every 2 weeks from May 2010 to October 2012. The CO 2 fluxes were 40% higher at the footslope than the shoulder landscape position, and CH 4 fluxes were similar in both landscape positions. Soil CO 2 and CH 4 fluxes averaged over the sampling dates were not impacted by N rates. Seasonal variations showed highest CO 2 release and CH 4 uptake in summer and fall, likely due to warmer and moist soil conditions. Higher CH 4 release was observed in winter possibly due to increased anaerobic conditions. However, year to year (2010–2012) variations in soil CO 2 and CH 4 fluxes were more pronounced than the variations due to the impact of landscape positions and N rates. Drought conditions reported in 2012, with higher annual temperature and lower soil moisture than long-term average, resulted in higher summer and fall CO 2 fluxes (between 1.3 and 3 times) than in 2011 and 2010. These conditions also promoted a net CH 4 uptake in 2012 in comparison to 2010 when there was net CH 4 release. Results from this study conclude that landscape positions, air temperature, and soil moisture content strongly influenced soil CO 2 fluxes, whereas soil moisture impacted the direction of CH 4 fluxes (uptake or release). However, a comprehensive life cycle analysis would be appropriate to evaluate environmental impacts associated with switchgrass production under local environmental conditions. 相似文献
9.
Forest soils and canopies are major components of ecosystem CO 2 and CH 4 fluxes. In contrast, less is known about coarse woody debris and living tree stems, both of which function as active surfaces for CO 2 and CH 4 fluxes. We measured CO 2 and CH 4 fluxes from soils, coarse woody debris, and tree stems over the growing season in an upland temperate forest. Soils were CO 2 sources (4.58 ± 2.46 µmol m ?2 s ?1, mean ± 1 SD) and net sinks of CH 4 (?2.17 ± 1.60 nmol m ?2 s ?1). Coarse woody debris was a CO 2 source (4.23 ± 3.42 µmol m ?2 s ?1) and net CH 4 sink, but with large uncertainty (?0.27 ± 1.04 nmol m ?2 s ?1) and with substantial differences depending on wood decay status. Stems were CO 2 sources (1.93 ± 1.63 µmol m ?2 s ?1), but also net CH 4 sources (up to 0.98 nmol m ?2 s ?1), with a mean of 0.11 ± 0.21 nmol m ?2 s ?1 and significant differences depending on tree species. Stems of N. sylvatica, F. grandifolia, and L. tulipifera consistently emitted CH 4, whereas stems of A. rubrum, B. lenta, and Q. spp. were intermittent sources. Coarse woody debris and stems accounted for 35% of total measured CO 2 fluxes, whereas CH 4 emissions from living stems offset net soil and CWD CH 4 uptake by 3.5%. Our results demonstrate the importance of CH 4 emissions from living stems in upland forests and the need to consider multiple forest components to understand and interpret ecosystem CO 2 and CH 4 dynamics. 相似文献
10.
Terrestrial ecosystems in northern high latitudes exchange large amounts of methane (CH 4) with the atmosphere. Climate warming could have a great impact on CH 4 exchange, in particular in regions where degradation of permafrost is induced. In order to improve the understanding of the present and future methane dynamics in permafrost regions, we studied CH 4 fluxes of typical landscape structures in a small catchment in the forest tundra ecotone in northern Siberia. Gas fluxes were measured using a closed‐chamber technique from August to November 2003 and from August 2006 to July 2007 on tree‐covered mineral soils with and without permafrost, on a frozen bog plateau, and on a thermokarst pond. For areal integration of the CH 4 fluxes, we combined field observations and classification of functional landscape structures based on a high‐resolution Quickbird satellite image. All mineral soils were net sinks of atmospheric CH 4. The magnitude of annual CH 4 uptake was higher for soils without permafrost (1.19 kg CH 4 ha −1 yr −1) than for soils with permafrost (0.37 kg CH 4 ha −1 yr −1). In well‐drained soils, significant CH 4 uptake occurred even after the onset of ground frost. Bog plateaux, which stored large amounts of frozen organic carbon, were also a net sink of atmospheric CH 4 (0.38 kg CH 4 ha −1 yr −1). Thermokarst ponds, which developed from permafrost collapse in bog plateaux, were hot spots of CH 4 emission (approximately 200 kg CH 4 ha −1 yr −1). Despite the low area coverage of thermokarst ponds (only 2.1% of the total catchment area), emissions from these sites resulted in a mean catchment CH 4 emission of 3.8 kg CH 4 ha −1 yr −1. Export of dissolved CH 4 with stream water was insignificant. The results suggest that mineral soils and bog plateaux in this region will respond differently to increasing temperatures and associated permafrost degradation. Net uptake of atmospheric CH 4 in mineral soils is expected to gradually increase with increasing active layer depth and soil drainage. Changes in bog plateaux will probably be much more rapid and drastic. Permafrost collapse in frozen bog plateaux would result in high CH 4 emissions that act as positive feedback to climate warming. 相似文献
11.
Upland soils are important sinks for atmospheric methane (CH 4), a process essentially driven by methanotrophic bacteria. Soil CH 4 uptake often depends on land use, with afforestation generally increasing the soil CH 4 sink. However, the mechanisms driving these changes are not well understood to date. We measured soil CH 4 and N 2O fluxes along an afforestation chronosequence with Norway spruce (Picea abies L.) established on an extensively grazed subalpine pasture. Our experimental design included forest stands with ages ranging from 25 to >120 years and included a factorial cattle urine addition treatment to test for the sensitivity of soil CH 4 uptake to N application. Mean CH 4 uptake significantly increased with stand age on all sampling dates. In contrast, CH 4 oxidation by sieved soils incubated in the laboratory did not show a similar age dependency. Soil CH 4 uptake was unrelated to soil N status (but cattle urine additions stimulated N 2O emission). Our data indicated that soil CH 4 uptake in older forest stands was driven by reduced soil water content, which resulted in a facilitated diffusion of atmospheric CH 4 into soils. The lower soil moisture likely resulted from increased interception and/or evapotranspiration in the older forest stands. This mechanism contrasts alternative explanations focusing on nitrogen dynamics or the composition of methanotrophic communities, although these factors also might be at play. Our findings further imply that the current dramatic increase in forested area increases CH 4 uptake in alpine regions. 相似文献
12.
Wetlands are the largest source of methane (CH 4) globally, yet our understanding of how process‐level controls scale to ecosystem fluxes remains limited. It is particularly uncertain how variable soil properties influence ecosystem CH 4 emissions on annual time scales. We measured ecosystem carbon dioxide (CO 2) and CH 4 fluxes by eddy covariance from two wetlands recently restored on peat and alluvium soils within the Sacramento–San Joaquin Delta of California. Annual CH 4 fluxes from the alluvium wetland were significantly lower than the peat site for multiple years following restoration, but these differences were not explained by variation in dominant climate drivers or productivity across wetlands. Soil iron (Fe) concentrations were significantly higher in alluvium soils, and alluvium CH 4 fluxes were decoupled from plant processes compared with the peat site, as expected when Fe reduction inhibits CH 4 production in the rhizosphere. Soil carbon content and CO 2 uptake rates did not vary across wetlands and, thus, could also be ruled out as drivers of initial CH 4 flux differences. Differences in wetland CH 4 fluxes across soil types were transient; alluvium wetland fluxes were similar to peat wetland fluxes 3 years after restoration. Changing alluvium CH 4 emissions with time could not be explained by an empirical model based on dominant CH 4 flux biophysical drivers, suggesting that other factors, not measured by our eddy covariance towers, were responsible for these changes. Recently accreted alluvium soils were less acidic and contained more reduced Fe compared with the pre‐restoration parent soils, suggesting that CH 4 emissions increased as conditions became more favorable to methanogenesis within wetland sediments. This study suggests that alluvium soil properties, likely Fe content, are capable of inhibiting ecosystem‐scale wetland CH 4 flux, but these effects appear to be transient without continued input of alluvium to wetland sediments. 相似文献
13.
Microbial metabolism of the thawing organic carbon stores in permafrost results in a positive feedback loop of greenhouse gas emissions. CO 2 and CH 4 fluxes and the associated microbial communities in Arctic cryosols are important in predicting future warming potential of the Arctic. We demonstrate that topography had an impact on CH 4 and CO 2 flux at a high Arctic ice-wedge polygon terrain site, with higher CO 2 emissions and lower CH 4 uptake at troughs compared to polygon interior soils. The pmoA sequencing suggested that USCα cluster of uncultured methanotrophs is likely responsible for observed methane sink. Community profiling revealed distinct assemblages across the terrain at different depths. Deeper soils contained higher abundances of Verrucomicrobia and Gemmatimonadetes, whereas the polygon interior had higher Acidobacteria and lower Betaproteobacteria and Deltaproteobacteria abundances. Genome sequencing of isolates from the terrain revealed presence of carbon cycling genes including ones involved in serine and ribulose monophosphate pathways. A novel hybrid network analysis identified key members that had positive and negative impacts on other species. Operational Taxonomic Units (OTUs) with numerous positive interactions corresponded to Proteobacteria, Candidatus Rokubacteria and Actinobacteria phyla, while Verrucomicrobia and Acidobacteria members had negative impacts on other species. Results indicate that topography and microbial interactions impact community composition. 相似文献
14.
Tropical forests on upland soils are assumed to be a methane (CH 4) sink and a weak source of nitrous oxide (N 2O), but studies of wetland forests have demonstrated that tree stems can be a substantial source of CH 4, and recent evidence from temperate woodlands suggests that tree stems can also emit N 2O. Here, we measured CH 4 and N 2O fluxes from the soil and from tree stems in a semi‐evergreen tropical forest on upland soil. To examine the influence of seasonality, soil abiotic conditions and substrate availability (litter inputs) on trace greenhouse gas (GHG) fluxes, we conducted our study during the transition from the dry to the wet season in a long‐term litter manipulation experiment in Panama, Central America. Trace GHG fluxes were measured from individual stem bases of two common tree species and from soils beneath the same trees. Soil CH 4 fluxes varied from uptake in the dry season to minor emissions in the wet season. Soil N 2O fluxes were negligible during the dry season but increased markedly after the start of the wet season. By contrast, tree stem bases emitted CH 4 and N 2O throughout the study. Although we observed no clear effect of litter manipulation on trace GHG fluxes, tree species and litter treatments interacted to influence CH 4 fluxes from stems and N 2O fluxes from stems and soil, indicating complex relationships between tree species traits and decomposition processes that can influence trace GHG dynamics. Collectively, our results show that tropical trees can act as conduits for trace GHGs that most likely originate from deeper soil horizons, even when they are growing on upland soils. Coupled with the finding that the soils may be a weaker sink for CH 4 than previously thought, our research highlights the need to reappraise trace gas budgets in tropical forests. 相似文献
15.
Atmospheric concentrations of methane (CH 4) and nitrous oxide (N 2O) have increased over the last 150 years because of human activity. Soils are important sources and sinks of both potent greenhouse gases where their production and consumption are largely regulated by biological processes. Climate change could alter these processes thereby affecting both rate and direction of their exchange with the atmosphere. We examined how a rise in atmospheric CO 2 and temperature affected CH 4 and N 2O fluxes in a well‐drained upland soil (volumetric water content ranging between 6% and 23%) in a semiarid grassland during five growing seasons. We hypothesized that responses of CH 4 and N 2O fluxes to elevated CO 2 and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO 2 increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO 2 and warming would have opposing effects on CH 4 and N 2O fluxes. Methane was taken up throughout the growing season in all 5 years. A bell‐shaped relationship was observed with soil moisture with highest CH 4 uptake at intermediate soil moisture. Both N 2O emission and uptake occurred at our site with some years showing cumulative N 2O emission and other years showing cumulative N 2O uptake. Nitrous oxide exchange switched from net uptake to net emission with increasing soil moisture. In contrast to our hypothesis, both elevated CO 2 and warming reduced the sink of CH 4 and N 2O expressed in CO 2 equivalents (across 5 years by 7% and 11% for elevated CO 2 and warming respectively) suggesting that soil moisture changes were not solely responsible for this reduction. We conclude that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH 4 and N 2O expressed in CO 2‐equivalents. 相似文献
16.
Microbial oxidation in aerobic soils is the primary biotic sink for atmospheric methane (CH 4), a powerful greenhouse gas. Although tropical forest soils are estimated to globally account for about 28% of annual soil CH 4 consumption (6.2 Tg CH 4 year ?1), limited data are available on CH 4 exchange from tropical montane forests. We present the results of an extensive study on CH 4 exchange from tropical montane forest soils along an elevation gradient (1,000, 2,000, 3,000 m) at different topographic positions (lower slope, mid-slope, ridge position) in southern Ecuador. All soils were net atmospheric CH 4 sinks, with decreasing annual uptake rates from 5.9 kg CH 4–C ha ?1 year ?1 at 1,000 m to 0.6 kg CH 4–C ha ?1 year ?1 at 3,000 m. Topography had no effect on soil atmospheric CH 4 uptake. We detected some unexpected factors controlling net methane fluxes: positive correlations between CH 4 uptake rates, mineral nitrogen content of the mineral soil and with CO 2 emissions indicated that the largest CH 4 uptake corresponded with favorable conditions for microbial activity. Furthermore, we found indications that CH 4 uptake was N limited instead of inhibited by NH 4 +. Finally, we showed that in contrast to temperate regions, substantial high affinity methane oxidation occurred in the thick organic layers which can influence the CH 4 budget of these tropical montane forest soils. Inclusion of elevation as a co-variable will improve regional estimates of methane exchange in these tropical montane forests. 相似文献
17.
Background and aimsThe litter layer is a major source of CO2, and it also influences soil-atmosphere exchange of N2O and CH4. So far, it is not clear how much of soil greenhouse gas (GHG) emission derives from the litter layer itself or is litter-induced. The present study investigates how the litter layer controls soil GHG fluxes and microbial decomposer communities in a temperate beech forest. MethodsWe removed the litter layer in an Austrian beech forest and studied responses of soil CO2, CH4 and N2O fluxes and the microbial community via phospholipid fatty acids (PLFA). Soil GHG fluxes were determined with static chambers on 22 occasions from July 2012 to February 2013, and soil samples collected at 8 sampling events. ResultsLitter removal reduced CO2 emissions by 30 % and increased temperature sensitivity (Q10) of CO2 fluxes. Diffusion of CH4 into soil was facilitated by litter removal and CH4 uptake increased by 16 %. This effect was strongest in autumn and winter when soil moisture was high. Soils without litter turned from net N2O sources to slight N2O sinks because N2O emissions peaked after rain events in summer and autumn, which was not the case in litter-removal plots. Microbial composition was only transiently affected by litter removal but strongly influenced by seasonality. ConclusionsLitter layers must be considered in calculating forest GHG budgets, and their influence on temperature sensitivity of soil GHG fluxes taken into account for future climate scenarios. 相似文献
18.
Seasonal fluxes of dissolved oxygen, inorganic carbon and methane were measured in microcosms containing vegetated ( Vallisneria spiralis L.) and unvegetated sediments under controlled laboratory conditions. We tested if measured fluxes were affected by a moderate (6% as loss on ignition, LOI) and an elevated (10%) organic matter content (OM) in sediments. Microcosms were set up with plants and sediments collected from two riverine sites, upstream (moderate OM load) and downstream (elevated OM load) of a wastewater treatment plant. Light and dark fluxes were measured and V. spiralis net primary production and respiration rates were calculated. Unvegetated sediments were always net heterotrophic and behaved as methane sources to the water column, with significantly higher CH 4 release during summer from sediment with elevated OM load. Vegetated sediments were always net autotrophic with attenuated or negative CH 4 fluxes, suggesting the occurrence of processes within the rhizosphere that inhibit methane production or favor its oxidation. Vegetated sediments had an unbalanced O 2 to DIC stoichiometry, with average photosynthetic quotients varying between 0.30 and 0.68, significantly below one. The missing oxygen amount varied seasonally, with a minimum in the summer coinciding with the highest water temperature, but was not dependent upon the two OM levels. Overall these results suggest that V. spiralis is likely to transport a significant proportion of photosynthetically produced oxygen to the rhizosphere. 相似文献
20.
In arctic and alpine ecosystems, soil nitrogen (N) dynamics can differ markedly between winter and summer months, and nitrogen
losses can be measurable during the spring and fall transitions. To explore the effect of seasonality on biogeochemical processes
in a temperate alpine environment, we used a combination of field incubations (year-round) and 15N tracer additions (late fall, early spring, summer) to characterize soil N dynamics in a wet and dry meadow in the Sierra
Nevada, California. The snowmelt to early summer season marked a period of high 15N uptake and turnover in the two soils, coincident with the increase in microbial N pools at the start of snowmelt (wet and
dry meadow); an increase in net N mineralization and net nitrification as snowmelt progressed (wet meadow only); and measureable
net production of 15N-NH 4
+ in mid-summer (wet and dry meadow). Whereas fluctuations in microbial biomass were generally synchronous between the wet
and dry meadow soils, only wet meadow soils appeared to mineralize N in response to declines in the microbial N pool. Net
N mineralization and net nitrification rates in the dry meadow soil were negligible on all but one sampling date, in spite
of periodic decreases in biomass of up to 60%. Across both sites, high 15N recoveries in microbial biomass N, rapid 15N-NH 4
+ turnover, and low or negative net 15N-NH 4
+ fluxes suggested tight cycling of N, particularly in the late fall and early spring. 相似文献
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