Micrometeorological Measurements Reveal Large Nitrous Oxide Losses during Spring Thaw in Alberta

Mediative Mechanism of Freezing/Thawing on Greenhouse Gas Emissions in an Inland Saline-Alkaline Wetland: a Metagenomic Analysis

Inland saline-alkaline wetlands distributed in the mid-high latitude have repeatedly experienced freezing and thawing. However, the response of greenhouse gas (GHG) emission and microbially-mediated carbon and nitrogen cycle to freezing and thawing remains unclear. We monitored the GHG flux in an inland saline-alkaline wetland and found that, compared with the growth period, the average CO2 flux decreased from 171.99 to 76.61-80.71 mg/(m2‧h), the average CH4 flux decreased from 10.72 to 1.96-3.94 mg/(m2‧h), and the average N2O flux decreased from 56.17 to - 27.14 to - 20.70 μg/(m2‧h). Freezing and thawing significantly decreased the relative abundance of functional genes involved in carbon and nitrogen cycles. The aceticlastic methanogenic pathway was the main methanogenic pathway, whereas the Candidatus Methylomirabilis oxyfera was the most abundant methane oxidizer in the wetland. Ammonia-oxidizing archaea and denitrifier belonging to proteobacteria was the major microbial N2O source, while bacteria within clade II nosZ was the major microbial N2O sink. Freezing and thawing reduced the relative abundance of these genes, leading to a decrease in GHG flux.

N2O emission associated with shifts of bacterial communities in riparian wetland during the spring thawing periods

Soil freeze-thaw processes lead to high nitrous oxide (N2O) emissions and exacerbate the greenhouse effect. The wetlands of the Inner Mongolia Plateau are in the pronounced seasonal freeze-thaw zone, but the effect of spring thaw on N2O emissions and related microbial mechanisms is still unclear. We investigated the effects of different periods (freeze, freeze-thaw, and thaw) on soil bacterial community diversity and composition and greenhouse gas emissions during the spring freeze-thaw in the XiLin River riparian wetlands in China by amplicon sequencing and static dark box methods. The results showed that the freeze-thaw periods predominantly impact on the diversity and composition of the bacterial communities. The phyla composition of the soil bacteria communities of the three periods is similar in level, with Proteobacteria, Chloroflexi, Actinobacteria, and Acidobacteria dominating the microbial communities. The alpha-diversity of bacterial communities in different periods varies that the freezing period is higher than that of the freeze-thaw period (p < .05). Soil total carbon, soil water content, and microbial biomass carbon were the primary factors regulating the abundance and compositions of the bacterial communities during spring thawing periods. Based on functional predictions, the relative abundance of nitrification and denitrification genes was higher in the freezing period than in the thawing period, while the abundance was lowest in the freeze-thawing period. The correlation results found that N2O emissions were significantly correlated with amoA and amoB in nitrification genes, indicating that nitrification may be the main process of N2O production during spring thaw. This study reveals potential microbial mechanisms of N2O emission during spring thaw and provides data support and theoretical basis for further insight into the mechanism of N2O emission during spring thaw.

Contribution of the nongrowing season to annual N2O emissions from the permafrost wetland in Northeast China

Permafrost regions store large amounts of soil organic carbon and nitrogen, which are major sources of greenhouse gas. With climate warming, permafrost is thawing and releasing an abundance of greenhouse gases into the atmosphere and contributing to climate warming. Numerous studies have shown the mechanism of nitrous oxide (N₂O) emissions from the permafrost region during the growing season. However, little is known about the temporal pattern and drivers of nongrowing season N₂O emissions from the permafrost region. In this study, N₂O emissions from the permafrost region were investigated from June 2016 to June 2018 using the static opaque chamber method. We aimed to quantify the seasonal dynamics of nongrowing season N₂O emissions and their contribution to the annual budget. The results showed that the N₂O emissions ranged from - 35.75 to 74.16 μg m^-2 h^-1 with 0.89 to 1.44 kg ha^-1 being released into the atmosphere during the nongrowing season in the permafrost region. The permafrost wetland types had no significant influence on the nongrowing season N₂O emissions due to the nitrate content. The cumulative N₂O emissions during the nongrowing season contributed to 41.96-53.73% of the annual budget, accounting for almost half of the annual emissions in the permafrost region. The driving factors of N₂O emissions were different among the nongrowing season, growing season, and entire period. The N₂O emissions from the nongrowing season and total 2-year observation period were mainly affected by soil temperature, which could explain 3.01-9.54% and 6.07-14.48% of the temporal variation in N₂O emissions, respectively. In contrast, the N₂O emissions from the growing season were controlled by soil temperature, water table level, pH, NH₄⁺-N, NO₃^--N, total nitrogen, total organic carbon, and C/N ratio, which could explain 14.51-45.72% of the temporal variation of N₂O emissions. Nongrowing season N₂O emissions are an essential component of annual emissions and cannot be ignored in the permafrost region.

Global Research Alliance N2 O chamber methodology guidelines: Introduction, with health and safety considerations

Non-steady-state (NSS) chamber techniques have been used for decades to measure nitrous oxide (N₂ O) fluxes from agricultural soils. These techniques are widely used because they are relatively inexpensive, easy to adopt, versatile, and adaptable to varying conditions. Much of our current understanding of the drivers of N₂ O emissions is based on studies using NSS chambers. These chamber techniques require decisions regarding multiple methodological aspects (e.g., chamber materials and geometry, deployment, sample analysis, and data and statistical analysis), each of which may significantly affect the results. Variation in methodological details can lead to challenges in comparing results between studies and assessment of reliability and uncertainty. Therefore, the New Zealand Government, in support of the objectives of the Livestock Research Group of the Global Research Alliance on Agricultural Greenhouse Gases (GRA), funded two international projects to, first, develop standardized guidelines on the use of NSS chamber techniques and, second, refine them based on the most up to date knowledge and methods. This introductory paper summarizes a collection of papers that represent the revised guidelines. Each article summarizes existing knowledge and provides guidance and minimum requirements on chamber design, deployment, sample collection, storage and analysis, automated chambers, flux calculations, statistical analysis, emission factor estimation and data reporting, modeling, and "gap-filling" approaches. The minimum requirements are not meant to be highly prescriptive but instead provide researchers with clear direction on best practices and factors that need to be considered. Health and safety considerations of NSS chamber techniques are also provided with this introductory paper.

Global Research Alliance N2 O chamber methodology guidelines: Guidelines for gap-filling missing measurements

Nitrous oxide (N₂ O) is a potent greenhouse gas that is primarily emitted from agriculture. Sampling limitations have generally resulted in discontinuous N₂ O observations over the course of any given year. The status quo for interpolating between sampling points has been to use a simple linear interpolation. This can be problematic with N₂ O emissions, since they are highly variable and sampling bias around these peak emission periods can have dramatic impacts on cumulative emissions. Here, we outline five gap-filling practices: linear interpolation, generalized additive models (GAMs), autoregressive integrated moving average (ARIMA), random forest (RF), and neural networks (NNs) that have been used for gap-filling soil N₂ O emissions. To facilitate the use of improved gap-filling methods, we describe the five methods and then provide strengths and challenges or weaknesses of each method so that model selection can be improved. We then outline a protocol that details data organization and selection, splitting of data into training and testing datasets, building and testing models, and reporting results. Use of advanced gap-filling methods within a standardized protocol is likely to increase transparency, improve emission estimates, reduce uncertainty, and increase capacity to quantify the impact of mitigation practices.

Emissions of CO2, CH4, and N2O Fluxes from Forest Soil in Permafrost Region of Daxing'an Mountains, Northeast China

With global warming, the large amount of greenhouse gas emissions released by permafrost degradation is important in the global carbon and nitrogen cycle. To study the feedback effect of greenhouse gases on climate change in permafrost regions, emissions of CO₂, CH₄, and N₂O were continuously measured by using the static chamber-gas chromatograph method, in three forest soil ecosystems (Larix gmelinii, Pinus sylvestris var. mongolica, and Betula platyphylla) of the Daxing’an Mountains, northeast China, from May 2016 to April 2018. Their dynamic characteristics, as well as the key environmental affecting factors, were also analyzed. The results showed that the flux variation ranges of CO₂, CH₄, and N₂O were 7.92 ± 1.30~650.93 ± 28.12 mg·m⁻²·h⁻¹, −57.71 ± 4.65~32.51 ± 13.03 ug·m⁻²·h⁻¹, and −3.87 ± 1.35~31.1 ± 2.92 ug·m⁻²·h⁻¹, respectively. The three greenhouse gas fluxes showed significant seasonal variations, and differences in soil CO₂ and CH₄ fluxes between different forest types were significant. The calculation fluxes indicated that the permafrost soil of the Daxing’an Mountains may be a potential source of CO₂ and N₂O, and a sink of CH₄. Each greenhouse gas was controlled using different key environmental factors. Based on the analysis of Q₁₀ values and global warming potential, the obtained results demonstrated that greenhouse gas emissions from forest soil ecosystems in the permafrost region of the Daxing’an Mountains, northeast China, promote the global greenhouse effect.

N2O Emissions From Two Agroecosystems: High Spatial Variability and Long Pulses Observed Using Static Chambers and the Flux-Gradient Technique

With the addition of nitrogen (N), agricultural soils are the main anthropogenic source of N2O, but high spatial and temporal variabilities make N2O emissions difficult to characterize at the field scale. This study used flux-gradient measurements to continuously monitor N2O emissions at two agricultural fields under different management regimes in the inland Pacific Northwest of Washington State, USA. Automated 16-chamber arrays were also deployed at each site; chamber monitoring results aided the interpretation of the flux gradient results. The cumulative emissions over the six-month (1 April-30 September) monitoring period were 2.4 ± 0.7 and 2.1 ± 2 kg N2O-N/ha at the no-till and conventional till sites, respectively. At both sites, maximum N2O emissions occurred following the first rainfall event after N fertilization, and both sites had monthlong emission pulses. The no-till site had a larger N2O emission factor than the Intergovernmental Panel on Climate Change Tier 1 emission factor of 1% of the N input, while the conventional-till site's emission factor was close to 1% of the N input. However, these emission factors are likely conservative. We estimate that the global warming potential of the N2O emissions at these sites is larger than that of the no-till conversion carbon uptake. We recommend the use of chambers to investigate spatiotemporal controls as a complementary method to micrometeorological monitoring, especially in systems with high variability. Continued monitoring coupled with the use of models is necessary to investigate how changing management and environmental conditions will affect N2O emissions.