Greenhouse Gas Budget Research Articles

Implications of the consumption-based accounting for future national emissions budgets

ABSTRACT As a complement of production-based accounting (PBA), consumption-based accounting (CBA) provides an alternative perspective to understand national emissions and responsibilities for global climate change. Existing studies assessed historical national emissions under CBA. However, how CBA might guide future mitigation under the Paris Agreement goals has not been well understood. By combining global multi-region input-output analysis and the equity principle of ‘equal cumulative per capita emissions’ (ECPCE), this study explores potential implications of CBA for future national emissions budgets to limit global warming to below 1.5°C. Historically, consumption-based greenhouse gas (GHG) emissions were higher than production-based emissions in most developed countries while lower in many developing countries. When CBA is used instead of PBA to allocate emissions budgets by ECPCE, future GHG mitigation burdens would become more stringent in 70% of Annex I (mostly developed) countries but slightly smaller in 55% of non-Annex I (mostly developing) countries. In particular, more than half of Annex I countries might need to achieve negative GHG budgets under 1.5°C. Our analysis emphasizes that, as net emission importers, developed countries could strive to pay equal attention to achieving consumption-based and production-based emissions reductions, and strengthen technological and financial support to developing countries that undertake production outsourcing. As net emission exporters, with international support and domestic investment, developing countries could pragmatically promote energy structure optimization and technological innovation to accelerate the decarbonization of production. To support the Paris Agreement goals, mitigation contributions initiated from the perspective of CBA could also be welcomed. Key policy insights Developed countries shift emissions to developing countries through international trade. CBA increases historical responsibilities of developed countries for climate warming from the PBA assessments. Compared to PBA, CBA has clear implications on future national emissions budgets under responsibility-based allocations, but does not change the urgency of national mitigation required to achieve the Paris Agreement goals. Allocated by ECPCE, future GHG budgets for over 50% of the world’s countries would be run out before 2040 under 1.5°C at the 2015 emissions rates, using either CBA or PBA. According to different roles in global supply chains, developed countries should step up support to help developing countries improve emissions and energy intensities, which is a key to reducing emissions embodied in trade.

Prairie wetlands as sources or sinks of nitrous oxide: Effects of land use and hydrology

National and global greenhouse gas (GHG) budgets are continually being refined as data become available. Primary sources of the potent GHG nitrous oxide (N2O) include agricultural soil management and burning of fossil fuels, but comprehensive N2O budgets also incorporate less prominent factors such as wetlands. Freshwater wetland GHG flux estimates, however, have high uncertainty, and wetlands have been identified as both sources and sinks. Here, we analyzed a regional database of >26,000 N2O chamber flux measurements sampled across >150 wetlands from the Prairie Pothole Region (PPR) in the Great Plains of North America. Our goal was to identify important land use and hydrologic drivers of N2O flux to help reduce uncertainty in N2O models, and to incorporate these drivers into an upscaled estimate of wetland N2O emissions from the U.S. portion of the PPR. Within individual wetlands, exposed soils with no standing water, such as along wetland edges, were hotspots that accounted for greater than 90% of wetland N2O emissions. In contrast wet (i.e., ponded) areas had minimal or negative N2O flux. N2O flux from wetlands nested within croplands (16.3–17.3 μg N2O m−2 hr−1) was, in some instances, nearly double that from wetlands within grasslands (9.2–14.4 μg N2O m−2 h−1). We estimated that seasonal N2O flux from PPR wetlands equated to roughly 0.2% (1.04 Tg CO2 equivalents) of the U.S. N2O budget (c. 2019). Overall, even though PPR wetlands are a small net source of N2O to the atmosphere, their emissions are negligible relative to agricultural soil management. Policy and management to restore wetland hydrology and surrounding uplands from cropland to grasslands can reduce landscape N2O fluxes. Future activities focused on wetland N2O flux would benefit from inclusion of adjacent land use and hydrologic factors, as well as from incorporation of temporally dynamic ponded wetland areas.

Effects of soil warming and straw return on soil organic matter and greenhouse gas fluxes in winter wheat seasons in the North China Plain

As one of the greatest political and scientific issues, global warming is affecting the terrestrial carbon (C) and nitrogen (N) cycles and subsequently greenhouse gases (GHGs) fluxes. Straw return is considered as an environmentally friendly management to increase soil health and agricultural productivity. However, little is known about their interactive effects on soil organic matter (SOM) and GHGs emissions. Therefore, we collected two-year data from a field experiment in the North China Plain (NCP) to elucidate the effects of soil warming (ambient temperature vs. warming +3.8 °C) and straw management (straw removal vs. straw return) on soil properties, nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) fluxes during winter wheat seasons. Soil warming stimulated the available nutrients (NH4+, NO3−, and dissolved organic C), and thereby increased cumulative CO2 emission by 17.5–25.0% compared with ambient temperature, while had no effect on N2O fluxes. Straw return enhanced soil CO2 and N2O emissions by 11.5–28.3% and 37.1–48.4% compared with straw removal, respectively. This was mainly due to increased organic matter input stimulating microbial activity, thereby accelerating soil C and N cycling. The NCP was found to be a CH4 sink, and soil warming and straw return increased the CH4 uptake. Finally, soil warming combined straw return resulted in the highest GHGs budget, and we estimated that almost 7.1 t CO2-eq ha−1 per wheat season would be lost in response to soil warming and straw return in the NCP. In the second growing season, soil organic C (SOC) and total N (TN) were reduced under warming by 6.1% and 4.9% due to the stimulated decomposition and mineralization, respectively. However, the decreases considerably counteracted by straw return, which enhanced the SOC and TN by 5.8% and 9.4%, respectively. Our study suggested that soil warming and straw return will lead to stimulated GHGs emissions in wheat seasons in the NCP, while straw return may contribute to the mitigation of global warming through increasing SOM accumulation.

Comparing national greenhouse gas budgets reported in UNFCCC inventories against atmospheric inversions

Abstract. In support of the global stocktake of the Paris Agreement on climate change, this study presents a comprehensive framework to process the results of an ensemble of atmospheric inversions in order to make their net ecosystem exchange (NEE) carbon dioxide (CO2) flux suitable for evaluating national greenhouse gas inventories (NGHGIs) submitted by countries to the United Nations Framework Convention on Climate Change (UNFCCC). From inversions we also deduced anthropogenic methane (CH4) emissions regrouped into fossil and agriculture and waste emissions, as well as anthropogenic nitrous oxide (N2O) emissions. To compare inversion results with national reports, we compiled a new global harmonized database of emissions and removals from periodical UNFCCC inventories by Annex I countries, and from sporadic and less detailed emissions reports by non-Annex I countries, given by national communications and biennial update reports. No gap filling was applied. The method to reconcile inversions with inventories is applied to selected large countries covering ∼90 % of the global land carbon uptake for CO2 and top emitters of CH4 and N2O. Our method uses results from an ensemble of global inversions produced by the Global Carbon Project for the three greenhouse gases, with ancillary data. We examine the role of CO2 fluxes caused by lateral transfer processes from rivers and from trade in crop and wood products and the role of carbon uptake in unmanaged lands, both not accounted for by NGHGIs. Here we show that, despite a large spread across the inversions, the median of available inversion models points to a larger terrestrial carbon sink than inventories over temperate countries or groups of countries of the Northern Hemisphere like Russia, Canada and the European Union. For CH4, we find good consistency between the inversions assimilating only data from the global in situ network and those using satellite CH4 retrievals and a tendency for inversions to diagnose higher CH4 emission estimates than reported by NGHGIs. In particular, oil- and gas-extracting countries in central Asia and the Persian Gulf region tend to systematically report lower emissions compared to those estimated by inversions. For N2O, inversions tend to produce higher anthropogenic emissions than inventories for tropical countries, even when attempting to consider only managed land emissions. In the inventories of many non-Annex I countries, this can be tentatively attributed to a lack of reporting indirect N2O emissions from atmospheric deposition and from leaching to rivers, to the existence of natural sources intertwined with managed lands, or to an underestimation of N2O emission factors for direct agricultural soil emissions. Inversions provide insights into seasonal and interannual greenhouse gas fluxes anomalies, e.g., during extreme events such as drought or abnormal fire episodes, whereas inventory methods are established to estimate trends and multi-annual changes. As a much denser sampling of atmospheric CO2 and CH4 concentrations by different satellites coordinated into a global constellation is expected in the coming years, the methodology proposed here to compare inversion results with inventory reports (e.g., NGHGIs) could be applied regularly for monitoring the effectiveness of mitigation policy and progress by countries to meet the objective of their pledges. The dataset constructed by this study is publicly available at https://doi.org/10.5281/zenodo.5089799 (Deng et al., 2021).

KGML-ag: a modeling framework of knowledge-guided machine learning to simulate agroecosystems: a case study of estimating N<sub>2</sub>O emission using data from mesocosm experiments

Abstract. Agricultural nitrous oxide (N2O) emission accounts for a non-trivial fraction of global greenhouse gas (GHG) budget. To date, estimating N2O fluxes from cropland remains a challenging task because the related microbial processes (e.g., nitrification and denitrification) are controlled by complex interactions among climate, soil, plant and human activities. Existing approaches such as process-based (PB) models have well-known limitations due to insufficient representations of the processes or uncertainties of model parameters, and due to leverage recent advances in machine learning (ML) a new method is needed to unlock the “black box” to overcome its limitations such as low interpretability, out-of-sample failure and massive data demand. In this study, we developed a first-of-its-kind knowledge-guided machine learning model for agroecosystems (KGML-ag) by incorporating biogeophysical and chemical domain knowledge from an advanced PB model, ecosys, and tested it by comparing simulating daily N2O fluxes with real observed data from mesocosm experiments. The gated recurrent unit (GRU) was used as the basis to build the model structure. To optimize the model performance, we have investigated a range of ideas, including (1) using initial values of intermediate variables (IMVs) instead of time series as model input to reduce data demand; (2) building hierarchical structures to explicitly estimate IMVs for further N2O prediction; (3) using multi-task learning to balance the simultaneous training on multiple variables; and (4) pre-training with millions of synthetic data generated from ecosys and fine-tuning with mesocosm observations. Six other pure ML models were developed using the same mesocosm data to serve as the benchmark for the KGML-ag model. Results show that KGML-ag did an excellent job in reproducing the mesocosm N2O fluxes (overall r2=0.81, and RMSE=3.6 mgNm-2d-1 from cross validation). Importantly, KGML-ag always outperforms the PB model and ML models in predicting N2O fluxes, especially for complex temporal dynamics and emission peaks. Besides, KGML-ag goes beyond the pure ML models by providing more interpretable predictions as well as pinpointing desired new knowledge and data to further empower the current KGML-ag. We believe the KGML-ag development in this study will stimulate a new body of research on interpretable ML for biogeochemistry and other related geoscience processes.

Soil greenhouse gas fluxes following conversion of tropical forests to fertilizer-based sugarcane systems in northwestern Uganda

Tropical deforestation for fertilizer-based agriculture has greatly increased in the last decades resulting in significant greenhouse gas (GHG; carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)) emissions. Unfortunately, empirical studies on soil GHG fluxes from African deforestation hotspots are still limited, creating uncertainties in global GHG budgets. Therefore, we assessed how soil GHG fluxes along with their auxiliary controls (water filled pore space (WFPS), temperature, and mineral nitrogen (N)) differed between the forest and sugarcane plantations. This assessment was based on monthly (forest) and intensive (sugarcane) GHG and auxiliary measurements between May 2019 and June 2020. Measurements were conducted in four reference forest plots and 12 sugarcane plots randomly assigned to three fertilization treatment groups (low, standard, and high), representing the fertilization gradient used by sugarcane farmers in Uganda. Despite the use of different fertilization rates as treatments for the sugarcane experiment, neither auxiliary controls nor soil GHG fluxes significantly differed among the treatments. Soil CO2 effluxes were higher under sugarcane (17.6 ± 0.0 Mg C ha-1 yr-1) compared to forest (14.5 ± 0.1 Mg C ha-1 yr-1; p < 0.001) because of the higher autotrophic respiration from the sugarcane’s fine root biomass and the microbial decomposition of the sugarcane’s larger soil organic carbon (SOC) stocks. Conversely, soil CH4 uptake under sugarcane (−1.1 ± 0.0 kg C ha-1 yr-1) was three times lower than under forest (−3.1 ± 0.0 kg C ha-1 yr-1; p < 0.001), owing to the likely alteration of methanotroph abundance upon conversion. Likewise, soil N2O emissions were smaller under sugarcane (1.3 ± 0.0 kg N ha-1 yr-1) compared to forest (1.8 ± 0.0 kg N ha-1 yr-1; p < 0.001) because excess N from fertilizer addition in the sugarcane was either lost through leaching or taken up by the sugarcane crop. Only seasonal variability in WFPS, among the auxiliary controls, affected CH4 uptake at both sites (p < 0.001) and soil CO2 effluxes under sugarcane (p = 0.018). Noteworthy, soil N2O fluxes from both sites were unaltered by the seasonality-mediated changes in auxiliary controls. We conclude that even with the increased SOC sequestration and the lower N2O emissions under sugarcane compared to forest, the forest-sugarcane conversion resulted in a yearly net C loss of 2.8 Mg CO2-eq ha-1 from soils to atmosphere, largely arising from the higher soil CO2 effluxes and to a smaller extent from a reduced CH4 uptake under sugarcane.

Reference for different sensitivities of greenhouse gases effluxes to warming climate among types of desert biological soil crust

There is much uncertainty about how climate warming will impact greenhouse gases (GHG) budget in dry environments due to the lack of available data for desert biocrust soil. We implemented a 2.5-year field measurement of CO2, CH4 and N2O effluxes in cyanobacteria-dominated, moss-dominated and mixed (cyanobacteria, moss and lichen) biocrust soils using open-top-chambers to simulate climate warming (1.2 °C on average). Desert biocrust soils generally acted as a weak sink of atmospheric CH4 and N2O. Although warming effects on daily CO2, CH4, and N2O effluxes varied depending on sampling date and biocrust soil, there was no significant difference in daily, monthly and seasonal average CO2, CH4 and N2O effluxes between warming and control in most cases for three biocrust soils. However, warming caused a marginal (p = 0.06) decrease (14.2%) in annual accumulative CO2 efflux in moss-dominated biocrust soil due to the drought effects caused by warming indirectly and OTC sheltering of precipitation directly, while there was no significant difference between warming and control for cyanobacteria-dominated and mixed biocrust soils, implying a neutral response of GHG effluxes to climate warming. These results suggest that the GHG budget in arid desert biocrust soil would not be significantly changed in the warmer future when the direct negative effects of drought on CO2 effluxes were excluded. Therefore, a marginal decrease of accumulative CO2 effluxes in response to warming coupled with drought for moss-dominated biocrust soil might offer a weak negative feedback to warming and drier climate change pattern.

Sugarcane residue and N-fertilization effects on soil GHG emissions in south-central, Brazil

Ethanol derived from sugarcane (Saccharum officinarum) could replace a substantial amount of fossil fuel in Brazil, provided the greenhouse gas (GHG) budget is balanced. Green harvest systems have significantly increased the potential amount of sugarcane residue (straw) available for harvest and bioenergy production, but several questions about how those practices will affect field GHG fluxes. In this sense, we quantified GHG emissions associated with four straw harvest rates (12 [no removal]; 6 [moderate removal]; 3 [high removal] and 0 Mg ha−1 [total removal] of dry matter of straw left on the soil), with and without 100 L ha−1 vinasse + N-fertilizer [80 kg ha−1 (NH4)2SO4]. GHG fluxes were measured for 60 days using static chambers. Our results showed that total straw removal reduced CO2 and N2O emissions by 15% and 25%, respectively. After vinasse and fertilizer additions, CO2 emissions were 2.5-fold higher, and N2O were 5-fold higher, regardless of straw rates. In additional, synergic effect of vinasse + fertilizer and straw removal decreased 60% CH4 uptakes. Direct N2O emission factor was estimated at 0.32%. Net GHG emissions induced by vinasse + fertilizer applied in the sugarcane plantation were 220 g m−2 CO2e (or 60 g m−2 C-equivalent), disregarding straw harvest rate. Therefore, we conclude that maintaining at least 6 Mg ha-1 of sugarcane straw in the field will result in a ‘win-win situation’ by mitigating GHG emissions, sustaining soil, and still saving part of the raw-material for bioenergy production.

Cooperation mechanisms for a competitive, sustainable food supply chain to reduce greenhouse gas emissions.

The food industry is a major source of greenhouse gases (GHG). Given that consumers in this industry are aware of the negative consequences of GHG emissions, such as global warming and climate change, members of the food supply chain should consider mechanisms to reduce GHG emissions. The purpose of this paper is to examine the effects of supply chain structure and cooperation methods on the objectives and decisions of a sustainable food supply chain composed of one manufacturer and two suppliers. In the first scenario, a single-level problem is solved for a centralized supply chain. Other scenarios consider the decentralized structure, in which supply chain members face constraints such as maximum additive use and GHG emission, budget, and capacity. The bi-level programming is used to model competition between members of the sustainable food supply chain. It is demonstrated first that the lower-level models in decentralized scenarios can be converted to a single-level model, and then the proposed bi-level model is converted to a single-level one using the KKT method. Computational results show that the alliance of the manufacturer and the first supplier yields the highest total profit for all decentralized scenarios. Because the lowest GHG emission rate among decentralized scenarios is achieved through alliance and cost-sharing mechanisms, the use of these mechanisms concurrently is appropriate for environmental purposes. In scenarios where supply chain members compete with each other, it is found to be cost-effective to increase the budget. Additional considerations can be made regarding the effects of other variables such as distance and advertising on demand as well as alternative forms of the demand function.

Carbon dioxide uptake overrides methane emission at the air-water interface of algae-shellfish mariculture ponds: Evidence from eddy covariance observations

Mariculture ponds are widely distributed along the coastal regions and have been increasingly recognized as biogeochemical hotspots of air-water greenhouse gas (GHG) fluxes, but their source/sink dynamics and climate benefits have not been well understood. Due to strong temporal variations of GHG fluxes over mariculture ponds, previous studies based on short-term or discrete flux measurements have large uncertainty in assessing GHG budgets and their radiative effects. In this study, we examined the temporal variations of air-water GHG fluxes, net CO2 exchange (NEE) and net CH4 exchange (NME), and their environmental controls, based on one-year (2020) continuous eddy covariance (EC) measurements over algae-shellfish mariculture ponds (razor clam) in a subtropical estuary of Southeast China. The results showed that (a) annually the ponds acted as a strong CO2 sink of −227.7 g CO2-C m−2 and a weak CH4 source of 1.44 g CH4-C m−2, and thus the NME-induced warming effect offset 25.9% (12.1%) of the NEE-induced cooling effect at a 20-year (100-year) time horizon using the metric of sustained-flux global warming potential; (b) two GHG fluxes showed different diurnal and seasonal variations but both had stronger source/sink capacity in summer and more fluctuating fluxes in winter; (c) temporal variations of NEE and NME tended to be more regulated by photosynthetically active radiation and tidal salinity, respectively, but both of them were affected by water temperature and area proportion of algae ponds within the EC footprint. This is the first study to disentangle temporal variations of air-water GHG fluxes over mariculture ponds based on simultaneous EC measurements of CO2 and CH4 fluxes. This study highlights the climate benefits of algae-shellfish mariculture ponds as biogeochemical hotspots by exerting a net radiative cooling effect dominated by the CO2 sink.

Grazing Reduces the Soil-Atmosphere Exchange of Greenhouse Gases During Freeze-Thaw Cycles in Meadow Steppes in Inner Mongolia

Both livestock grazing and soil freeze-thaw cycles (FTCs) can affect the soil-atmosphere exchange of greenhouse gases (GHGs) in grasslands. However, the combined effects of grazing and FTCs on GHG fluxes in meadow steppe soils remain unclear. In this study, we collected soils from paired grazing and enclosed sites and conducted an incubation experiment to investigate the effect of grazing on soil GHG fluxes in the meadow steppes of Inner Mongolia during three FTCs. Our results showed that FTCs substantially stimulated the emissions of soil N2O and CO2 and the uptake of CH4 in the meadow steppes. However, compared with enclosure treatments, grazing significantly reduced the cumulative N2O, CO2 and CH4 fluxes by 13.3, 14.6, and 26.8%, respectively, during the entire FTCs experiment. The soil dissolved organic carbon (DOC) and nitrogen (DON), NH4+-N and NO3–-N, significantly increased after three FTCs and showed close correlations with N2O and CO2 emissions. Structural equation modeling (SEM) revealed that the increase in NO3–-N induced by FTCs dominated the variance in N2O emissions and that DOC strongly affected CO2 emissions during thawing periods. However, long-term grazing reduced soil substrate availability and microbial activity and increased soil bulk density, which in turn decreased the cumulative GHG fluxes during FTCs. In addition, the interaction between grazing and FTCs significantly affected CO2 and CH4 fluxes but not N2O fluxes. Our results indicated that livestock grazing had an important effect on soil GHG fluxes during FTCs. The combined effect of grazing and FTCs should be taken into account in future estimations of GHG budgets in both modeling and experimental studies.

Assessment of GHG Interactions in the Vicinity of the Municipal Waste Landfill Site—Case Study

Landfills have been identified as one of the major sources of greenhouse gas (GHG) emissions and as a contributor to climate change. Landfill facilities exhibit considerable spatial and temporal variability of both methane (CH4) and carbon dioxide (CO2) rates. The present work aimed to evaluate the spatial distribution of CH4 and CO2 and their δ13C isotopic composition originating from a municipal landfill site, to identify its contribution to the local GHG budget and the potential impact on the air quality of the immediate surroundings in a short-term response to environmental conditions. The objective was met by performing direct measurements of atmospheric CO2 and CH4 at the selected monitoring points on the surface and applying a binary mixing model for the determination of carbon isotopic ratios in the vicinity of the municipal waste landfill site. Air samples were collected and analysed for isotopic composition using flask sampling with a Picarro G2201-I Cavity Ring-Down Spectroscopy (CRDS) technique. Kriging and Inverse distance weighting (IDW) methods were used to evaluate the values at unsampled locations and to map the excess of GHGs emitted from the landfill surface. The large off-site dispersion of methane from the landfill site at a 500 m distance was identified during field measurements using isotopic data. The mean δ13C of the landfill biogas emitted to the surrounded atmosphere was −53.9 ± 2.2‰, which corresponded well to the microbial degradation processes during acetate fermentation in the waste deposits. The calculated isotopic compositions of CO2 (δ13C = −18.64 ± 1.75‰) indicate the domination of biogenic carbon reduction by vegetation surrounding the landfill. Finally, amounts of methane escaping into the air can be limited by the appropriate landfill management practices (faster covers active quarter through separation layer), and CH4 reduction can be achieved by sealing the cover on the leachate tank.

From sink to source: high inter-annual variability in the carbon budget of a Southern African wetland.

We report on three years of continuous monitoring of carbon dioxide (CO2) and methane (CH4) emissions in two contrasting wetland areas of the Okavango Delta, Botswana: a perennial swamp and a seasonal floodplain. The hydrographic zones of the Okavango Delta possess distinct attributes (e.g. vegetation zonation, hydrology) which dictate their respective greenhouse gas (GHG) temporal emission patterns and magnitude. The perennial swamp was a net source of carbon (expressed in CO2-eq units), while the seasonal swamp was a sink in 2018. Despite differences in vegetation types and lifecycles, the net CO2 uptake was comparable at the two sites studied in 2018/2020 (−894.2 ± 127.4 g m−2 yr−1 at the perennial swamp, average of the 2018 and 2020 budgets, and −1024.5 ± 134.7 g m−2 yr−1 at the seasonal floodplain). The annual budgets of CH4 were however a factor of three larger at the permanent swamp in 2018 compared to the seasonal floodplain. Both ecosystems were sensitive to drought, which switched these sinks of atmospheric CO2 into sources in 2019. This phenomenon was particularly strong at the seasonal floodplain (net annual loss of CO2 of 1572.4 ± 158.1 g m−2), due to a sharp decrease in gross primary productivity. Similarly, drought caused CH4 emissions at the seasonal floodplain to decrease by a factor of 4 in 2019 compared to the previous year, but emissions from the perennial swamp were unaffected. Our study demonstrates that complex and divergent processes can coexist within the same landscape, and that meteorological anomalies can significantly perturb the balance of the individual terms of the GHG budget. Seasonal floodplains are particularly sensitive to drought, which exacerbate carbon losses to the atmosphere, and it is crucial to improve our understanding of the role played by such wetlands in order to better forecast how their emissions might evolve in a changing climate. Studying such hydro-ecosystems, particularly in the data-poor tropics, and how natural stressors such as drought affect them, can also inform on the potential impacts of man-made perturbations (e.g. construction of hydro-electric dams) and how these might be mitigated. Given the contrasting effects of drought on the CO2 and CH4 flux terms, it is crucial to evaluate an ecosystem's complete carbon budget instead of treating these GHGs in isolation.This article is part of a discussion meeting issue ‘Rising methane: is warming feeding warming? (part 2)’.

Impact of organic fertilizer substitution and biochar amendment on net greenhouse gas budget in a tea plantation

Tailoring agricultural practices to enhance the soil carbon (C) stock is seen as a promising mitigation tactic to offset greenhouse gas (GHG) emissions in croplands. Tea plantations are not only the important part of economic industry but also one of the crucial agricultural sources of non-carbon dioxide emissions. Although many studies have measured GHG emissions from tea plantations, it remains unknown about the effect of knowledge-based mitigation options on the entire C budget from Chinese rapidly expanding tea plantations. Thus, we carried out a 2-year field trial to provide an insight into the influence of organic fertilizer substitution for synthetic fertilizer and biochar amendment on net ecosystem carbon budget (NECB), net greenhouse gas budget (NGB), and yield-scaled greenhouse gas intensity (GHGI) from a subtropical tea plantation. Results showed that when averaged a 2-year experimental period, both full organic substitution and biochar amendment contributed significantly to the increment in NECB, mainly due to the enhanced soil organic C content in the tea field. Compared with the conventional farm practice, the application of full organic substitution can induce a 52% decrease in both NGB and GHGI. Regardless of fertilizer type, both NGB and GHGI were negative and 2.4 times lower in the treatments with biochar amendment relative to the control. In addition to their roles in maintaining soil health and alleviating soil acidification, our results suggest that organic fertilizer substitution and biochar addition may achieve low carbon development for tea plantations. Our findings will inform efforts to implement and evaluate these tailored mitigation options in tea plantations at a national scale.

Microbial activity, methane production, and carbon storage in Early Holocene North Sea peats

Abstract. Northern latitude peatlands act as important carbon sources and sinks, but little is known about the greenhouse gas (GHG) budgets of peatlands that were submerged beneath the North Sea during the last glacial–interglacial transition. We found that whilst peat formation was diachronous, commencing between 13 680 and 8360 calibrated years before the present, stratigraphic layering and local vegetation succession were consistent across a large study area. Large carbon stores were measured. In situ methane (CH4) concentrations of sediment pore waters were widespread but low at most sites, with the exception of two locations. Incubation experiments in the laboratory revealed molecular signatures of methanogenic archaea, with strong increases in rates of activity upon methylated substrate amendment. Remarkably, methanotrophic activity and the respective diagnostic molecular signatures could not be detected. Heterotrophic Bathyarchaeota dominated the archaeal communities, and bacterial populations were dominated by candidate phylum JS1 bacteria. In the absence of active methanogenic microorganisms, we conclude that these sediment harbour low concentrations of widespread millennia-old CH4. The presence of large widespread stores of carbon and in situ methanogenic microorganisms, in the absence of methanotrophic microorganisms, holds the potential for microbial CH4 production if catalysed by a change in environmental conditions.

Significant Emissions From Forest Drainage Ditches—An Unaccounted Term in Anthropogenic Greenhouse Gas Inventories?

AbstractForestry is a natural climate solution for removing atmospheric carbon dioxide (CO2) and reaching net zero emissions. Managed boreal forests typically have extensive drainage ditch networks, and these can be hotspots of anthropogenic greenhouse gas (GHG) emissions, potentially offsetting the terrestrial carbon gain. However, there is a lack of data detailing GHG emissions from ditches on mineral soils, where most boreal forestry occurs. Here, we address this knowledge gap using two approaches. First, we conducted a synoptic campaign to measure summer GHG fluxes from 109 boreal forest ditches draining mineral soils within one local region. We found a clear control of ditch water level on methane (CH4), with zero emissions from dry ditches and variable, but often high, emissions from water‐filled ditches. Almost all ditches acted as sources of CO2, regardless of water status. Second, we reanalyzed a published data set of boreal forest ditches and streams across three regions where GHG concentrations had been repeatedly measured and detailed catchment information was available. Within this data set we categorized 76 ditches into mineral and peatland catchments and detected no difference in mean CH4 and CO2 concentrations between the two soil types. GHG emissions from ditches draining mineral forest soils can be as large as those from peatland forest ditches. Using literature values for forest GHG uptake we demonstrate that ditch CH4 emissions are particularly important and can offset the terrestrial CH4 uptake. Ignoring ditch emissions, which are anthropogenic in origin, will lead to incorrect estimates of the landscape‐scale forest GHG budget.

Biophysical permafrost map indicates ecosystem processes dominate permafrost stability in the Northern Hemisphere

The stability of permafrost is of fundamental importance to socio-economic well-being and ecological services, involving broad impacts to hydrological cycling, global budgets of greenhouse gases and infrastructure safety. This study presents a biophysical permafrost zonation map that uses a rule-based geographic information system (GIS) model integrating global climate and ecological datasets to classify and map permafrost regions (totaling 19.76 × 106 km2, excluding glaciers and lakes) in the Northern Hemisphere into five types: climate-driven (CD) (19% of area), CD/ecosystem-modified (41%), CD/ecosystem protected (3%), ecosystem-driven (29%), and ecosystem-protected (8%). Overall, 81% of the permafrost regions in the Northern Hemisphere are modified, driven, or protected by ecosystems, indicating the dominant role of ecosystems in permafrost stability in the Northern Hemisphere. Permafrost driven solely by climate occupies 19% of permafrost regions, mainly in High Arctic and high mountains areas, such as the Qinghai–Tibet Plateau. This highlights the importance of reducing ecosystem disturbances (natural and human activity) to help slow permafrost degradation and lower the related risks from a warming climate.

Sulfur Constraints on the Carbon Cycle of a Blanket Bog Peatland

AbstractThe reduction of sulfate (SO42−) represents an alternative terminal electron acceptor for the oxidation of organic matter in peat soils. The greenhouse gas budget in peatlands will be constrained by how much a peatland can utilize SO42− reduction as an alternative to methanogenesis. Using records of atmospheric deposition and stream chemistry coupled with elemental analysis of peat soil, vegetation, particulate organic matter (POM) and dissolved organic matter (DOM), this study estimated a 23‐years long sulfur (S) budget for a blanket bog‐covered catchment in the North Pennines, England. The study showed that: (a) Atmospheric deposition of total S significantly declined over the study period from 2.4 to 0.5 t S/km2/yr. (b) Long term accumulation of S into deep peat at 1 m depth averaged 127 kg S/km2/yr. (c) Total S fluvial flux peaked as 4.5 t S/km2/yr with an average of 0.7 t S/km2/yr. (d) On average, over 23 years, 0.25 t S/km2/yr were reduced to either mineral sulphides or hydrogen sulphide; however, in eight out of the 23 years the catchment was a net producer of S to the streams of the catchment. At maximum observed, S reduction capacity the peatland was capable of a net removal of 71% of atmospheric S deposition. Allowing for the efficiency of energy transfer in the redox process and the oxidation state of peat organic matter means that for every mole of SO42− reduced, 1.69 moles of CO2 were produced, and an average of 0.47 t C/km2/yr are diverted from methanogenesis.

Effects of land-use conversion from Masson pine forests to tea plantations on net ecosystem carbon and greenhouse gas budgets

A five-year in situ field experiment was conducted to explore the effects of land-use conversion from Masson pine forests to tea plantations on the net ecosystem carbon balance (NECB) and net greenhouse gas (GHG) budget. Compared with forests, tea plantations decreased soil methane (CH4) uptake by 23.3–73.1% and increased heterotrophic respiration (Rh) and nitrous oxide (N2O) emissions by 1.4–60.0% and 263.2–2796.7%, respectively, over the five-year study period. The forests acted as net carbon sinks (0.04 Mg C ha−1 yr−1 on average). Converting the forests to tea fields in the first and second years led to net carbon losses of 27.94 and 2.28 Mg C ha−1 yr−1, respectively. These losses could primarily be attributed to the sharp decrease in net primary productivity (NPP) by removal of the forests and increase in Rh outweighing organic fertilizer application in the first year, and Rh exceeding NPP in the second year. Tea plantations served as net carbon sinks (0.27–0.69 Mg C ha−1 yr−1) in the third to fifth years following land-use conversion, which was mainly due to organic fertilizer application and NPP outweighing Rh. The forests showed net GHG uptake (−0.07 Mg CO2-eq. ha−1 yr−1 on average). However, tea plantations functioned as net GHG emissions for the first and second years (104.3 and 8.9 Mg CO2-eq. ha−1 yr−1, respectively) following their conversion from the forests primarily due to net C losses and N2O emissions. Subsequently, tea fields transformed into net GHG sinks (−0.29 to −0.06 Mg CO2-eq. ha−1 yr−1) from the third to fifth years following land-use conversion, primarily due to the NECB overweighing N2O emissions. On a 5-year average, converting Masson pine forests to tea plantations resulted in net C losses and net GHG emissions of 5.76 Mg C ha−1 yr−1 and 22.53 Mg CO2-eq. ha−1 yr−1, respectively. In conclusion, net C losses and net GHG emissions in the first two years following the conversion of Masson pine forests to tea plantations should be considered when assessing the environmental impacts on land-use conversion.

Microbial community dynamics behind major release of methane in constructed wetlands

It is global practice to construct wetlands in urban environments to treat pollutants from stormwater and overland runoff. However, constructed wetlands can also trigger climatic consequences by releasing a considerable amount of greenhouse gas emissions via diffusion and ebullition. While diffusive emissions are broadly measured, assessing the extent of ebullitive emissions is less common. This is due to the stochastic nature of ebullitive emissions and the complexity in their measurement. In addition, the soil microbiome community involved in greenhouse gas transport pathway has been generally understudied in Constructed wetlands although widely recognized in natural wetlands. Here we investigated diffusive and ebullitive CO2 and CH4 and ebullitive N2O emissions and the prokaryotic community from four municipal Surface Flow Constructed wetlands in southeast Australia over a 4-week period. We found that the total Global Warming Potential over a 100-year horizon of diffusive and ebullitive emissions was 0.44 kg CO2-equivalents m-2 day-1, which is 3-fold higher than the European estimates that accounted for diffusive emissions alone. Among the microbial functional groups that drove the main differences in β-diversity, putative methanotrophs represented the 3.6%, nitrogen-cycling microbes the 44.5% and sulfate/iron cycling microbes the 13.5%, indicating that several taxa, other than methanogens, can be key in microbial dynamics, nutrient turnover and in the shift in the production of greenhouse gas thus amplifying the role of constructed wetlands as sources of CH4. We present evidence that Constructed wetlands can be major sources of greenhouse gases, particularly ebullitive CH4 that accounted for more than two-thirds of the total emissions. As such, ebullitive flux rate measurements are critical to assemble national greenhouse gas budgets, particularly from Constructed wetlands.