A better understanding of the surface-earth nitrogen (N) cycle is crucial in the Anthropocene because it strongly interacts with environmental quality, food production, biosphere and climate changes. Over the past century, atmospheric N loading has become a major driver of air pollution, ozone-layer destruction, elevated N deposition, and associated negative impacts on ecosystem structure and functions (e.g., biodiversity, acidification, eutrophication, and carbon balance). Nitrogen oxides (NOx) are major components of reactive N pollutants. Its concentrations and deposition fluxes have been remarkably elevated since the industrial revolution, which has been attributed to fossil fuel NOx emissions dominated by coal and oil combustion. Recently, non-fossil fuel NOx emissions from biomass burning and microbial N cycle have been recognized as important sources of atmospheric NOx. However, due to the incomplete or missing NOx emissions from the microbial N cycle in the land and ocean, there is great uncertainty in global NOx emissions. To accurately constrain global NOx emissions is pivotal to mitigate NOx emissions, budget nitrate (NO3-) deposition fluxes, and evaluate the eco-environmental and climatic effects of atmospheric NOx loading.
In the land environment, there have been observations and simulations of NOx emissions from microbial N cycle in natural and agricultural soils. However, it remains challenging to observe NOx emissions accurately and comprehensively from microbial N cycle from other substrates (e.g., the surface water of rivers, lakes, swamps, etc.) and emission sources (e.g., wastewater, water treatment systems, solid wastes, etc.). In the ocean environment, there are very sporadic observations of NOx emissions from seawater and thus lack of an estimate on the ocean microbial NOx emission. Previously, the oil combustion of marine traffic transportation has been considered as the dominant source of ocean NOx emissions. Stable isotope methods have been successfully used to trace global water and many other biogeochemical cycles. It is necessary to explore new N isotope methods to comprehensively constrain ocean and land NOx emissions from microbial N cycle.
Based on the above issues and background, the nitrogen isotope research team of Tianjin University collected and analyzed the global observation data on nitrogen isotopes of NO3- in atmospheric particulates. By using its ocean-land differences, they constrained the nitrogen isotope signals of particulate NO3- that were purely derived from ocean NOx emissions. Furthermore, they constructed a new N isotope method to quantify the relative contributions of major NOx emission sources by constraining N isotope effects of atmospheric NOx transformations to particulate NO3- and combining the N isotope ratios of NOx from dominant emission sources, including coal combustion, oil combustion, biomass burning, and microbial N cycle. Then, combining the known fossil fuel NOx emissions, they accomplished estimates on NOx emissions from microbial N cycle in the land and ocean, respectively (Fig. 1).
Fig. 1 Emission fluxes from major NOx sources in the atmosphere and NOy transportation and deposition fluxes
Their results show that NOx emissions from the microbial N cycle account for about 24%, 58%, and 31% of the total NOx emissions in the land, ocean, and globe, equivalent to 0.5, 1.4, and 0.6 times of the corresponding fossil fuel NOx emissions. This study fills the data gap of NOx emissions from microbial N cycle in the ocean and updates fluxes of NOx emissions from microbial N cycle in the land and globe. They confirm the significant contribution of microbial N cycle to global NOx emissions. It should be considered into current and future atmospheric NOx emission reduction policy formulation and eco-environmental and climatic effects assessment.
This study was supported by National Natural Science Foundation of China (Nos. 42125301, 41730855, 42073005) and the Coordinated Research Project of IAEA (F32008).
Full article: Wei Song, Xue-Yan Liu, Benjamin Z Houlton, Cong-Qiang Liu. (2022) Isotopic constraints confirm the significant role of microbial nitrogen oxides emissions from the land and ocean environment. National Science Review, nwac106, https://doi.org/10.1093/nsr/nwac106.
