The nitrogen isotope research team of Tianjin University collected and analyzed the global observation data on nitrogen isotopes of NO3-in atmospheric particulates.
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 NOxemissions dominated by coal and oil combustion. Recently, non-fossil fuel NOxemissions from biomass burning and microbial N cycle have been recognized as important sources of atmospheric NOx. However, due to the incomplete or missing NOxemissions from the microbial N cycle in the land and ocean, there is great uncertainty in global NOxemissions. To accurately constrain global NOxemissions is pivotal to mitigate NOxemissions, budget nitrate (NO3-) deposition fluxes, and evaluate the eco-environmental and climatic effects of atmospheric NOxloading.
In the land environment, there have been observations and simulations of NOxemissions from microbial N cycle in natural and agricultural soils. However, it remains challenging to observe NOxemissions 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 NOxemissions from seawater and thus lack of an estimate on the ocean microbial NOxemission. Previously, the oil combustion of marine traffic transportation has been considered as the dominant source of ocean NOxemissions. 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 NOxemissions 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 and constrained the nitrogen isotope signals of particulate NO3-that were purely derived from ocean NOxemissions by using its ocean-land differences. Furthermore, they constructed a new N isotope method to quantify the relative contributions of major NOxemission sources by constraining N isotope effects of atmospheric NOxtransformations to particulate NO3-and combining the N isotope ratios of NOxfrom dominant emission sources, including coal combustion, oil combustion, biomass burning, and microbial N cycle. Then, combining the known fossil fuel NOxemissions, they accomplished estimates on NOxemissions from microbial N cycle in the land and ocean, respectively (Fig. 1).
Fig. 1Emission fluxes from major NOxsources in the atmosphere and NOytransportation and deposition fluxes
Their results show that NOxemissions from the microbial N cycle account for about 24%, 58%, and 31% of the total NOxemissions in the land, ocean, and globe, equivalent to 0.5, 1.4, and 0.6 times of the corresponding fossil fuel NOxemissions. This study fills the data gap of NOxemissions from microbial N cycle in the ocean and updates fluxes of NOxemissions from microbial N cycle in the land and globe. They confirm the significant contribution of microbial N cycle to global NOxemissions. It should be considered into current and future atmospheric NOxemission 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.
By School of Earth System Science
Editor: Sun Xiaofang
