Soil microbial nitrous acid (HONO) production is being increasingly recognized as a significant source of atmospheric reactive nitrogen, which affects air quality and climate. Recent research published in Nature Communications reveals alarming trends connecting climate change and fertilizer usage to rising soil HONO emissions. Over the past four decades, global soil HONO emissions have surged from 9.4 teragrams (Tg) of nitrogen (N) in 1980 to 11.5 Tg N by 2016, signaling the need for urgent action to address air quality and vegetation impacts.
The study, which utilized advanced chemistry-climate model simulations, illustrated the ramifications of increased soil HONO emissions on global surface ozone (O3) levels and vegetation. It was found these emissions increased global surface O3 mixing ratios by approximately 2.5% on average, with spikes reaching up to 29% in some regions, coinciding with areas of lower anthropogenic emissions. The results indicate heightened risks to vegetation due to elevated O3 levels, particularly from 1980 through 2016.
"This finding highlights the urgent need for managing soil HONO emissions to mitigate their adverse impacts on air quality and ecosystems," wrote the authors of the article. They recommend incorporating soil HONO emissions assessments within strategies aimed at curtailing global air pollution.
HONO emissions are estimated to contribute between 17% and 80% of atmospheric HONO mixing ratios. Without considering the canopy reduction factor (CRF), which accounts for the impact of vegetation on gas emissions, total global soil emissions of HONO were estimated at 13.4 Tg N yr−1 for 2016. Applying the CRF decreased this figure to 11.5 Tg N yr−1.
The geographic distribution of soil HONO emissions showed significant concentration within low-latitude regions, with approximately 69% attributed to areas between 30°S and 30°N and 28% occurring poleward of 30°N. Asia alone accounted for 37.2% of global emissions, with India contributing 1.74 Tg N yr−1 and China 0.55 Tg N yr−1.
Throughout the period from 1980 to 2016, global soil HONO emissions have consistently increased at a rate of 62.9 Gg N yr−1 (0.7% per year), reflecting significant changes driven by trends in agricultural fertilizer usage and climate-induced shifts in soil conditions. The study employed the Community Atmosphere Model with Chemistry (CAM-Chem) to model and quantify these increasing impacts on air quality.
Importantly, the influence of soil HONO emissions on the atmosphere did not remain localized but has far-reaching effects, propagaging globally by altering atmospheric mixing ratios. For example, the emission of soil HONO was responsible for increases in O3 levels across regions with previously less pollution—specifically, the southern hemisphere—where increases reached 15%, 9%, and 8% across Australia, Africa, and South America respectively.
Seasonal analyses illustrated HONO emissions peaked during summer months, aligning with agricultural fertilization practices and higher soil temperatures. It was during these periods of intensified plant growth and fertilizer application where soil emissions highlighted their role as significant contributors to elevated OH concentrations, directly affecting atmospheric conditions.
The study also assessed vegetation exposure to ozone using the AOT40 metric—an index reflecting the accumulated O3 exposure exceeding 40 parts per billion (ppbv). The findings demonstrated increased O3 exposure resulting from soil HONO emissions, leading to higher exceedance rates particularly pronounced in regions like India, China, and North America, all of which are noted for their high grain production and population.
With global surface temperatures having risen by 1.09 °C since industrialization and projected to increase by 1.4–4.4 °C by the century's end, the resulting interactions between climate change and fertilizer-induced emissions forecast worsening air quality. The continuing rise of soil HONO emissions underlines the pressing necessity for the agricultural sector to implement more efficient fertilization practices. This transition is necessary not only to meet the growing global food demands but also to alleviate environmental impacts such as air pollution.
Given the substantial potential of soil emissions to adversely impact air quality, vegetation health, and food security, this research advocates for innovative agricultural management strategies, such as careful timing of fertilizer applications and enhanced nitrogen use efficiency. Through such practices, it is possible to aim for sustainable agricultural productivity without compromising environmental integrity.