Atmospheric methane and nitrous oxide: challenges alongthe path to Net Zero

Mechanisms of mitigating nitrous oxide emission during composting by biochar and calcium carbonate addition

To investigate the mechanisms underlying effects of biochar and calcium carbonate (CaCO3) addition on nitrous oxide (N2O) emissions during composting, this paper conducted a systematic study on mineral nitrogen (N), dissolved organic carbon (C) and N, sources of N2O, and functional genes. Biochar and CaCO3 addition decreased N2O emissions by 26.5-47.8% (9.5-96.9 mg N kg-1 dw) and 13.9-37.4% (12.0-121.0 mg N kg-1 dw) compared to the control (14.3-179.7 mg N kg-1 dw), respectively. The mitigation of N2O emission was caused by decreased contribution of ammonia-oxidizing bacteria (AOB) and fungi to N2O production due to diminished AOB amoA, fungal nirK and P450 gene abundances, or by stimulated N2O reduction to N2 owing to increased abundances of nosZⅠ and nosZⅠⅠ genes under biochar and CaCO3 addition. The findings suggest that the addition of biochar or CaCO3 is effective in mitigating N2O emission during composting.

Dynamic chamber as a more reliable technique for measuring methane emissions from aquatic ecosystems

Aquatic ecosystems are the largest natural source of atmospheric methane ("CH₄") worldwide. However, the current estimation of CH₄ emissions from aquatic ecosystems still has extensive uncertainty due to large spatiotemporal variations in CH₄ emissions as well as significant uncertainty in measurement methods. In this study, we initially investigated CH₄ fluxes from a simulated eutrophic water body by using static chamber method ("SC") during an incubation period of 36 days. Approximately 23 % of the total flux measurements were unsuccessful because they lacked a linear correlation between the accumulation of CH₄ concentrations and enclosure time. CH₄ fluxes could be achieved for most measurements. However, 5 min after enclosing, the initial CH₄ concentrations measured in the chambers were too high (up to 507.4 ppm) to greatly suppress CH₄ emissions from the diffusion process. Therefore, a dynamic chamber method ("DC") was developed to overcome the shortcomings of the SC. To achieve the DC, air samples must be continuously collected at the inlet and outlet of the dynamic chamber at fixed flow rates. In contrast to the SC, effective CH₄ flux data could be obtained by the DC for each measurement at different frequencies. The DC measured the diel and daily variations in CH₄ fluxes and the displayed CH₄ emissions from the simulated water were highly irregular. The displayed emissions had variations up to more than two orders of magnitude. These results implied that the SC measured few intermittent fluxes that were difficult to represent the actual CH₄ emissions from eutrophic water. The DC developed in this study considers the temporal variations in CH₄ emissions from aquatic ecosystems. Thus, the DC is expected to be applicable in the field flux measurements of CH₄ as well as other greenhouse gases to reduce emissions uncertainties.

CleanEx: A Versatile Automated Methane Preconcentration Device for High-Precision Analysis of 13CH4, 12CH3D, and 13CH3D

The relative abundance of methane isotopologues offers key insights into the global methane (CH₄) cycle. Advances in laser spectroscopy enable routine high-precision measurements even for rare deuterated methane isotopologues, ¹²CH₃D and ¹³CH₃D, provided there are sufficiently high methane amount fractions and reproducible measurement conditions, which can be achieved by CH₄ adsorption-desorption techniques. We present a new cryogen-free automated preconcentration device─CleanEx─designed for quantitative extraction of CH₄ from large volumes of sample gas and for cleaning by stepwise temperature-controlled desorption to separate interferant gases. We show that CleanEx has the capability to preconcentrate methane by almost 2000-fold from ∼18 L of air. The performance is demonstrated in a range of methane amount fractions between 2 ppm (μmol mol^-1), which corresponds to the present-day ambient air, up to 1000 ppm, representative for close to source or process conditions. Advantages over existing devices are a significantly larger primary adsorption trap and a secondary cryo-focusing step, which ensures separation of methane from major atmospheric compounds, i.e., O₂, Ar, and CO₂. We have demonstrated quantitative extraction of methane, with no significant isotopic fractionation and high repeatability of 0.2‰, 0.6‰, and 0.8‰ (n = 42) for the studied isotopologue ratios, ¹³CH₄/¹²CH₄, ¹²CH₃D/¹²CH₄, and ¹³CH₃D/¹²CH₄, during cryogenic adsorption-desorption on HayeSep D material. The developed device in combination with a suitable laser spectrometer offers a robust and autonomous method for precise continuous monitoring of δ¹³C-CH₄ and δD-CH₄ in ambient air and optionally Δ¹³CH₃D in process-derived methane.

Wilde S. Isotopic signatures of methane emissions from tropical fires, agriculture and wetlands: the MOYA and ZWAMPS flights

We report methane isotopologue data from aircraft and ground measurements in Africa and South America. Aircraft campaigns sampled strong methane fluxes over tropical papyrus wetlands in the Nile, Congo and Zambezi basins, herbaceous wetlands in Bolivian southern Amazonia, and over fires in African woodland, cropland and savannah grassland. Measured methane δ¹³C_CH4 isotopic signatures were in the range -55 to -49‰ for emissions from equatorial Nile wetlands and agricultural areas, but widely -60 ± 1‰ from Upper Congo and Zambezi wetlands. Very similar δ¹³C_CH4 signatures were measured over the Amazonian wetlands of NE Bolivia (around -59‰) and the overall δ¹³C_CH4 signature from outer tropical wetlands in the southern Upper Congo and Upper Amazon drainage plotted together was -59 ± 2‰. These results were more negative than expected. For African cattle, δ¹³C_CH4 values were around -60 to -50‰. Isotopic ratios in methane emitted by tropical fires depended on the C3 : C4 ratio of the biomass fuel. In smoke from tropical C3 dry forest fires in Senegal, δ¹³C_CH4 values were around -28‰. By contrast, African C4 tropical grass fire δ¹³C_CH4 values were -16 to -12‰. Methane from urban landfills in Zambia and Zimbabwe, which have frequent waste fires, had δ¹³C_CH4 around -37 to -36‰. These new isotopic values help improve isotopic constraints on global methane budget models because atmospheric δ¹³C_CH4 values predicted by global atmospheric models are highly sensitive to the δ¹³C_CH4 isotopic signatures applied to tropical wetland emissions. Field and aircraft campaigns also observed widespread regional smoke pollution over Africa, in both the wet and dry seasons, and large urban pollution plumes. The work highlights the need to understand tropical greenhouse gas emissions in order to meet the goals of the UNFCCC Paris Agreement, and to help reduce air pollution over wide regions of Africa. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.