Fluxes of methane, carbon dioxide and nitrous oxide in an alpine wetland and an alpine grassland of the Tianshan Mountains, China

With growing concerns on impacts of human activities and global warming on Alpine grasslands,comprehensive understanding of the sources and sinks of greenhouse gases becomes increasingly more important. The understanding is closely related to the progress on biogeochemical cycles of carbon and nitrogen in terrestrial ecosystems. Carbon dioxide, methane and nitrous oxide are the three most important greenhouse gases,which are considered to account for 80% contribution to global warming potential. The alpine grassland of Xinjiang is a typical temperate arid region of grasslands. The study was conducted at the Bayinbuluk Grassland Eco-system Research Station,Chinese Academy of Sciences( 83°43' E,42° 54' N). Bayinbuluk alpine grassland is located in the southern Tianshan mountains. Xinjiang Uygur AutonoMous Region,central Asia and covers a total area of approximately 2. 3 × 104km2. Bayinbuluk alpine grassland is the typicaltemperate arid alpine grassland,which is the second largest grassland of China after Inner Mongolia Grassland. As we all know,the grassland ecosystem has degenerated seriously and grazing prohibition is a frequently-used solution to prevent grass grassland degradation. While,it is still unknown that grazing prohibition impacts greenhouse gases fluxes in some degree. The study of carbon dioxide,methane and nitrous oxide of long-term grazing-prohibition grass( 1984),short-term grazing-prohibition grass( 2005) and free grazing grass in Bayinbuluk alpine grassland is meaningful,which will deepen our understanding of greenhouse gases fluxes in the alpine grassland ecosystem,help us assess global warming,parameterize Earth System models and get more comprehensive grasp of the impact of grazing prohibition on the grassland ecosystem. Using opaque,static,manual stainless steel chambers and gas chromatography,the fluxes of carbon dioxide,methane and nitrous oxide of long-term grazing-prohibition grass,short-term grazing-prohibition grass and free grazing grass were measured through the continuous experiment in situ from May 2010 to October 2011( no sampling in January and February 2011 because of the very low temperatures,about-40℃). Four times per month during the growing season( from May to October) and twice per month during non-growing( from November to next year April) season at all sites. According to the results of field experiment,the alpine grassland of Xinjiang is the sources of carbon dioxide and nitrous oxide; it is the sinks of methane. In the growing season,CO2average fluxes of short-term grazing-prohibition,long-term grazing-prohibition and free grazing are( 89.8 ± 49.3),( 52.8 ± 28.7),( 57.0 ± 30.7) mg·m-2·h-1; CH4fluxes averaged out to(- 66. 3 ± 21. 3),(-104.5±32.8),(-103.0±39.0) μg·m-2·h-1; CH4fluxes averaged out to( 21.2 ± 11.8),( 13.6 ± 6.9),( 13.2 ± 6.2) μg·m-2·h-1. Our results indicated that:( 1) Nitrous oxide fluxes showed a significant correlation with carbon dioxide fluxes in three kinds of grasslands.( 2) In the growing season,the difference of greenhouse gases fluxes between long-term grazing-prohibition grass and free grazing grass were not significant,while short-term grazing-prohibition grass has higher fluxes of carbon dioxide and nitrous oxide and lower fluxes of methane.( 3) In growing season,the fluxes of methane of short-term grazing-prohibition grass showed significant difference with long-term grazing-prohibition grass and free grazing grass. But the difference of growing-season average carbon dioxide and nitrous oxide fluxes did not reach the significance level of 0.05.( 4) In non-growing season,no significant differences between the fluxes of carbon dioxide,methane and nitrous oxide were found in long-term grazing-prohibition grassland,short-term grazing-prohibition grassland and free grazing grassland.

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.

Seasonal changes in the spatial structures of N2O, CO2, and CH4 fluxes from Acacia mangium plantation soils in Indonesia

Diffusive greenhouse gases fluxes from the surface of the Three Gorges Reservoir: Study at a site in Fuling

Context Conversion of grasslands to croplands can usually result in the degradation of soils and increased greenhouse gas (GHG) emissions such as carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4). However, little is known about the impacts of grassland conversion to recently tilled croplands on soils and GHG fluxes. Aims A field experiment was established in 2016 to evaluate the impacts of grassland conversion to tilled cropland under different landscape positions (upslope, backslope, and footslope) on select soil properties and soil GHG fluxes. Key results The findings showed that the grassland conversion significantly increased soil bulk density and electrical conductivity but reduced pH and total nitrogen (TN). The conversion impacted soil biome community grassland and tilled croplands. The landscape position significantly impacted soil pH (footslope < upslope) and TN (footslope > upslope). The grassland conversion significantly decreased soil CO2 fluxes, but increased soil CH4 and N2O fluxes. The landscape position significantly impacted soil CO2 (footslope > upslope and backslope) and CH4 (upslope > footslope and backslope) fluxes for some periods. Soil CO2 and N2O fluxes generally followed upward and downward trends over time, respectively. Conclusions These results indicate that grassland conversion was able to lose soil N, increase soil compaction, acidity, salts, and soil N2O and CH4 fluxes, and decrease the diversity of abundant genera and CO2 fluxes. Footslope increased TN, soil acidity, CO2, and CH4 fluxes, compared with upslope and backslope. CO2 fluxes under grassland and tilled cropland significantly increased over time, whereas N2O fluxes under grassland significantly reduced. Implications Conversion of grassland to tilled cropland significantly impacted on sol quality. It caused a loss in soil N and increased soil compaction, acidity and salts. Grassland conversion also decreased the abundance and diversity soil microbiome.

Abstract. A profound understanding of temporal and spatial variabilities of soil carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes between terrestrial ecosystems and the atmosphere is needed to reliably quantify these fluxes and to develop future mitigation strategies. For managed grassland ecosystems, temporal and spatial variabilities of these three soil greenhouse gas (GHG) fluxes occur due to changes in environmental drivers as well as fertilizer applications, harvests and grazing. To assess how such changes affect soil GHG fluxes at Swiss grassland sites, we studied three sites along an altitudinal gradient that corresponds to a management gradient: from 400 m a.s.l. (intensively managed) to 1000 m a.s.l. (moderately intensive managed) to 2000 m a.s.l. (extensively managed). The alpine grassland was included to study both effects of extensive management on CH4 and N2O fluxes and the different climate regime occurring at this altitude. Temporal and spatial variabilities of soil GHG fluxes and environmental drivers on various timescales were determined along transects of 16 static soil chambers at each site. All three grasslands were N2O sources, with mean annual soil fluxes ranging from 0.15 to 1.28 nmol m−2 s−1. Contrastingly, all sites were weak CH4 sinks, with soil uptake rates ranging from −0.56 to −0.15 nmol m−2 s−1. Mean annual soil and plant respiration losses of CO2, measured with opaque chambers, ranged from 5.2 to 6.5 μmol m−2 s−1. While the environmental drivers and their respective explanatory power for soil N2O emissions differed considerably among the three grasslands (adjusted r2 ranging from 0.19 to 0.42), CH4 and CO2 soil fluxes were much better constrained (adjusted r2 ranging from 0.46 to 0.80) by soil water content and air temperature, respectively. Throughout the year, spatial heterogeneity was particularly high for soil N2O and CH4 fluxes. We found permanent hot spots for soil N2O emissions as well as locations of permanently lower soil CH4 uptake rates at the extensively managed alpine site. Including hot spots was essential to obtain a representative mean soil flux for the respective ecosystem. At the intensively managed grassland, management effects clearly dominated over effects of environmental drivers on soil N2O fluxes. For CO2 and CH4, the importance of management effects did depend on the status of the vegetation (LAI).

Abstract. An experiment was conducted to investigate the effect of seasonally asymmetric warming on CO2, CH4, and N2O fluxes in alpine grassland of Tianshan Mountains of Central Asia, from October 2016 to September 2019. Our results indicated that the CO2, CH4 and N2O fluxes varied in the range 0.56–98.03 mg C m−2 h−1, −94.30–0.23 μg C m−2 h−1, and −1.28–10.09 μg N m−2 h−1, respectively. The CO2 and N2O fluxes were negatively correlated with soil temperature, but the CH4 fluxes decreased with the increase in temperature. Furthermore, the variation in greenhouse gas flux under seasonally asymmetric warming was different between the growing season (June to September) and the non-growing season (October to May). In addition, the response rates of CO2 and N2O fluxes to temperature increases was significantly reduced due to warming throughout the year. Warming during the growing season led to a significant decrease in the response rate of CO2 flux to temperature increases. However, warming during the non-growing season caused a significant increase in the response rate of CO2 flux to temperature increases. The response rate of CH4 flux was insensitive to temperature increase under seasonally asymmetric warming. Thus, the main finding of our results was that seasonally asymmetric warming resulted in different responses in the fluxes of individual greenhouse gases to rising temperatures in the alpine grassland.

Carbon dioxide and methane fluxes: Seasonal dynamics from inland riparian ecosystems, northeast China

Seasonal changes of CO2, CH4 and N2O fluxes in different types of alpine grassland in the Qinghai-Tibetan Plateau of China

Atmospheric concentrations of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are predicted to increase as a consequence of fossil fuel emissions and the impact on biosphere–atmosphere interactions. Forest ecosystems in general, and forest soils in particular, can be sinks or sources for CO2, CH4, and N2O. Environmental studies traditionally target soil temperature and moisture as the main predictors of soil greenhouse gas (GHG) flux from different ecosystems; however, these emissions are primarily biologically driven. Thus, little is known about the degree of regulation by soil biotic vs. abiotic factors on GHG emissions, particularly under predicted increase in global temperatures, and changes in intensity and frequency of precipitation events. Here we measured net CO2, CH4 and N2O fluxes after 5 years of experimental warming (+3.4°C), and 2 years of ≈45% summer rainfall reduction, in two forest sites in a boreal–temperate ecotone under different habitat conditions (closed or open canopy) in Minnesota, USA. We evaluated the importance of microbial gene abundance and climo‐edaphic factors (soil texture, canopy, seasonality, climate, and soil physicochemical properties) driving GHG emissions. We found that changes in CO2 fluxes were predominantly determined abiotically by temperature and moisture, after accounting for bacterial abundance. Methane fluxes on the other hand, were determined both abiotically, by gas diffusivity (via soil texture) and microbially, by methanotroph pmoA gene abundance, whereas, N2O emissions showed only a strong biotic regulation via ammonia‐oxidizing bacteria amoA gene abundance. Warming did not significantly alter CO2 and CH4 fluxes after 5 years of manipulation, while N2O emissions were greater with warming under open canopy. Our findings provide evidence that soil GHG emissions result from multiple direct and indirect interactions of microbial and abiotic drivers. Overall, this study highlights the need to include both microbial and climo‐edaphic properties in predictive models in order to provide improved mechanistic understanding for the development of future mitigation strategies. A plain language summary is available for this article.

Tree stems exchange greenhouse gases with the atmosphere but the magnitude, variability and drivers of these fluxes remain poorly understood. Here, we report stem fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in a boreal riparian forest, and investigate their spatiotemporal variability and ecosystem level importance. For two years, we measured CO2 and CH4 fluxes on a monthly basis in 14 spruces (Picea abies) and 14 birches (Betula pendula) growing near a headwater stream affected by historic ditching. We also measured N2O fluxes on three occasions. All tree stems were net emitters of CO2 and CH4, while N2O fluxes were around zero. CO2 fluxes correlated strongly with air temperature and peaked in summer. CH4 fluxes correlated modestly with air temperature and solar radiation and peaked in late winter and summer. Trees with larger stem diameter emitted more CO2 and less CH4 and trees closer to the stream emitted more CO2 and CH4. The CO2 and CH4 fluxes did not differ between spruce and birch, but correlations of CO2 fluxes with stem diameter and distance to stream differed between the tree species. The absence of vertical trends in CO2 and CH4 fluxes along the stems and their low correlation with groundwater levels and soil CO2 and CH4 partial pressures suggest tree internal production as the primary source of stem emissions. At the ecosystem level, the stem CO2, CH4 and N2O emissions represented 52 ± 16 % of the forest floor CO2 emissions and 3 ± 1 % and 11 ± 40 % of the forest floor CH4 and N2O uptake, respectively, during the snow-free period (median ± SE). The six month snow-cover period contributed 11 ± 45 % and 40 ± 29 % to annual stem CO2 and CH4 emissions, respectively. Overall, the stem gas fluxes were more typical for upland rather than wetland ecosystems likely due to historic ditching and subsequent groundwater level decrease.

Soil–atmosphere fluxes of trace gases (especially nitrous oxide (N2O)) can be significant during winter and at snowmelt. We investigated the effects of decreases in snow cover on soil freezing and trace gas fluxes at the Hubbard Brook Experimental Forest, a northern hardwood forest in New Hampshire, USA. We manipulated snow depth by shoveling to induce soil freezing, and measured fluxes of N2O, methane (CH4) and carbon dioxide (CO2) in field chambers monthly (bi‐weekly at snowmelt) in stands dominated by sugar maple or yellow birch. The snow manipulation and measurements were carried out in two winters (1997/1998 and 1998/1999) and measurements continued through 2000. Fluxes of CO2 and CH4 showed a strong seasonal pattern, with low rates in winter, but N2O fluxes did not show strong seasonal variation. The snow manipulation induced soil freezing, increased N2O flux and decreased CH4 uptake in both treatment years, especially during winter. Annual N2O fluxes in sugar maple treatment plots were 207 and 99 mg N m−2 yr−1 in 1998 and 1999 vs. 105 and 42 in reference plots. Tree species had no effect on N2O or CO2 fluxes, but CH4 uptake was higher in plots dominated by yellow birch than in plots dominated by sugar maple. Our results suggest that winter fluxes of N2O are important and that winter climate change that decreases snow cover will increase soil:atmosphere N2O fluxes from northern hardwood forests.

Under the umbrella of international and European Union climate policies and agreements aimed at achieving climate neutrality and thus reducing greenhouse gas (GHG) emissions from drained organic soils (including the Paris agreement, the European Green Deal and the Nature Restoration Law), it is urgently necessary to estimate GHG fluxes from former peat extraction fields to provide measurement-based recommendations for further management of these areas. In addition, there is lack of quantitative estimates of contribution of peatland plant cultivation, including berries, to total GHG emissions and climate change mitigation. Here, we compared carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes from nutrient-poor organic soils (Histosols) in former peat extraction fields currently used for cranberry (Vaccinium macrocarpon) and highbush blueberry (Vaccinium corymbosum) plantations, active peat extraction fields and pristine raised bogs. GHG flux measurements were conducted over two years using a manual chamber technique at 16 study sites (at least three sites of each land-use type) across 11 different raised bogs in the hemiboreal vegetation region of Europe (in Latvia). Across the studied land-use types, mean annual net CO2 fluxes, calculated as the difference between the annual soil heterotrophic respiration and the annual C input into soil with plant litter, ranged from near zero (-0.07 ± 0.68 t CO2-C ha-1 y-1) in the pristine raised bogs to 1.56 ± 0.19 t CO2-C ha-1 y-1 in active peat extraction fields. Furthermore, net CO2 fluxes had the largest contribution to total GHG emissions in both active peat extraction fields and berry plantations. The lowest annual CH4 fluxes were observed in cranberry plantations (6.65 ± 1.77 kg CH4-C ha-1 yr-1), while the highest were in pristine raised bogs (128.0 ± 27.5 kg CH4-C ha-1 yr-1), where CH4 fluxes accounted for the largest share of total GHG emissions. Annual N2O fluxes did not exceed 0.65 ± 0.33 kg N2O-N ha-1 yr-1 (in highbush blueberry plantations) and made a relatively low contribution to total GHG emissions compared to net CO2 and CH4 fluxes. Across the studied land-use types, the highest total GHG fluxes (the sum of annual net CO2, CH4 and N2O fluxes considering global warming potential values for a 100-year time horizon) were observed in active peat extraction fields (6.23 t CO2 eq. ha-1 yr-1), while the lowest were in cranberry plantations (1.50 t CO2 eq. ha-1 yr-1).Acknowledgments: The research was conducted within the scope of the European Commission LIFE Climate Action Programme Project “Peatland restoration for greenhouse gas emission reduction and carbon sequestration in the Baltic Sea region” (LIFE21 - CCM - LV - LIFE PeatCarbon, Project number: 101074396).

The cycling of greenhouse gases in forest ecosystems is significantly influenced by tree stems. Yet, little is known about the variability and drivers of stem-atmosphere greenhouse gas fluxes, especially in managed boreal riparian ecosystems where environmental conditions vary substantially at small spatial scales and throughout the year. Here, we report magnitudes and drivers of tree stem-atmosphere fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in a riparian buffer zone of a Swedish boreal forest that has been subject to recent forest clearcutting and historic ditching. For two full years, we conducted CO2 and CH4 flux chamber measurements on a monthly basis in 14 spruce trees (Picea abies) and 14 birch trees (Betula pendula) that grew between one and fifteen meters from a headwater stream. We also performed N2O flux measurements during three occasions. All trees were net emitters of CO2 and CH4 over the majority of the year, while N2O fluxes were close to zero. CO2 fluxes correlated strongly and positively with air temperature and followed distinct seasonal cycles peaking in summer. CH4 fluxes correlated modestly with air temperature and solar radiation and peaked in late winter and summer. Trees with larger stem diameter released more CO2 and less CH4, and trees that were nearer the stream released more CO2 and CH4. The CO2 and CH4 fluxes did not differ between spruce and birch in general, but correlations of CO2 fluxes with stem diameter and distance to stream differed between the tree species. The absence of distinct vertical trends in the CO2 and CH4 fluxes along the stems and their lack of correlation with groundwater levels and groundwater greenhouse gas concentrations point to tree internal production as the primary source of the tree stem gas emissions. Upscaled to the ecosystem, the tree stem CO2, CH4 and N2O emissions represented 52% of the forest floor CO2 emissions and 2.5% and 11.3% of the forest floor CH4  and N2O uptake, respectively, during the snow-free season. The snow cover season contributed 15% and 35% to annual tree stem CO2 and CH4 emissions, respectively. In contrast to other riparian zone studies, the stem gas fluxes in our study generally exhibited characteristics of an upland rather than a wetland ecosystem, likely because of historical ditching and subsequent groundwater level declines.

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.