Fluxes of nitrous oxide and methane in different coastal Suaeda salsa marshes of the Yellow River estuary, China

There is substantial interest in restoring tidal wetlands because of their high rates of long-term soil carbon sequestration and other valued ecosystem services. However, these wetlands are sometimes net sources of greenhouse gases (GHG) that may offset their climate cooling potential. GHG fluxes vary widely within and across tidal wetlands, so it is essential to better understand how key environmental drivers, and importantly, land management, affect GHG dynamics. We measured methane (CH4) and nitrous oxide (N2O) fluxes at 26 reference and restored tidal wetland sites and eight nontidal pastures (mostly diked former tidal wetlands) in five estuaries in the Pacific Northwest (PNW), USA. We measured fluxes 7-8 times over one year to assess the effects of environmental drivers, wetland type, and land management on CH4 and N2O fluxes. Linear relationships between CH4 fluxes and environmental drivers were poor, but a machine-learning approach with boosted regression trees provided strong predictability for fluxes based upon wetland surface elevation, water-table level, and salinity. Less important variables were groundwater pH, wetland type, and temperature. Under oligohaline conditions, CH4 fluxes were variable and sometimes very high, but fluxes at salinities above 2 ppt were relatively low on an annual basis. Fluxes of CH4 were higher in restored tidal marshes and wet pastures than in reference tidal marshes, tidal swamps, and dry pastures. The N2O model had lower predictive power than the CH4 model, with wetland type as the most important factor, although N2O fluxes across all wetland types were low (median of zero). Our results indicate that estuarine hydrologic gradients are a key driver of CH4 fluxes and that wetland land use impacts on CH4 fluxes are largely mediated by their varying environmental conditions. In the PNW, estuarine wetlands that have low salinity, lower elevation, and have high water tables are more likely to have increased CH4 emissions that may offset their carbon sequestration benefits until they gain enough elevation through accretion. This study also provides a transferrable modeling approach to predict the consequences of coastal wetland management on GHG fluxes using monitoring data from a limited set of key environmental drivers.

Comprising 45% of global forest cover, tropical forests are pivotal in the GHG budgets. Emerging research highlights the significance of tropical trees as CH4 sources, yet tree-foliage emissions have been minimally investigated. Moreover, the N2O fluxes from tropical tree foliage remain almost completely unexamined.Objectives: This study presents a comprehensive survey of foliar CH4 and N2O fluxes across tropical forest tree species using integrated output spectroscopy and a purpose-built cuvette system for accurate in-situ flux rate measurements. It tests two key hypotheses: (1) broadleaf trees in well-drained soils of tropical forests exhibit foliar CH4 oxidation; (2) foliar CH4 and N2O flux patterns vary systematically among ecological and phylogenetic groups.Methods: We measured foliar fluxes from 120 trees across 40 species within Lawachara National Park, Bangladesh, an upland mixed-tropical-evergreen forest, prioritizing diverse shade-tolerant canopy trees. We utilized a dynamic leaf chamber (CS-LC7000) with continuous gas flow and portable CH4 (LGR 915-001) and N2O (LI-7820) analyzers, alongside concurrent measurements of CO2 and H2O flux. In addition to gas flux data, our study incorporated leaf trait measurements (of leaf mass per area and leaf N content).Results: Across all samples, the mean CH4 flux of 0.016 nmol m-2 s-1 did not display a significant deviation from zero (t = 19.44, df = 827, p > 0.05). In contrast, the mean N2O flux 0.54 nmol m-2 s-1, exhibited a significant elevation above zero (t = 19.42, df = 827, p < 0.001), indicating notable N2O emissions on average. Methane flux varied among species and various ecological successional groups, namely pioneer, mid-successional, and late successional species (F = 5.99, df = 2, p < 0.01). Pioneer species, which were sources of CH­4, demonstrated significantly higher CH4 flux compared to both mid (p < 0.01) and late successional (p < 0.05) species, which both acted as weak CH4 sinks. All ecological groups were sources of N2O, with significant variations among the ecological successional groups (F = 12.97, df = 2, p < 0.01). Pioneer species were identified as the highest emitters of N2O, followed by mid and late-successional species.A comparative CH4 flux analysis among the 28 families revealed significant variability (F = 47.7, df = 27, p < 0.01), with certain species acting as sources and others as sinks of CH4. Notably, 11 families were classified as CH4 sources, while the remainder functioned as sinks. Meliaceae emerged as having the highest average CH4 emissions, and Thymeliaceae the greatest CH4 consumption. Similarly, a distinct variation in N2O flux was observed among families (F = 6.57, df = 27, p < 0.01), with Sapindaceae showing the highest, and Rubiaceae and Euphorbiaceae the lowest N2O emissions.Conclusions: This study on foliar CH4 and N2O fluxes in tropical forests reveals trees' crucial role in greenhouse gas emissions. Pioneer species emerge as major emitters of both CH4 and N2O, suggesting that foliar emissions of these GHGs may be pronounced in secondary forests, and hence the importance of conserving intact forests dominated by later-successional species.

Plants have been suggested to have significant effects on methane (CH4) and nitrous oxide (N2O) fluxes from littoral wetlands, but it remains unclear in subtropical lakes. We conducted in situ measurement of CH4 and N2O fluxes for two years. To distinguish between the effects of shoots and roots, three treatments (i.e., intact plants as control, shoot clipping, and root exclusion) were used. Effects of plant biomass, temperature, and soil moisture on CH4 and N2O fluxes were analyzed. The mean ecosystem CH4 emission rate was 36 μg CH4 m−2 h−1 for drying periods, but 8219 μg CH4 m−2 h−1 for drying-wetting transition periods. CH4 fluxes were positively correlated with below-ground and total biomass, but not with above-ground biomass. Clipping did not significantly alter CH4 flux rate, but root exclusion decreased the CH4 flux by 116 % as compared to the control. N2O emissions were similar for both the drying and drying-wetting transition periods, with a mean rate of 20 μg N2O m−2 h−1. Both clipping and root exclusion significantly increased N2O fluxes as compared to the control. There was no significant correlation between CH4 and N2O fluxes. Roots dominated plant-mediated enhancement in CH4 fluxes, but played almost an equal role as shoots in plant-regulated suppression on N2O fluxes in this Carex meadow during drawdown periods.

The invasions of the alien species such as Spartina alterniflora along the northern Jiangsu coastlines have posed a threat to biodiversity and the ecosystem function. Yet, limited attention has been given to their potential influence on greenhouse gas (GHG) emissions, including the diurnal variations of GHG fluxes that are fundamental in estimating the carbon and nitrogen budget. In this study, we examined the diurnal variation in fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from a S. alterniflora intertidal flat in June, October, and December of 2013 and April of 2014 representing the summer, autumn, winter, and spring seasons, respectively. We found that the average CH4 fluxes on the diurnal scale were positive during the growing season while negative otherwise. The tidal flat of S. alterniflora acted as a source of CH4 in summer (June) and a combination of source and sink in other seasons. We observed higher diurnal variations in the CO2 and N2O fluxes during the growing season (1 536.5 mg CO2 m–2 h–1 and 25.6 μg N2O m–2 h–1) compared with those measured in the non-growing season (379.1 mg CO2 m–2 h–1 and 16.5 μg N2O m–2 h–1). The mean fluxes of CH4 were higher at night than that in the daytime during all the seasons but October. The diurnal variation in the fluxes of CO2 in June and N2O in December fluctuated more than that in October and April. However, two peak curves in October and April were observed for the diurnal changes in CO2 and N2O fluxes (prominent peaks were found in the morning of October and in the afternoon of April, respectively). The highest diurnal variation in the N2O fluxes took place at 15:00 (86.4 μg N2O m–2 h–1) in June with an unimodal distribution. Water logging in October increased the emission of CO2 (especially at nighttime), yet decreased N2O and CH4 emissions to a different degree on the daily scale because of the restrained diffusion rates of the gases. The seasonal and diurnal variations of CH4 and CO2 fluxes did not correlate to the air and soil temperatures, whereas the seasonal and diurnal variation of the fluxes of N2O in June exhibited a significant correlation with air temperature. When N2O and CH4 fluxes were converted to CO2-e equivalents, the emissions of N2O had a remarkable potential to impact the global warming. The mean daily flux (MF) and total daily flux (TDF) were higher in the growing season, nevertheless, the MF and TDF of CO2 were higher in October and those of CH4 and N2O were higher in June. In spite of the difference in the optimal sampling times throughout the observation period, our results obtained have implications for sampling and scaling strategies in estimating the GHG fluxes in coastal saline wetlands.

To investigate the spatial and seasonal variations of nitrous oxide (N2O) fluxes and understand the key controlling factors, we explored N2O fluxes and environmental variables in high marsh (HM), middle marsh (MM), low marsh (LM), and mudflat (MF) in the Yellow River estuary throughout a year. Fluxes of N2O differed significantly between sampling periods as well as between sampling positions. During all times of day and the seasons measured, N2O fluxes ranged from −0.0051 to 0.0805 mg N2O m−2 h−1, and high N2O emissions occurred during spring (0.0278 mg N2O m−2 h−1) and winter (0.0139 mg N2O m−2 h−1) while low fluxes were observed during summer (0.0065 mg N2O m−2 h−1) and autumn (0.0060 mg N2O m−2 h−1). The annual average N2O flux from the intertidal zone was 0.0117 mg N2O m−2 h−1, and the cumulative N2O emission throughout a year was 113.03 mg N2O m−2, indicating that coastal marsh acted as N2O source. Over all seasons, N2O fluxes from the four marshes were significantly different (p < 0.05), in the order of HM (0.0256 ± 0.0040 mg N2O m−2 h−1) > MF (0.0107 ± 0.0027 mg N2O m−2 h−1) > LM (0.0073 ± 0.0020 mg N2O m−2 h−1) > MM (0.0026 ± 0.0011 mg N2O m−2 h−1). Temporal variations of N2O emissions were related to the vegetations (Suaeda salsa, Phragmites australis, and Tamarix chinensis) and the limited C and mineral N in soils during summer and autumn and the frequent freeze/thaw cycles in soils during spring and winter, while spatial variations were mainly affected by tidal fluctuation and plant composition at spatial scale. This study indicated the importance of seasonal N2O contributions (particularly during non-growing season) to the estimation of local N2O inventory, and highlighted both the large spatial variation of N2O fluxes across the coastal marsh (CV = 158.31 %) and the potential effect of exogenous nitrogen loading to the Yellow River estuary on N2O emission should be considered before the annual or local N2O inventory was evaluated accurately.

Comparative accounting of methane and nitrous oxide fluxes with related soil parameters of degraded mangrove wetlands and adjacent rice fields in Sundarban, India

Methane (CH4) and nitrous oxide (N2O) fluxes from tree stems are still poorly quantified in temperate floodplain forests. Methane and N2O fluxes were repeatedly measured at 0.3, 1.6 and 3.6 m stem height at three sites along a landscape gradient ranging from non-flooded to frequently flooded forest sites. The non-flooded forest was dominated by Fraxinus excelsior and the infrequently and frequently flooded sites by Populus alba. Stem surfaces were net CH4 and N2O sources at all sites. The CH4 source strength increased towards the wetter sites (non-flooded 2.51±12.71, infrequently-flooded 5.2±17.26, and frequently-flooded 11.15±24.04 μg-C-m−2 h−1), but flooding had no immanent effects on CH4 and N2O fluxes. Methane fluxes from poplar stems were highest at the stem base (0.3 m) and decreased with increasing measurement height. Methane fluxes from ash stems were lowest at the stem base and gradually increased until 3.6 m height. Nitrous oxide fluxes were low and did not show clear spatial patterns. The presence of mosses had no significant effects on CH4 and N2O fluxes. Stem fluxes were small when compared to the corresponding soil fluxes at the non-flooded and infrequently flooded site, but significantly reduced the soil CH4 sink capacity at the frequently-flooded site. Methane flux strongly varied between 0.3 and 3.6 m stem height and showed distinctive tree species specific patterns. Our results therefore suggest that measuring at more than a single location near the stem base is inevitable to obtain any reliable CH4 or N2O flux estimate of a whole tree stem.

Using high-frequency soil oxygen sensors to predict greenhouse gas emissions from wetlands

IntroductionFarmlands are key sources of greenhouse gas (GHG) emissions, which are susceptible to changes in precipitation regimes. The soils of seasonal fallow contribute approximately half of annual GHG emissions from farmlands, but the effect of precipitation frequency on soil GHG emissions from seasonal fallow croplands remains virtually unknown.Materials and MethodsWe conducted a microcosm study to evaluate the response of nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) fluxes from typical paddy and upland soils to the changes in watering frequency simulating precipitation scenarios of subtropical regions during seasonal fallow. We also analyzed changes of soil properties and biotic characteristics associated with GHG emissions, including abundances of soil denitrifiers (nirK, nirS, nosZI and nosZII genes), methanotrophs (pmoA gene) and methanogens (mcrA gene) to altered watering frequency.ResultsIncreased watering frequency led to overall increases in soil N2O and CO2 fluxes compared with low frequency. Compared with low frequency, high watering frequency decreased CH4 flux from the paddy soil by 3.5 times, while enhanced CH4 flux from the upland soil by 60%. Furthermore, the increased watering frequency had positive effects on cumulative N2O and CO2 fluxes from the upland soil, whereas no similar trend was observed for the paddy soil. Hierarchical partitioning analyses showed that N2O fluxes from the paddy soil were mostly related to nitrogen availability, and mcrA gene abundance had more than 90% of relative independent effects on CH4 and CO2 fluxes from the paddy soil. For the upland soil, nosZ (60.34%), pmoA (53.18%) and nir (47.07%) gene abundances were important predictors of N2O, CH4 and CO2 fluxes, respectively.ConclusionOur results demonstrate that increased watering frequency facilitates GHG emissions by changing soil properties and functional gene abundances. These findings provide new insights into GHG fluxes from seasonal fallow croplands in response to altered precipitation patterns.

The main purpose of this study was to explore the dynamic changes of greenhouse gas (GHG) from grasslands under different degradation levels during the growing seasons of Inner Mongolia, China. Grassland degradation is associated with the dynamics of GHG fluxes, e.g., CO2, CH4 and N2O fluxes. As one of the global ecological environmental problems, grassland degradation has changed the vegetation productivity as well as the accumulation and decomposition rates of soil organic matter and thus will influence the carbon and nitrogen cycles of ecosystems, which will affect the GHG fluxes between grassland ecosystems and the atmosphere. Therefore, it is necessary to explore how the exchanges of CO2, CH4 and N2O fluxes between soil and atmosphere are influenced by the grassland degradation. We measured the fluxes of CO2, CH4 and N2O in lightly degraded, moderately degraded and severely degraded grasslands in Inner Mongolia of China during the growing seasons from July to September in 2013 and 2014. The typical semi-arid grassland of Inner Mongolia plays a role as the source of atmospheric CO2 and N2O and the sink for CH4. Compared with CO2 fluxes, N2O and CH4 fluxes were relatively low. The exchange of CO2, N2O and CH4 fluxes between the grassland soil and the atmosphere may exclusively depend on the net exchange rate of CO2 in semi-arid grasslands. The greenhouse gases showed a clear seasonal pattern, with the CO2 fluxes of –33.63 –386.36 mg/(m•h), CH4 uptake fluxes of 0.113–0.023 mg/(m•h) and N2O fluxes of –1.68 –19.90 μg/(m•h). Grassland degradation significantly influenced CH4 uptake but had no significant influence on CO2 and N2O emissions. Soil moisture and temperature were positively correlated with CO2 emissions but had no significant effect on N2O fluxes. Soil moisture may be the primary driving factor for CH4 uptake. The research results can be in help to better understand the impact of grassland degradation on the ecological environment.

Quantifying and predicting spatio-temporal variability of soil CH4 and N2O fluxes from a seemingly homogeneous Australian agricultural field

Much uncertainty exists in spatial and temporal variations of nitrous oxide (N2O) emissions from coastal marshes in temperate regions. To investigate the spatial and temporal variations of N2O fluxes and determine the environmental factors influencing N2O fluxes across the coastal marsh dominated by Suaeda salsa in the Yellow River estuary, China, in situ measurements were conducted in high marsh (HM), middle marsh (MM), low marsh (LM), and mudflat (MF) in autumn and winter during 2011–2012. Results showed that mean N2O fluxes and cumulative N2O emission indicated intertidal zone of the examined marshes as N2O sources over all sampling seasons with range of 0.0051 to 0.0152 mg N2O m−2 h−1 and 7.58 to 22.02 mg N2O m−2, respectively. During all times of day and the seasons measured, N2O fluxes from the intertidal zone ranged from −0.0004 to 0.0644 mg N2O m−2 h−1. The freeze/thaw cycles in sediments during early winter (frequent short-term cycle) and midwinter (long-term cycle) were one of main factors affecting the temporal variations of N2O emission. The spatial variations of N2O fluxes in autumn were mainly dependent on tidal fluctuation and plant composition. The ammonia-nitrogen (NH4 +–N) in sediments of MF significantly affected N2O emissions (p < 0.05), and the high concentrations of Fe in sediments might affect the spatial variation of N2O fluxes. This study highlighted the large spatial variation of N2O fluxes across the coastal marsh (coefficient of variation (CV) = 127.86 %) and the temporal variation of N2O fluxes during 2011–2012 (CV = 137.29 %). Presently, the exogenous C and N loadings of the Yellow River estuary are increasing due to human activities; thus, the potential effects of exogenous C and N loadings on N2O emissions during early winter should be paid more attention as the N2O inventory is assessed precisely.

Methane and N 2 O are gases that are several times more radiatively active than CO 2 . It is well known that flooded rice ( Oryza sativa L.) soils are a globally important source of atmospheric CH 4 . Mitigation strategies for CH 4 flux, such as mid‐season drainage, might have the opposite effect on N 2 O emissions. An automated chamber system at the International Rice Research Institute in the Philippines measured CH 4 and N 2 O fluxes from flooded rice and fallow rice fields essentially 24 h a day between December 1992 and April 1994. This period included two irrigated dry rice‐growing seasons (DS) and one wet rice‐growing season (WS). Nitrous oxide fluxes were generally barely detectable during the growing seasons, but small peaks (maximum 3.5 mg N 2 O‐N m ‐2 d ‐1 ) appeared after N fertilizer applications. Methane fluxes, on the other hand, were evident throughout the rice‐growing seasons. Organic matter additions as straw (5.5 t ha ‐1 , dry) or green manure (GM; Sesbania rostrata L.; 12 t ha ‐1 , wet) stimulated CH 4 flux severalfold. Seasonal CH 4 flux with ammonium sulfate (AS) was one‐fourth to one‐third the flux with urea. During the DS, however, the seasonal N 2 O flux was 2.5 times higher with AS than with urea. Mid‐season drainage (2‐wk duration) at either mid‐tillering or panicle initiation was very successful in suppressing CH 4 flux up to 60%. However, N 2 O flux increased sharply during the drainage period at mid‐tillering until reflooding, when it dropped back to near zero.

Spartina alterniflora invasions impact CH4 and N2O fluxes from a salt marsh in eastern China

Static chamber measurements of nitrous oxide (N2O) and methane (CH4) fluxes were made from five characteristic vegetation types, representing three different natural ecosystems (grasslands, deciduous forest and wetlands) in the Pannonian basin, Hungary. The main objective of the study was to determine the drivers of average seasonal, annual and interannual N2O and CH4 fluxes in these different ecosystems to enable more accurate predictions of responses to future climatic conditions. Investigations into the response of net N2O and CH4 emission rates to soil temperature and soil water content were carried out over a 2‐year period. Both N2O and CH4 fluxes covered a wide range. Yearly average N2O emissions ranged from 4.38 mg N m−2 year−1 for wetland poplar forest to 242 mg N m−2 year−1 for mountain oak forest. Yearly average soil fluxes of CH4 varied from oxidation, −106 mg CH4 m−2 year−1, for loess grassland to an emission of 129 mg CH4 m−2 year−1 for a wetland Glyceria stand. Multiple regression analyses showed that N2O fluxes from the Pannonian grasslands and oak forest were more dependent than CH4 fluxes on the key soil variables water content and temperature. The largest seasonal mean N2O emission, 319 mg N m−2 year−1, from a mountain oak forest, was observed in summer, and the largest seasonal mean CH4 emission, equivalent to an annual rate of 423 mg CH4 m−2 year−1, from a wetland Glyceria stand, was found in the spring of the wet year.