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1.
Nitrous oxide (N 2O) emissions are subject to intra‐ and interannual variation due to changes in weather and management. This creates significant uncertainties when quantifying estimates of annual N 2O emissions from grazed grasslands. Despite these uncertainties, the majority of studies are short‐term in nature (<1 year) and as a consequence, there is a lack of data on interannual variation in N 2O emissions. The objectives of this study were to (i) quantify annual N 2O emissions and (ii) assess the causes of interannual variation in emissions from grazed perennial ryegrass/white clover grassland. Nitrous oxide emissions were measured from fertilized and grazed perennial ryegrass/white clover grassland (WC) and from perennial ryegrass plots that were not grazed and did not receive N input (GB), over 4 years from 2008 to 2012 in Ireland (52°51′N, 08°21′W). The annual N 2O‐N emissions (kg ha ?1; mean ± SE) ranged from 4.4 ± 0.2 to 34.4 ± 5.5 from WC and from 1.7 ± 0.8 to 6.3 ± 1.2 from GB. Interannual variation in N 2O emissions was attributed to differences in annual rainfall, monthly (December) soil temperatures and variation in N input. Such substantial interannual variation in N 2O emissions highlights the need for long‐term studies of emissions from managed pastoral systems. 相似文献
2.
Chronic elevated nitrogen deposition has increased nitrogen availability in many forest ecosystems globally, and this phenomenon has been suggested to increase soil nitrification. Although it is believed that increased nitrogen availability would also increase nitrous oxide (N2O) emissions from forest ecosystems, its impact on N2O flux is poorly known. In this study, 3-years monitoring of N2O emissions was performed in a forested watershed receiving elevated nitrogen deposition and located in the suburbs of Tokyo, Japan. In addition, a comparative field survey was carried out in nine temperate forest sites with varying nitrogen availabilities. In the intensively studied forest site showing typical nitrogen saturation, the average annual N2O emissions from the whole watershed were estimated to be 0.88 kg N ha−1 year−1, comparable to the highest observed levels for temperate forests except for some very high emission sites in Europe. Although no correlation was found for humid spots with WFPS > 60%, a clear positive correlation was noted between N2O flux and net nitrification rate in situ for plots with water-filled pore space (WFPS) < 60%. The N2O flux varied across nine forest sites almost in proportional to the stream water NO3− concentration in the watershed that ranged from 0.14 to 1.64 mg N/L. We conclude that N2O emissions are related to nitrification in moist temperate forest, which may be associated with the magnitude of nitrogen saturation. 相似文献
3.
Ecosystem nitrous oxide (N(2) O) emissions respond to changes in climate and CO(2) concentration as well as anthropogenic nitrogen (N) enhancements. Here, we aimed to quantify the responses of natural ecosystem N(2) O emissions to multiple environmental drivers using a process-based global vegetation model (DyN-LPJ). We checked that modelled annual N(2) O emissions from nonagricultural ecosystems could reproduce field measurements worldwide, and experimentally observed responses to step changes in environmental factors. We then simulated global N(2) O emissions throughout the 20th century and analysed the effects of environmental changes. The model reproduced well the global pattern of N(2) O emissions and the observed responses of N cycle components to changes in environmental factors. Simulated 20th century global decadal-average soil emissions were c. 8.2-9.5?Tg?N?yr(-1) (or 8.3-10.3?Tg?N?yr(-1) with N deposition). Warming and N deposition contributed 0.85?±?0.41 and 0.80?±?0.14?Tg?N?yr(-1) , respectively, to an overall upward trend. Rising CO(2) also contributed, in part, through a positive interaction with warming. The modelled temperature dependence of N(2) O emission (c. 1?Tg?N?yr(-1) K(-1) ) implies a positive climate feedback which, over the lifetime of N(2) O (114?yr), could become as important as the climate-carbon cycle feedback caused by soil CO(2) release. 相似文献
5.
Aim This study examines the impact of changing nitrogen (N) fertilizer application rates, land use and climate on N fertilizer-derived direct nitrous oxide (N 2O) emissions in Irish grasslands. Methods A set of N fertilizer application rates, land use and climate change scenarios were developed for the baseline year 2000 and then for the years 2020 and 2050. Direct N 2O emissions under the different scenarios were estimated using three different types of emission factors and a newly developed Irish grassland N 2O emissions empirical model. Results There were large differences in the predicted N 2O emissions between the methodologies, however, all methods predicted that the overall N 2O emissions from Irish grasslands would decrease by 2050 (by 40–60 %) relative to the year 2000. Reduced N fertilizer application rate and land-use changes resulted in decreases of 19–34 % and 11–60 % in N 2O emission respectively, while climate change led to an increase of 5–80 % in N 2O emission by 2050. Conclusions It was observed in the study that a reduction in N fertilizer and a reduction in the land used for agriculture could mitigate emissions of N 2O, however, future changes in climate may be responsible for increases in emissions causing the positive feedback of climate on emissions of N 2O. 相似文献
6.
Based on a review of N 2O field studies in Europe, major soil, climate and management controls of N 2O release from agricultural mineral soils in the European Union have been identified. Data for these N 2O emission drivers can easily be gathered from statistical services. Using stepwise multivariate linear regression analysis, empirical first order models of N 2O emissions have been established which allow – in contrast to existing large-scale approaches – a regionally disaggregated estimation of N 2O emissions at sub-national, national and continental level in the temperate and boreal climate regions of Europe. Arable soils showed lower mean and maximum emissions in oceanic temperate climate (Temperate West) than in pre-alpine temperate and sub-boreal climate (Sub-boreal Europe). Therefore, two separate regression models were developed. Nitrous oxide emissions from arable soils the Temperate West amount to an average flux rate below 2 kg N 2O-N ha –1 yr –1 and rarely exceed 5 kg N 2O-N ha –1 yr –1. They are modelled by the parameters fertiliser, topsoil organic carbon and sand content. In Sub-boreal European arable soils, N 2O emissions vary in a much wider range between 0 and 27 kg N 2O-N ha –1 yr –1 in dependence of available nitrogen, represented in the model by fertiliser and topsoil nitrogen content. Compared to existing methods for large scale inventories, the regression models allow a better regional fit to measured values since they integrate additional driving forces for N 2O emissions. For grasslands, a fertiliser-based model was established which yields higher emission estimates than existing ones. Due to an extreme variability, no climate, soil nor management parameters could be included in the empirical grasslands model. 相似文献
7.
Nitrous oxide (N 2O) emissions can be significantly affected by the amounts and forms of nitrogen (N) available in soils, but the effect is highly dependent on local climate and soil conditions in specific ecosystem. To improve our understanding of the response of N 2O emissions to different N sources of fertilizer in a typical semiarid temperate steppe in Inner Mongolia, a 2-year field experiment was conducted to investigate the effects of high, medium and low N fertilizer levels (HN: 200 kg N?ha -1y -1, MN: 100 kg N ha -1y -1, and LN: 50 kg N ha -1y -1) respectively and N fertilizer forms (CAN: calcium ammonium nitrate, AS: ammonium sulphate and NS: sodium nitrate) on N 2O emissions using static closed chamber method. Our data showed that peak N 2O fluxes induced by N treatments were concentrated in short periods (2 to 3 weeks) after fertilization in summer and in soil thawing periods in early spring; there were similarly low N 2O fluxes from all treatments in the remaining seasons of the year. The three N levels increased annual N 2O emissions significantly ( P?<?0.05) in the order of MN > HN > LN compared with the CK (control) treatment in year 1; in year 2, the elevation of annual N 2O emissions was significant ( P?<?0.05) by HN and MN treatments but was insignificant by LN treatments ( P?>?0.05). The three N forms also had strong effects on N 2O emissions. Significantly ( P?<?0.05) higher annual N 2O emissions were observed in the soils of CAN and AS fertilizer treatments than in the soils of NS fertilizer treatments in both measured years, but the difference between CAN and AS was not significant ( P?>?0.05). Annual N 2O emission factors (EF) ranged from 0.060 to 0.298% for different N fertilizer treatments in the two observed years, with an overall EF value of 0.125%. The EF values were by far less than the mean default EF proposed by the Intergovernmental Panel on Climate Change (IPCC). 相似文献
8.
Seasonal and interannual variations in nitrous oxide (N 2O) losses from agricultural soils hamper the accurate quantification of the N 2O source strength of these soils. This study focuses on a quantification of seasonal and interannual variations in N 2O losses from managed grasslands. Special attention was paid to N 2O losses during the growing season and off-season as affected by grassland management. Fluxes of N 2O from grasslands with three different types of management and on four different soil types in the Netherlands were measured weekly during two consecutive years, using flux chambers. There were distinct seasonal patterns in N 2O losses, with large losses during spring, summer, and autumn but relatively small losses during the winter. These seasonal variations were related to fertilizer N application, grazing and weather conditions. Measurements of N 2O concentrations in soil profiles showed that a rise in groundwater level was accompanied by increased N 2O concentrations in the soil. Disregarding off-season losses would underestimate total annual losses by up to 20%, being largest for unfertilized grassland and smallest for N-fertilized grazed grassland. Total annual N 2O losses ranged from 0.5 to 12.9 kg N ha -1 yr -1 for unfertilized grasslands to 7.3 to 42.0 kg N ha -1 yr -1 for N-fertilized grazed grasslands. Despite the considerable interannual variations in N 2O losses, this study indicates that the results of measurements carried out in one year have predictive power for estimating N 2O losses in other years. 相似文献
9.
Understanding nitrous oxide (N 2O) emissions from agricultural soils in semi‐arid regions is required to better understand global terrestrial N 2O losses. Nitrous oxide emissions were measured from a rain‐fed, cropped soil in a semi‐arid region of south‐western Australia for one year on a sub‐daily basis. The site included N‐fertilized (100 kg N ha ?1 yr ?1) and nonfertilized plots. Emissions were measured using soil chambers connected to a fully automated system that measured N 2O using gas chromatography. Daily N 2O emissions were low (?1.8 to 7.3 g N 2O‐N ha ?1 day ?1) and culminated in an annual loss of 0.11 kg N 2O‐N ha ?1 from N‐fertilized soil and 0.09 kg N 2O‐N ha ?1 from nonfertilized soil. Over half (55%) the annual N 2O emission occurred from both N treatments when the soil was fallow, following a series of summer rainfall events. At this time of the year, conditions were conducive for soil microbial N 2O production: elevated soil water content, available N, soil temperatures generally >25 °C and no active plant growth. The proportion of N fertilizer emitted as N 2O in 1 year, after correction for the ‘background’ emission (no N fertilizer applied), was 0.02%. The emission factor reported in this study was 60 times lower than the IPCC default value for the application of synthetic fertilizers to land (1.25%), suggesting that the default may not be suitable for cropped soils in semi‐arid regions. Applying N fertilizer did not significantly increase the annual N 2O emission, demonstrating that a proportion of N 2O emitted from agricultural soils may not be directly derived from the application of N fertilizer. ‘Background’ emissions, resulting from other agricultural practices, need to be accounted for if we are to fully assess the impact of agriculture in semi‐arid regions on global terrestrial N 2O emissions. 相似文献
10.
Changes in precipitation in the Amazon Basin resulting from regional deforestation, global warming, and El Niño events may affect emissions of carbon dioxide (CO 2), methane (CH 4), nitrous oxide (N 2O), and nitric oxide (NO) from soils. Changes in soil emissions of radiatively important gases could have feedback implications for regional and global climates. Here we report results of a large‐scale (1 ha) throughfall exclusion experiment conducted in a mature evergreen forest near Santarém, Brazil. The exclusion manipulation lowered annual N 2O emissions by >40% and increased rates of consumption of atmospheric CH 4 by a factor of >4. No treatment effect has yet been detected for NO and CO 2 fluxes. The responses of these microbial processes after three rainy seasons of the exclusion treatment are characteristic of a direct effect of soil aeration on denitrification, methanogenesis, and methanotrophy. An anticipated second phase response, in which drought‐induced plant mortality is followed by increased mineralization of C and N substrates from dead fine roots and by increased foraging of termites on dead coarse roots, has not yet been detected. Analyses of depth profiles of N 2O and CO 2 concentrations with a diffusivity model revealed that the top 25 cm soil is the site of most of the wet season production of N 2O, whereas significant CO 2 production occurs down to 100 cm in both seasons, and small production of CO 2 occurs to at least 1100 cm depth. The diffusivity‐based estimates of CO 2 production as a function of depth were strongly correlated with fine root biomass, indicating that trends in belowground C allocation may be inferred from monitoring and modeling profiles of H 2O and CO 2. 相似文献
11.
Changes in precipitation in the Amazon Basin resulting from regional deforestation, global warming, and El Niño events may affect emissions of carbon dioxide (CO 2), methane (CH 4), nitrous oxide (N 2O), and nitric oxide (NO) from soils. Changes in soil emissions of radiatively important gases could have feedback implications for regional and global climate. Here, we report the final results of a 5‐year, large‐scale (1 ha) throughfall exclusion experiment, followed by 1 year of recovery with natural throughfall, conducted in a mature evergreen forest near Santarém, Brazil. The exclusion manipulation lowered annual N 2O emissions in four out of five treatment years (a natural drought year being the exception), and then recovered during the first year after the drought treatment stopped. Similarly, consumption of atmospheric CH 4 increased under drought treatment, except during a natural drought year, and it also recovered to pretreatment values during the first year that natural throughfall was permitted back on the plot. No treatment effect was detected for NO emissions during the first 3 treatment years, but NO emissions increased in the fourth year under the extremely dry conditions of the exclusion plot during a natural drought. Surprisingly, there was no treatment effect on soil CO 2 efflux in any year. The drought treatment provoked significant tree mortality and reduced the allocation of C to stems, but allocation of C to foliage and roots were less affected. Taken together, these results suggest that the dominant effect of throughfall exclusion on soil processes during this 6‐year period was on soil aeration conditions that transiently affected CH 4, N 2O, and NO production and consumption. 相似文献
12.
Aim This study aimed at better characterising background nitrous oxide (N 2O) emissions (BNE) in agricultural and natural lands. Methods We compiled and analysed field-measured data for annual background N 2O emission in agricultural (BNEA) and natural (BNEN) lands from 600 and 307 independent experimental studies, respectively. Results There were no significant differences between BNEA (median: 0.70 & mean: 1.52 kg N 2O???N ha ?1 yr ?1) and BNEN (median:0.31 & mean:1.75 kg N 2O???N ha ?1 yr ?1) ( P?>?0.05). A simultaneous comparison across all BNEA and BNEN indicated that BNEs from riparian, vegetable crop fields and intentional fallow areas were significantly higher than from boreal forests ( P?<?0.05). Correlation and regression analyses supported the underlying associations of soil organic carbon (C), nitrogen (N), pH, bulk density (BD),and/or air temperature (AT) with BNEs to a varying degree as a function of land-use or ecosystem type ( Ps?<?0.05). Conclusions Although overall BNEN tended to be lower than BNEA on median basis, results in general suggest that land-use shifts between natural and managed production systems would not result in consistent changes in BNE. 相似文献
13.
Plant and Soil - Global nitrogen deposition alters grassland ecosystems. Whether added nitrogen changes root production and turnover by root orders is unclear. We compared the root dynamics across... 相似文献
14.
Aims A field experiment was conducted to quantify annual nitrous oxide (N 2O) fluxes from control and fertilized plots under open-air and greenhouse vegetable cropping systems in southeast China. We compiled the reported global field annual N 2O flux measurements to estimate the emission factor of N fertilizer for N 2O and its background emissions from vegetable fields. Methods Fluxes of N 2O were measured using static chamber-GC techniques over the 2010–2011 annual cycle with multiple cropping seasons. Results The mean annual N 2O fluxes from the controls were 46.1?±?2.3 μg N 2O-N m ?2 hr ?1 and 68.3?±?4.1 μg N 2O-N m ?2 hr ?1 in the open-air and greenhouse vegetable systems, respectively. For the plots receiving 900 kg?N?ha ?1, annual N 2O emissions averaged 90.6?±?8.9 μg N 2O-N m ?2 hr ?1 and 106.4?±?6.6 μg N 2O-N?m ?2 hr ?1 in the open-air and greenhouse vegetable systems, respectively. By pooling published field N 2O flux measurements taken over or close to a full year, the N 2O emission factor for N fertilizer averaged 0.63?±?0.09 %, with a background emission of 2.67?±?0.80 kg N 2O-N ha ?1 in Chinese vegetable fields. Annual N 2O emissions from Chinese vegetable systems were estimated to be 84.7 Gg N 2O-N yr ?1, consisting of 72.5 Gg N 2O-N yr ?1 and 12.2 Gg N 2O-N yr ?1 in the open-air and greenhouse vegetable systems, respectively. Conclusions While N 2O emissions from the greenhouse vegetable cropping system tended to be slightly higher compared to the open-air system in our experiment, the synthesis of literature data suggests that N 2O emissions would be greater at low N-rates but smaller at high N-rates in greenhouse systems than in open-air vegetable cropping systems. The estimates of this study suggest that vegetable cropping systems covering 11.4 % in national total cropping area, contributed 21–25 % to the total N 2O emission from Chinese croplands. 相似文献
15.
We studied the export of inorganic carbon and nitrous oxide (N 2O) from a Danish freshwater wetland. The wetland is situated in an agricultural catchment area and is recharged by groundwater enriched with nitrate (NO 3
–) (1000 M). NO 3
– in recharging groundwater was reduced (57.5 mol NO 3
– m –2 yr –) within a narrow zone of the wetland. Congruently, the annual efflux of carbon dioxide (CO 2) from the sediment was 19.1 mol C m –2 when estimated from monthly in situ measurements. In comparison the CO 2 efflux was 4.8 mol C m –2 yr –1 further out in the wetland, where no NO 3
– reduction occurred. Annual exports of inorganic carbon in groundwater and surface water was 78.4 mol C m –2 and 6.1 mol C m –2 at the two sites, respectively. N 2O efflux from the sedimenst was detectable on five out of twelve sampling dates and was significantly ( P < 0.0001) higher in the NO 3
– reduction zone (0.35–9.40 mol m –2 h –1, range of monthly means) than in the zone without NO 3
– reduction (0.21–0.41 mol m –2 h –1). No loss of dissolved N 2O could be measured. Total annual export of N 2O was not estimated. The reduction of oxygen (O 2) in groundwater was minor throughout the wetland and did not exceed 0.2 mol 0 2 m –2yr –1. Sulfate (SO 4
––) was reduced in groundwater (2.1 mol SO 4
–– m –2 yr –1) in the zone without NO 3
– reduction. Although the NO 3
– in our wetland can be reduced along several pathways our results strongly suggest that NO 3
– loading of freshwater wetlands disturb the carbon balance of such areas, resulting in an accelerated loss of inorganic carbon in gaseous and dissolved forms. 相似文献
16.
The increasing atmospheric N 2O concentration and the imbalance in its global budget have triggered the interest in quantifying N 2O fluxes from various ecosystems. This study was conducted to estimate the annual N 2O emissions from a transitional grassland-forest region in Saskatchewan, Canada. The study region was stratified according to soil texture and land use types, and we selected seven landscapes (sites) to cover the range of soil texture and land use characteristics in the region. The study sites were, in turn, stratified into distinguishable spatial sampling units (i.e., footslope and shoulder complexes), which reflected the differences in soils and soil moisture regimes within a landscape. N 2O emission was measured using a sealed chamber method. Our results showed that water-filled pore space (WFPS) was the variable most correlated to N 2O fluxes. With this finding, we estimated the total N 2O emissions by using regression equations that relate WFPS to N 2O emission, and linking these regression equations with a soil moisture model for predicting WFPS. The average annual fluxes from fertilized cropland, pasture/hay land, and forest areas were 2.00, 0.04, and 0.02 kg N 2O-N ha –1 yr –1, respectively. The average annual fluxes for the medium- to fine-textured and sandy-textured areas were 1.40 and 0.04 kg N 2O-N ha –1 yr –1, respectively. The weighted-average annual flux for the study region is 0.95 kg N 2O-N ha –1yr –1. The fertilized cropped areas covered only 47% of the regional area but contributed about 98% of the regional flux. We found that in the clay loam, cropped site, 2% and 3% of the applied fertilizer were emitted as N 2O on the shoulders and footslopes, respectively.Contribution no. R824 of Saskatchewan Center for Soil Research, Saskatoon, Saskatchewan, Canada 相似文献
17.
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
18.
We studied the effect of mountain grassland afforestation with conifer trees ( Pinus sylvestris, Picea abies and Pinus cembra) on soil organic matter (SOM) cycling and carbon (C) isotopic composition in two contrasting climate areas using a regional approach. Seventeen paired sites (each containing at least 40 years prior afforested and grassland plots) were investigated in the mountains of Central Spain and Western Austria. Topsoil CO 2 effluxes were monitored under standardized conditions for six months as a proxy for soil organic carbon (SOC) mineralisation. The bulk C and nitrogen (N) concentrations and their isotopic composition in the soil and in the plants were assessed. The soil C:N ratio was consistently greater after afforestation in both regions, which in Spain was caused by a significant decrease in N concentration. No consistent effect was found on mineralisation rates due to vegetation change. Afforestation produced a more consistent soil 13C enrichment in the Spanish than in the Austrian sites. Our work strongly suggests that increasing altitude in Mediterranean mountain grasslands alleviates water limitation, favouring both plant growth and SOM decomposition, and ultimately accelerating C cycling. In contrast, temperate grassland areas at high altitudes were associated with severe temperature limitations, which constrained SOM transformation processes. In spite of the impact of afforestation on soil biogeochemical processes, C concentrations were marginally affected. We therefore conclude that grassland conversion to coniferous forests does not enhanced C sequestration in the mineral soil, for at least 40 years after land-use change. 相似文献
19.
Over the past two decades, the interest to decrease the emission levels of greenhouse gases (GHGs) has increased. The livestock sector has been put under continuous supervision and regulation because it is an important source of GHG emissions. In 2012, it was estimated that 3.46 Gton CO 2-eq was released from this sector, methane (CH 4) being the gas with the highest contribution (43 %), followed by nitrous oxide (21 %). In order to determine real emissions, it is necessary to use precise and reproducible measuring methods which can be complex and expensive. The challenges in these methods are focused on achieving an accurate assessment and monitoring of gas emissions, developing monitoring systems for the continuous measurement and implementation of methodologies for their validation in field in order to understand the complex nature of environmental variables affecting gas production. Different techniques for the measurement of CH 4 and nitrous oxide (N 2O) emissions are reviewed and discussed in this research. The passive flux sampling to measure emissions of these GHGs has been identified as an interesting alternative technique because it is practical, low cost and robust. This kind of sampler is highly adequate to measure emissions of N 2O and CH 4 originating from some sources of the livestock sector, but at this moment, no prototypes are commercially available and thus more research is necessary in this field. 相似文献
20.
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
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