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Farmed soils contribute substantially to global warming by emitting N 2 O (ref.  1 ), and mitigation has proved difficult 2 . Several microbial nitrogen transformations produce N 2 O, but the only biological sink for N 2 O is the enzyme NosZ, catalysing the reduction of N 2 O to N 2 (ref.  3 ). Although strengthening the NosZ activity in soils would reduce N 2 O emissions, such bioengineering of the soil microbiota is considered challenging 4,5 . However, we have developed a technology to achieve this, using organic waste as a substrate and vector for N 2 O-respiring bacteria selected for their capacity to thrive in soil 6-8 . Here we have analysed the biokinetics of N 2 O reduction by our most promising N 2 O-respiring bacterium, Cloacibacterium sp. CB-01, its survival in soil and its effect on N 2 O emissions in field experiments. Fertilization with waste from biogas production, in which CB-01 had grown aerobically to about 6 × 10 9 cells per millilitre, reduced N 2 O emissions by 50-95%, depending on soil type. The strong and long-lasting effect of CB-01 is ascribed to its tenacity in soil, rather than its biokinetic parameters, which were inferior to those of other strains of N 2 O-respiring bacteria. Scaling our data up to the European level, we find that national anthropogenic N 2 O emissions could be reduced by 5-20%, and more if including other organic wastes. This opens an avenue for cost-effective reduction of N 2 O emissions for which other mitigation options are lacking at present., (© 2024. The Author(s).)

We synthesized N2O emissions over North America using 17 bottom‐up (BU) estimates from 1980–2016 and five top‐down (TD) estimates from 1998 to 2016. The BU‐based total emission shows a slight increase owing to U.S. agriculture, while no consistent trend is shown in TD estimates. During 2007–2016, North American N2O emissions are estimated at 1.7 (1.0–3.0) Tg N yr−1 (BU) and 1.3 (0.9–1.5) Tg N yr−1 (TD). Anthropogenic emissions were twice as large as natural fluxes from soil and water. Direct agricultural and industrial activities accounted for 68% of total anthropogenic emissions, 71% of which was contributed by the U.S. Our estimates of U.S. agricultural emissions are comparable to the EPA greenhouse gas (GHG) inventory, which includes estimates from IPCC tier 1 (emission factor) and tier 3 (process‐based modeling) approaches. Conversely, our estimated agricultural emissions for Canada and Mexico are twice as large as the respective national GHG inventories. Plain Language Summary: Nitrous oxide (N2O) is the third most important greenhouse gase (GHG) after CO2 and CH4 causing global warming. Among world regions, North America (defined herein as U.S., Canada, and Mexico) is the second largest source of N2O emissions globally, and previous source estimates for this region vary widely. This study aims to provide a comprehensive N2O assessment over North America including all available estimates based on a number of approaches. We report total emissions, and emissions from four anthropogenic source sectors, over the past four decades. Agriculture and industry are two major N2O sources in North America. Our results show a minor increase in the total N2O emission due to agricultural trends in the U.S. Our bottom‐up estimate of U.S. agricultural N2O emissions are close to those in the EPA national GHG inventory that includes both empirical and model results. The high consistency suggests the need to take process‐based modeling results into account for future national GHG inventories. Key Points: North American N2O emissions during 2007–2016 are estimated at 0.9–3.0 Tg N yr−1 using a combination of bottom‐up and top‐down approachesNorth American anthropogenic N2O emissions grew by ∼0.2 Tg N during 1980–2016; U.S. agriculture was the largest cause of that growthOur modeled N2O fluxes reflect an IPCC tier 3 approach, and can improve greenhouse gase inventories that largely use tier 1 and tier 2 approaches [ABSTRACT FROM AUTHOR]

The relationship, on a global basis, between the amount of N fixed by chemical, biological or atmospheric processes entering the terrestrial biosphere, and the total emission of nitrous oxide (N2O), has been re-examined, using known global atmospheric removal rates and concentration growth of N2O as a proxy for overall emissions. For both the pre-industrial period and in recent times, after taking into account the large-scale changes in synthetic N fertiliser production, we find an overall conversion factor of 3-5% from newly fixed N to N2O-N. We assume the same factor to be valid for biofuel production systems. It is covered only in part by the default conversion factor for "direct" emissions from agricultural crop lands (1%) estimated by IPCC (2006), and the default factors for the "indirect" emissions (following volatilization/deposition and leaching/runoff of N: 0.35-0.45%) cited therein. However, as we show in the paper, when additional emissions included in the IPCC methodology, e.g. those from livestock production, are included, the total may not be inconsistent with that given by our "topdown" method. When the extra N2O emission from biofuel production is calculated in "CO2-equivalent" global warming terms, and compared with the quasi-cooling effect of "saving" emissions of fossil fuel derived CO2, the outcome is that the production of commonly used biofuels, such as biodiesel from rapeseed and bioethanol from corn (maize), depending on N fertilizer uptake efficiency by the plants, can contribute as much or more to global warming by N2O emissions than cooling by fossil fuel savings. Crops with less N demand, such as grasses and woody coppice species, have more favourable climate impacts. This analysis only considers the conversion of biomass to biofuel. It does not take into account the use of fossil fuel on the farms and for fertilizer and pesticide production, but it also neglects the production of useful co-products. Both factors partially compensate each other. This needs to be analyzed in a full life cycle assessment. [ABSTRACT FROM AUTHOR]

We discuss the likely impact of agricultural fertiliser N use on the global nitrous oxide (N2O) budget, as described by Crutzen et al. (2008). We then project the impact of full N2O accounting in biofuel net Greenhouse Gas (GHG) emissions using the life-cycle analyses from Farrell et al. (2006) (ERG Biofuels Analysis Meta-Model (EBAMM)), Liska et al. (2008) (Biofuel Energy Systems Simulator (BESS)) for corn-based ethanol production and Smith et al. (2006) (Bioethanol GHG Calculator (BGGC)) for wheat-based ethanol. USA biofuel production, even if nominally fulfilling the requirements, may trigger a net increase in global warming. [ABSTRACT FROM AUTHOR]