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1.
We report the effect of CH 4 and of CH 4 oxidation on nitrification in freshwater sediment from Hamilton Harbour, Ontario, Canada, a highly polluted ecosystem. Aerobic slurry experiments showed a high potential for aerobic N 2O production in some sites. It was suppressed by C 2H 2, correlated to NO 3- production, and stimulated by NH 4+ concentration, supporting the hypothesis of a nitrification-dependent source for this N 2O production. Diluted sediment slurries supplemented with CH 4 (1 to 24 μM) showed earlier and enhanced nitrification and N 2O production compared with unsupplemented slurries (≤1 μM CH 4). This suggests that nitrification by methanotrophs may be significant in freshwater sediment under certain conditions. Suppression of nitrification was observed at CH 4 concentrations of 84 μM and greater, possibly through competition for O 2 between methanotrophs and NH 4+ -oxidizing bacteria and/or competition for mineral N between these two groups of organisms. In Hamilton Harbour sediment, the very high CH 4 concentrations (1.02 to 6.83 mM) which exist would probably suppress nitrification and favor NH 4+ accumulation in the pore water. Indeed, NH 4+ concentrations in Hamilton Harbour sediment are higher than those found in other lakes. We conclude that the impact of CH 4 metabolism on N cycling processes in freshwater ecosystems should be given more attention. 相似文献
2.
Atmospheric nitrogen (N) deposition is rapidly increasing in tropical regions. We investigated how a decade of experimental N addition (125 kg N ha ?1 year ?1) to a seasonal lowland forest affected depth distribution and contents of soil nitrous oxide (N 2O), carbon dioxide (CO 2) and methane (CH 4), as well as natural abundance isotopic signatures of N 2O, nitrate (NO 3 ?) and ammonium (NH 4 +). In the control plots during dry season, we deduced limited N 2O production by denitrification in the topsoil (0.05–0.40 m) as indicated by: ambient N 2O concentrations and ambient 15N-N 2O signatures, low water-filled pore space (35–60%), and similar 15N signatures of N 2O and NO 3 ?. In the subsoil (0.40–2.00 m), we detected evidence of N 2O reduction to N 2 during upward diffusion, indicating denitrification activity. During wet season, we found that N 2O at 0.05–2.00 m was mainly produced by denitrification with substantial further reduction to N 2, as indicated by: lighter 15N-N 2O than 15N-NO 3 ? throughout the profile, and increasing N 2O concentrations with simultaneously decreasing 15N-N 2O enrichment with depth. These interpretations were supported by an isotopomer map and by a positive correlation between 18O-N 2O and 15N-N 2O site preferences. Long-term N addition did not affect dry-season soil N 2O-N contents, doubled wet-season soil N 2O-N contents, did not affect 15N signatures of NO 3 ?, and reduced wet-season 15N signatures of N 2O compared to the control plots. These suggest that the increased NO 3 ? concentrations have stimulated N 2O production and decreased N 2O-to-N 2 reduction. Soil CO 2-C contents did not differ between treatments, implying that N addition essentially did not influence soil C cycling. The pronounced seasonality in soil respiration was largely attributable to enhanced topsoil respiration as indicated by a wet-season increase in the topsoil CO 2-C contents. The N-addition plots showed reduced dry-season soil CH 4-C contents and threshold CH 4 concentrations were reached at a shallower depth compared to the control plots, revealing an N-induced stimulation of methanotrophic activity. However, the net soil CH 4 uptake rates remained similar between treatments possibly because diffusive CH 4 supply from the atmosphere largely limited CH 4 oxidation. 相似文献
3.
Temperate pastures are often managed with P fertilizers and N 2-fixing legumes to maintain and increase pasture productivity which may lead to greater nitrous oxide (N 2O) emissions and reduced methane (CH 4) uptake. However, the diel and inter-daily variation in N 2O and CH 4 flux in pastures is poorly understood, especially in relation to key environmental drivers. We investigated the effect of pasture productivity, rainfall, and changing soil moisture and temperature upon short-term soil N 2O and CH 4 flux dynamics during spring in sheep grazed pasture systems in southeastern Australia. N 2O and CH 4 flux was measured continuously in a High P (23 kg P ha ?1 yr ?1) and No P pasture treatment and in a sheep camp area in a Low P (4 kg P ha ?1 yr ?1) pasture for a four week period in spring 2005 using an automated trace gas system. Although pasture productivity was three-fold greater in the High P than No P treatment, mean CH 4 uptake was similar (?6.3?±?SE 0.3 to ?8.6?±?0.4 μg C m ?2 hr ?1) as were mean N 2O emissions (6.5 to 7.9?±?0.8 μg N m ?2 hr ?1), although N 2O flux in the No P pasture did not respond to changing soil water conditions. N 2O emissions were greatest in the Low P sheep camp (12.4 μg?±?1.1 N m ?2 hr ?1) where there were also net CH 4 emissions of 5.2?±?0.5 μg C m ?2 hr ?1. There were significant, but weak, relationships between soil water and N 2O emissions, but not between soil water and CH 4 flux. The diel temperature cycle strongly influenced CH 4 and N 2O emissions, but this was often masked by the confounding covariate effects of changing soil water content. There were no consistently significant differences in soil mineral N or gross N transformation rates, however, measurements of substrate induced respiration (SIR) indicated that soil microbial processes in the highly productive pasture are more N limited than P limited after >20 years of P fertilizer addition. Increased productivity, through P fertilizer and legume management, did not significantly increase N 2O emissions, or reduce CH 4 uptake, during this 4 week measurement period, but the lack of an N 2O response to rainfall in the No P pasture suggests this may be evident over a longer measurement period. This study also suggests that small compacted and nutrient enriched areas of grazed pastures may contribute greatly to the overall N 2O and CH 4 trace gas balance. 相似文献
4.
The two non-CO 2 greenhouse gases (GHGs) nitrous oxide (N 2O) and methane (CH 4) comprise 54.8% of total New Zealand emissions. Nitrous oxide is mainly generated from mineral N originating from animal dung and urine, applied fertiliser N, biologically fixed N 2, and mineralisation of soil organic N. Even though about 96% of the anthropogenic CH 4 emitted in New Zealand is from ruminant animals (methanogenesis), methane uptake by aerobic soils (methanotrophy) can significantly contribute to the removal of CH 4 from the atmpsphere, as the global estimates confirm. Both the net uptake of CH 4 by soils and N 2O emissions from soils are strongly influenced by changes in land use and land management. Quantitative information on the fluxes of these two non-CO 2 GHGs is required for a range of land-use and land-management ecosystems to determine their contribution to the national emissions inventory, and for assessing the potential of mitigation options. Here we report soil N 2O fluxes and CH 4 uptake for a range of land-use and land-management systems collated from published and unpublished New Zealand studies. Nitrous oxide emissions are highest in dairy-grazed pastures (10–12 kg N 2O–N ha ?1 year ? 1), intermediate in sheep-grazed pastures, (4–6 kg N 2O–N ha ?1 year ?1), and lowest in forest, shrubland and ungrazed pasture soils (1–2 kg N 2O–N ha ?1 year ?1). N deposited in the form of animal urine and dung, and N applied as fertiliser, are the principal sources of N 2O production. Generally, N 2O emissions from grazed pasture soils are high when the soil water-filled pore-space is above field capacity, and net CH 4 uptake is low or absent. Although nitrification inhibitors have shown some promise in reducing N 2O emissions from grazed pasture systems, their efficacy as an integral part of farm management has yet to be tested. Methane uptake was highest for a New Zealand Beech forest soil (10–11 kg CH 4 ha ?1 year ?1), intermediate in some pine forest soils (4–6 kg CH 4 ha ?1 year ?1), and lowest in most pasture (<1 kg CH 4 ha ?1 year ?1) and cropped soils (1.5 kg CH 4 ha ?1 year ?1). Afforestation /reforestation of pastures results in increases in soil CH 4 uptake, largely as a result of increases in soil aeration status and changes in the population and activities of methanotrophs. Soil CH 4 uptake is also seasonally dependent, being about two to three times higher in a dry summer and autumn than in a wet winter. There are no practical ways yet available to reduce CH 4 emissions from agricultural systems. The mitigation options to reduce gaseous emissions are discussed and future research needs identified. 相似文献
5.
The rapid expansion of intensively farmed vegetable fields has substantially contributed to the total N 2O emissions from croplands in China. However, to date, the mechanisms underlying this phenomenon have not been completely understood. To quantify the contributions of autotrophic nitrification, heterotrophic nitrification, and denitrification to N 2O production from the intensive vegetable fields and to identify the affecting factors, a 15N tracing experiment was conducted using five soil samples collected from adjacent fields used for rice-wheat rotation system (WF), or for consecutive vegetable cultivation (VF) for 0.5 (VF1), 6 (VF2), 8 (VF3), and 10 (VF4) years. Soil was incubated under 50% water holding capacity (WHC) at 25°C for 96 h after being labeled with 15NH 4NO 3 or NH 4 15 NO 3. The average N 2O emission rate was 24.2 ng N?kg ?1 h ?1 in WF soil, but it ranged from 69.6 to 507 ng N?kg ?1 h ?1 in VF soils. Autotrophic nitrification, heterotrophic nitrification and denitrification accounted for 0.3–31.4%, 25.4–54.4% and 22.5–57.7% of the N 2O emissions, respectively. When vegetable soils were moderately acidified (pH, 6.2 to ?≥?5.7), the increased N 2O emissions resulted from the increase of both the gross autotrophic and heterotrophic nitrification rates and the N 2O product ratio of autotrophic nitrification. However, once severe acidification occurred (as in VF4, pH?≤?4.3) and salt stress increased, both autotrophic and heterotrophic nitrification rates were inhibited to levels similar to those of WF soil. The enhanced N 2O product ratios of heterotrophic nitrification (4.84‰), autotrophic nitrification (0.93‰) and denitrification processes were the most important factors explaining high N 2O emission in VF4 soil. Data from this study showed that various soil conditions (e.g., soil salinity and concentration of NO 3 - or NH 4 + ) could also significantly affect the sources and rates of N 2O emission. 相似文献
6.
Combined measurements of nitrification activity and N 2O 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 N 2O emissions were available, nitrification was positively correlated with N 2O emission. Nitrification contributed significantly to N 2O 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 N 2O. The N 2O 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 N 2O 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 N 2O emissions from soils of terrestrial ecosystems. 相似文献
7.
During two intensive field campaigns in summer and autumn 2004 nitrogen (N 2O, NO/NO 2) and carbon (CO 2, CH 4) trace gas exchange between soil and the atmosphere was measured in a sessile oak ( Quercus petraea (Matt.) Liebl.) forest in Hungary. The climate can be described as continental temperate. Fluxes were measured with a fully automatic measuring system allowing for high temporal resolution. Mean N 2O emission rates were 1.5 μg N m −2 h −1 in summer and 3.4 μg N m −2 h −1 in autumn, respectively. Also mean NO emission rates were higher in autumn (8.4 μg N m −2 h −1) as compared to summer (6.0 μg N m −2 h −1). However, as NO 2 deposition rates continuously exceeded NO emission rates (−9.7 μg N m −2 h −1 in summer and −18.3 μg N m −2 h −1 in autumn), the forest soil always acted as a net NO
x
sink. The mean value of CO 2 fluxes showed only little seasonal differences between summer (81.1 mg C m −2 h −1) and autumn (74.2 mg C m −2 h −1) measurements, likewise CH 4uptake (summer: −52.6 μg C m −2 h −1; autumn: −56.5 μg C m −2 h −1). In addition, the microbial soil processes net/gross N mineralization, net/gross nitrification and heterotrophic soil respiration as well as inorganic soil nitrogen concentrations and N 2O/CH 4 soil air concentrations in different soil depths were determined. The respiratory quotient (ΔCO 2 resp ΔO 2 resp−1) for the uppermost mineral soil, which is needed for the calculation of gross nitrification via the Barometric Process Separation (BaPS) technique, was 0.8978 ± 0.008. The mean value of gross nitrification rates showed only little seasonal differences between summer (0.99 μg N kg −1 SDW d −1) and autumn measurements (0.89 μg N kg −1 SDW d −1). Gross rates of N mineralization were highest in the organic layer (20.1–137.9 μg N kg −1 SDW d −1) and significantly lower in the uppermost mineral layer (1.3–2.9 μg N kg −1 SDW d −1). Only for the organic layer seasonality in gross N mineralization rates could be demonstrated, with highest mean values in autumn, most likely caused by fresh litter decomposition. Gross mineralization rates of the organic layer were positively correlated with N 2O emissions and negatively correlated with CH 4 uptake, whereas soil CO 2 emissions were positively correlated with heterotrophic respiration in the uppermost mineral soil layer. The most important abiotic factor influencing C and N trace gas fluxes was soil moisture, while the influence of soil temperature on trace gas exchange rates was high only in autumn. 相似文献
8.
Rice is staple food of half of mankind and paddy soils account for the largest anthropogenic wetlands on earth. Ample of research is being done to find cultivation methods under which the integrative greenhouse effect caused by emitted CH 4 and N 2O would be mitigated. Whereas most of the research focuses on quantifying such emissions, there is a lack of studies on the biogeochemistry of paddy soils. In order to deepen our mechanistic understanding of N 2O and CH 4 fluxes in rice paddies, we also determined NO 3 ? and N 2O concentrations as well as N 2O isotope abundances and presence of O 2 along soil profiles of paddies which underwent three different water managements during the rice growing season(s) in (2010 and) 2011 in Korea. Largest amounts of N 2O (2 mmol m ?2) and CH 4 (14.5 mol m ?2) degassed from the continuously flooded paddy, while paddies with less flooding showed 30–60 % less CH 4 emissions and very low to negative N 2O balances. In accordance, the global warming potential (GWP) was lowest for the Intermittent Irrigation paddy and highest for the Traditional Irrigation paddy. The N 2O emissions could the best be explained (* P < 0.05) with the δ 15N values and N 2O concentrations in 40–50 cm soil depth, implying that major N 2O production/consumption occurs there. No significant effect of NO 3 ? on N 2O production has been found. Our study gives insight into the soil of a rice paddy and reveals areas along the soil profile where N 2O is being produced. Thereby it contributes to our understanding of subsoil processes of paddy soils. 相似文献
9.
Nitrous oxide (N 2O) in soils is produced through nitrification and denitrification. The N 2O produced is considered as a nitrogen (N) loss because it will most likely escape from the soil to the atmosphere as N 2O or N 2. Aim of the study was to quantify N 2O production in grassland on peat soils in relation to N input and to determine the relative contribution of nitrification and denitrification to N 2O production. Measurements were carried out on a weekly basis in 2 grasslands on peat soil (Peat I and Peat II) for 2 years (1993 and 1994) using intact soil core incubations. In additional experiments distinction between N 2O from nitrification and denitrification was made by use of the gaseous nitrification inhibitor methyl fluoride (CH 3F).Nitrous oxide production over the 2 year period was on average 34 kg N ha -1 yr -1 for mown treatments that received no N fertiliser and 44 kg N ha -1 yr -1 for mown and N fertilised treatments. Grazing by dairy cattle on Peat I caused additional N 2O production to reach 81 kg N ha -1 yr -1. The sub soil (20–40 cm) contributed 25 to 40% of the total N 2O production in the 0–40 cm layer. The N 2O production:denitrification ratio was on average about 1 in the top soil and 2 in the sub soil indicating that N 2O production through nitrification was important. Experiments showed that when ratios were larger than l, nitrification was the major source of N 2O. In conclusion, N 2O production is a significant N loss mechanism in grassland on peat soil with nitrification as an important N 2O producing process. 相似文献
10.
Currently, there is a lack of knowledge about GHG emissions, specifically N 2O and CH 4, in subtropical coastal freshwater wetland and mangroves in the southern hemisphere. In this study, we quantified the gas fluxes and substrate availability in a subtropical coastal wetland off the coast of southeast Queensland, Australia over a complete wet-dry seasonal cycle. Sites were selected along a salinity gradient ranging from marine (34 psu) in a mangrove forest to freshwater (0.05 psu) wetland, encompassing the range of tidal influence. Fluxes were quantified for CH 4 (range ?0.4–483 mg C–CH 4 h ?1 m ?2) and N 2O (?5.5–126.4 μg N–N 2O h ?1 m ?2), with the system acting as an overall source for CH 4 and N 2O (mean N 2O and CH 4 fluxes: 52.8 μg N–N 2O h ?1 m ?2 and 48.7 mg C–CH 4 h ?1 m ?2, respectively). Significantly higher N 2O fluxes were measured during the summer months (summer mean 64.2 ± 22.2 μg N–N 2O h ?1 m ?2; winter mean 33.1 ± 24.4 µg N–N 2O h –1 m ?2) but not CH 4 fluxes (summer mean 30.2 ± 81.1 mg C–CH 4 h ?1 m ?2; winter mean 37.4 ± 79.6 mg C–CH 4 h ?1 m ?2). The changes with season are primarily driven by temperature and precipitation controls on the dissolved inorganic nitrogen (DIN) concentration. A significant spatial pattern was observed based on location within the study site, with highest fluxes observed in the freshwater tidal wetland and decreasing through the mangrove forest. The dissolved organic carbon (DOC) varied throughout the landscape and was correlated with higher CH 4 fluxes, but this was a nonlinear trend. DIN availability was dominated by N–NH 4 and correlated to changes in N 2O fluxes throughout the landscape. Overall, we did not observe linear relationships between CH 4 and N 2O fluxes and salinity, oxygen or substrate availability along the fresh-marine continuum, suggesting that this ecosystem is a mosaic of processes and responses to environmental changes. 相似文献
11.
Here we report on a controlled environment experiment in which we applied 13C- and 15N-enrichment approaches to quantify methane oxidation rates and source partition N 2O production in a silt loam soil following application of NH 4NO 3, enabling us to look for potential interactions between methane oxidation and nitrifier-N 2O production. 15N-N 2O, 14+15N-N 2O and CO 2 fluxes and mineral N concentrations were measured over a 23-day period after application of NH 4NO 3 (5 at.% excess 15N) at rates of 0, 5, 10, 20, 30 and 40 g N m ?2 to a silt loam soil. Change in 12/13C-CH 4 concentrations (as indicative of 13C-CH 4 oxidation rates) and production of 13C-CO 2 were monitored over the first 72 h after addition of 1.7 ??l 13C-CH 4 l ?1 (10 at.% excess 13C) to these N treatments. Oxidation of applied 13C-CH 4 was slower in the 5, 10, 20 and 30 g N m ?2 (5 at.% excess 15N) treatments (0.24?C0.32 ??g 13C-CH 4 l ?1 day ?1) than in the control (0.40 ??g 13C-CH 4 l ?1 day ?1), suggesting that these N loadings inhibited oxidation. N 2O production was raised after N addition, and in the 10, 20 and 30 g N m ?2 treatments nitrification was the predominant source of N 2O accounting for 61, 83 and 57% of the total 15N-N 2O produced, respectively. Our results point towards the possibility of methylotrophs switching function to oxidise ammonia in the presence of N, which may result in greater atmospheric loading of both CH 4 and N 2O. 相似文献
12.
The occurrence of bottom-water hypoxia is increasing in bodies of water around the world. Hypoxia is of concern due to the way it negatively impacts lakes and estuaries at the whole ecosystem level. During 2015, we examined the influence of hypoxia on the Muskegon Lake ecosystem by collecting surface- and bottom-water nutrient samples, bacterial abundance counts, benthic fish community information, and performing profiles of chlorophyll and phycocyanin as proxies for phytoplankton and cyanobacterial growth, respectively. Several significant changes occurred in the bottom waters of the Muskegon Lake ecosystem as a result of hypoxia. Lake-wide concentrations of soluble reactive phosphorus (SRP) and total phosphorus increased with decreasing dissolved oxygen (DO). Bacterial abundance was significantly lower when DO was less than 2.2 mg L ?1. Whereas there were no drastic changes in surface chlorophyll a concentration through the season, phycocyanin increased threefold during and following a series of major wind-mixing events. Phycocyanin remained elevated for over 1.5 months despite several strong wind events, suggesting that high SRP concentrations in the bottom waters may have mixed into the surface waters, sustaining the bloom. The fish assemblage in the hypolimnion also changed in association with hypoxia. Overall fish abundance, number of species, and maximum length all decreased in catch as a function of bottom DO concentrations. The link between hypoxia and wind events appears to serve as a positive feedback loop by continuing internal loading and cyanobacterial blooms in the lake, while simultaneously eroding habitat quality for benthic fish. 相似文献
13.
Benthic biogeochemistry and macrofauna were investigated six times over 1 year in a shallow sub-tropical embayment. Benthic fluxes of oxygen (annual mean ?918 μmol O 2 m ?2 h ?1), ammonium (NH 4 +), nitrate (NO 3 ?), dissolved organic nitrogen, dinitrogen gas (N 2), and dissolved inorganic phosphorus were positively related to OM supply (N mineralisation) and inversely related to benthic light (N assimilation). Ammonium (NH 4 +), NO 3 ? and N 2 fluxes (annual means +14.6, +15.9 and 44.6 μmol N m ?2 h ?1) accounted for 14, 16 and 53 % of the annual benthic N remineralisation respectively. Denitrification was dominated by coupled nitrification–denitrification throughout the study. Potential assimilation of nitrogen by benthic microalgae (BMA) accounted for between 1 and 30 % of remineralised N, and was greatest during winter when bottom light was higher. Macrofauna biomass tended to be highest at intermediate benthic respiration rates (?1,000 μmol O 2 m ?2 h ?1), and appeared to become limited as respiration increased above this point. While bioturbation did not significantly affect net fluxes, macrofauna biomass was correlated with increased light rates of NH 4 + flux which may have masked reductions in NH 4 + flux associated with BMA assimilation during the light. Peaks in net N 2 fluxes at intermediate respiration rates are suggested to be associated with the stimulation of potential denitrification sites due to bioturbation by burrowing macrofauna. NO 3 ? fluxes suggest that nitrification was not significantly limited within respiration range measured during this study, however comparisons with other parts of Moreton Bay suggest that limitation of coupled nitrification–denitrification may occur in sub-tropical systems at respiration rates exceeding ?1,500 μmol O 2 m ?2 h ?1. 相似文献
14.
Methyl fluoride (CH 3F) and dimethyl ether (DME) inhibited nitrification in washed-cell suspensions of Nitrosomonas europaea and in a variety of oxygenated soils and sediments. Headspace additions of CH 3F (10% [vol/vol]) and DME (25% [vol/vol]) fully inhibited NO 2- and N 2O production from NH 4+ in incubations of N. europaea, while lower concentrations of these gases resulted in partial inhibition. Oxidation of hydroxylamine (NH 2OH) by N. europaea and oxidation of NO 2- by a Nitrobacter sp. were unaffected by CH 3F or DME. In nitrifying soils, CH 3F and DME inhibited N 2O production. In field experiments with surface flux chambers and intact cores, CH 3F reduced the release of N 2O from soils to the atmosphere by 20- to 30-fold. Inhibition by CH 3F also resulted in decreased NO 3- + NO 2- levels and increased NH 4+ levels in soils. CH 3F did not affect patterns of dissimilatory nitrate reduction to ammonia in cell suspensions of a nitrate-respiring bacterium, nor did it affect N 2O metabolism in denitrifying soils. CH 3F and DME will be useful in discriminating N 2O production via nitrification and denitrification when both processes occur and in decoupling these processes by blocking NO 2- and NO 3- production. 相似文献
15.
We examined controls of benthic dinitrogen (N 2) fixation and primary production in oligotrophic lakes in Arctic Alaska, Toolik Field Station (Arctic Long-Term Ecological Research Site). Primary production in many oligotrophic lakes is limited by nitrogen (N), and benthic processes are important for whole-lake function. Oligotrophic lakes are increasingly susceptible to low-level, non-point source nutrient inputs, yet the effects on benthic processes are not well understood. This study examines the results from a whole-lake fertilization experiment in which N and P were added at a relatively low level (4 times natural loading) in Redfield ratio to a shallow (3 m) and a deep (20 m) oligotrophic lake. The two lakes showed similar responses to fertilization: benthic primary production and respiration (each 50–150 mg C m ?2 day ?1) remained the same, and benthic N 2 fixation declined by a factor of three- to fourfold by the second year of treatment (from ~0.35 to 0.1 mg N m ?2 day ?1). This showed that the response of benthic N 2 fixation was de-coupled from the nutrient limitation status of benthic primary producers and raised questions about the mechanisms, which were examined in separate laboratory experiments. Bioassay experiments in intact cores also showed no response of benthic primary production to added N and P, but contrasted with the whole-lake experiment in that N 2 fixation did not respond to added N, either alone or in conjunction with P. This inconsistency was likely a result of nitrogenase activity of existing N 2 fixers during the relative short duration (9 days) of the bioassay experiment. N 2 fixation showed a positive saturating response when light was increased in the laboratory, but was not statistically related to ambient light level in the field, leading us to conclude that light limitation of the benthos from increasing water-column production was not important. Thus, increased N availability in the sediments through direct uptake likely caused a reduction in N 2 fixation. These results show the capacity of the benthos in oligotrophic systems to buffer the whole-system response to nutrient addition by the apparent ability for significant nutrient uptake and the rapid decline in N 2 fixation in response to added nutrients. Reduced benthic N 2 fixation may be an early indicator of a eutrophication response of lakes which precedes the transition from benthic to water-column-dominated systems. 相似文献
16.
At most sites the magnitude of soil-atmosphere exchange of nitrous dioxide (N 2O), carbon dioxide (CO 2) and methane (CH 4) was estimated based on a few chambers located in a limited area. Topography has been demonstrated to influence the production and consumption of these gases in temperate ecosystems, but this aspect has often been ignored in tropical areas. In this study, we investigated spatial variability of the net fluxes of these gases along a 100 m long slope of a evergreen broadleaved forest in southern China over a whole year. We expected that the lower part of slope would release more N 2O and CO 2, but take up less atmospheric CH 4 than the upper part due to different availability of water and nutrients. Our results showed that the soil moisture (Water Filled Pore Space, WFPS) decreased along the slope from bottom to top as we expected, but among the three gases only N 2O emissions followed this pattern. Annual means of WFPS ranged from 27.7% to 52.7% within the slope, and annual emissions of N 2O ranged from 2.0 to 4.4 kg N ha ?1 year ?1, respectively. These two variables were highly and positively correlated across the slope. Neither potential rates of net N mineralization and nitrification, nor N 2O emissions in the laboratory incubated soils varied with slope positions. Soil CO 2 release and CH 4 uptake appeared to be independent on slope position in this study. Our results suggested that soil water content and associated N 2O emissions are likely to be influenced by topography even in a short slope, which may need to be taken into account in field measurements and modelling. 相似文献
17.
We examined the possibility that microbial adaptation to temperature could affect rates of CO 2, N 2O and CH 4 release from soils. Laboratory incubations were used to determine the functional relationship between temperature and CO 2, N 2O and CH 4 fluxes for five soils collected across an elevational range in Hawaii. Initial rates of CO2 production and net N mineralization increased exponentially from 15 °C to 55 °C; initial rates of CH 4 and N 2O release were more complex. No optimum temperature (in which rates decline at higher and lower temperatures) was apparent for any of the gases, but respiration declined with time at higher temperatures, suggesting rapid depletion of readily available substrate. Mean Q 10S for respiration varied from 1.4 to 2.0, a typical range for tropical soils. The functional relationship between CO 2 production and temperature was consistent among all five soils, despite the substantial differences in mean annual temperature, soils, and land-use among the sites. Temperature responses of N 2O and CH 4 fluxes did not follow simple Q 10 relationships suggesting that temperature functions developed for CO 2 release from heterotrophic respiration cannot be simply extrapolated. Expanding this study to tropical heterotrophic respiration, the flux is more sensitive to changes in Q 10 than to changes in temperature on a per unit basis: the partial derivative with respect to temperature is 2.4 Gt C ·° C ?1 with respect to Q 10, it is 3.5 Gt C · Q 10 unit ?1. Therefore, what appears to be minor variability might still produce substantial uncertainty in regional estimates of gas exchange. 相似文献
18.
BackgroundThe greenhouse gas (GHG) mitigation is one of the most important environmental benefits of using bioenergy replacing fossil fuels. Nitrous oxide (N2O) and methane (CH4) are important GHGs and have drawn extra attention for their roles in global warming. Although there have been many works of soil emissions of N2O and CH4 from bioenergy crops in the field scale, GHG emissions in large area of marginal lands are rather sparse and how soil temperature and moisture affect the emission potential remains unknown. Therefore, we sought to estimate the regional GHG emission based on N2O and CH4 releases from the energy crop fields.ResultsHere we sampled the top soils from two Miscanthus fields and incubated them using a short-term laboratory microcosm approach under different conditions of typical soil temperatures and moistures. Based on the emission measurements of N2O and CH4, we developed a model to estimate annual regional GHG emission of Miscanthus production in the infertile Loess Plateau of China. The results showed that the N2O emission potential was 0.27 kg N ha?1 year?1 and clearly lower than that of croplands and grasslands. The CH4 uptake potential was 1.06 kg C ha?1 year?1 and was slightly higher than that of croplands. Integrated with our previous study on the emission of CO2, the net greenhouse effect of three major GHGs (N2O, CH4 and CO2) from Miscanthus fields was 4.08 t CO2eq ha?1 year?1 in the Loess Plateau, which was lower than that of croplands, grasslands and shrub lands.ConclusionsOur study revealed that Miscanthus production may hold a great potential for GHG mitigation in the vast infertile land in the Loess Plateau of China and could contribute to the sustainable energy utilization and have positive environmental impact on the region. 相似文献
19.
Northern lakes are a source of greenhouse gases to the atmosphere and contribute substantially to the global carbon budget. However, the sources of methane (CH4) to northern lakes are poorly constrained limiting our ability to the assess impacts of future Arctic change. Here we present measurements of the natural groundwater tracer, radon, and CH4 in a shallow lake on the Yukon-Kuskokwim Delta, AK and quantify groundwater discharge rates and fluxes of groundwater-derived CH4. We found that groundwater was significantly enriched (2000%) in radon and CH4 relative to lake water. Using a mass balance approach, we calculated average groundwater fluxes of 1.2 ± 0.6 and 4.3 ± 2.0 cm day−1, respectively as conservative and upper limit estimates. Groundwater CH4 fluxes were 7—24 mmol m−2 day−1 and significantly exceeded diffusive air–water CH4 fluxes (1.3–2.3 mmol m−2 day−1) from the lake to the atmosphere, suggesting that groundwater is an important source of CH4 to Arctic lakes and may drive observed CH4 emissions. Isotopic signatures of CH4 were depleted in groundwaters, consistent with microbial production. Higher methane concentrations in groundwater compared to other high latitude lakes were likely the source of the comparatively higher CH4 diffusive fluxes, as compared to those reported previously in high latitude lakes. These findings indicate that deltaic lakes across warmer permafrost regions may act as important hotspots for CH4 release across Arctic landscapes. 相似文献
20.
Background and aims The rice production is experiencing a shift from conventionally seedling-transplanted (TPR) to direct-seeded (DSR) cropping systems in Southeast Asia. Besides the difference in rice crop establishment, water regime is typically characterized as water-saving moist irrigation for DSR and flooding-midseason drainage-reflooding and moist irrigation for TPR fields, respectively. A field experiment was conducted to quantify methane (CH 4) and nitrous oxide (N 2O) emissions from the DSR and TPR rice paddies in southeast China. Methods Seasonal measurements of CH 4 and N 2O fluxes from the DSR and TPR plots were simultaneously taken by static chamber-GC technique. Results Seasonal fluxes of CH 4 averaged 1.58 mg m ?2 h ?1 and 1.02 mg m ?2 h ?1 across treatments in TPR and DSR rice paddies, respectively. Compared with TPR cropping systems, seasonal N 2O emissions from DSR cropping systems were increased by 49 % and 46 % for the plots with or without N application, respectively. The emission factors of N 2O were estimated to be 0.45 % and 0.69 % of N application, with a background emission of 0.65 and 0.95 kg N 2O-N ha ?1 under the TPR and DSR cropping regimes, respectively. Rice biomass and grain yield were significantly greater in the DSR than in the TPR cropping systems. The net global warming potential (GWP) of CH 4 and N 2O emissions were comparable between the two cropping systems, while the greenhouse gas intensity (GHGI) was significantly lower in the DSR than in the TPR cropping systems. Conclusions Higher grain yield, comparable GWP, and lower GHGI suggest that the DSR instead of conventional TPR rice cropping regime would weaken the radiative forcing of rice production in terms of per unit of rice grain yield in China, and DSR rice cropping regime could be a promising rice development alternative in mainland China. 相似文献
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