Denitrification is the major nitrous acid production pathway in boreal agricultural soils

Abstract

Nitrous acid (HONO) photolysis produces hydroxyl radicals—a key atmospheric oxidant. Soils are strong HONO emitters, yet HONO production pathways in soils and their relative contributions are poorly constrained. Here, we conduct 15N tracer experiments and isotope pool dilution assays on two types of agricultural soils in Finland to determine HONO emission fluxes and pathways. We show that microbial processes are more important than abiotic processes for HONO emissions. Microbial nitrate reduction (denitrification) considerably exceeded ammonium oxidation as a source of nitrite—a central nitrogen pool connected with HONO emissions. Denitrification contributed 97% and 62% of total HONO fluxes in low and high organic matter soil, respectively. Microbial ammonium oxidation only produced HONO in high organic matter soil (10%). Our findings indicate that microbial nitrate reduction is an important HONO production pathway in aerobic soils, suggesting that terrestrial ecosystems favouring it could be HONO emission hotspots, thereby influencing atmospheric chemistry.

Emissions of atmospherically reactive gases nitrous acid and nitric oxide from Arctic permafrost peatlands

“…The low HONO and NO emissions in the vegetated surfaces (⩽0.2 µg N m −2 h −1 ; figure 3(a), table S1) were associated with lower mineral N content than in the unvegetated surfaces (F(1,37) = 5.47, P = 0.0248). The HONO emissions measured from permafrost peatlands are smaller than the global median (18 µg N m −2 h −1 ) (Bhattarai et al 2021), but within the range reported for boreal peatlands drained for agriculture (1.7-6 µg HONO-N µg m −2 h −1 ) (Maljanen et al 2013). Notably, the mean NO emission from the unvegetated peat surfaces of the highest emitting Seida site (59.3 µg N m −2 h −1 ; table S1) is almost two-fold the global mean NO emission (30.8 µg N m −2 h −1 ) (Davidson and Kingerlee 1997) and more than two-fold the mean N 2 O emission (24.8 µg N m −2 h −1 ) from peatlands across the permafrost region (Voigt et al 2020), highlighting the importance of this newly revealed N gas emissions from Arctic permafrost peatlands.…”

“…humic acid) into gaseous NO and N2O in the process called chemo-denitrification (green arrow) (Coby and Picardal 2005). Nitrite can also react chemically with proton (H + ) to form HONO and NO (Cleemput and Samater 1995, Su et al 2011, Bhattarai et al 2021. The italic terms above the colored arrows indicating nitrogen transforming processes denote the genes encoding enzymes carrying out the respective processes.…”

“…Numerous studies have reported substantial HONO and NO emissions from agricultural soils (Davidson and Kingerlee 1997, Stehfest and Bouwman 2006, Su et al 2011, Maljanen et al 2013, Oswald et al 2013, Scharko et al 2015, Wu et al 2019, Bhattarai et al 2021, where fertilization and other management practices lead to the high availability of mineral N. The global emissions of NO x (NO + NO 2 ) from agricultural soils (3.7 Tg N yr −1 ) are dominated by NO emissions (Ciais et al 2013). Agricultural soils are also globally significant sources of the potent greenhouse gas N 2 O (3.8 Tg N yr −1 ) (Tian et al 2020), suggesting that the production mechanisms of HONO, NO, and N 2 O are interlinked and related to high N turnover rates in soil (Maljanen et al 2013, 2019, Wu et al 2019.…”

“…Agricultural soils are also globally significant sources of the potent greenhouse gas N 2 O (3.8 Tg N yr −1 ) (Tian et al 2020), suggesting that the production mechanisms of HONO, NO, and N 2 O are interlinked and related to high N turnover rates in soil (Maljanen et al 2013, 2019, Wu et al 2019. Indeed, studies have clearly shown the role of microbial N-transformation pathways, most importantly nitrification and denitrification, behind N 2 O, HONO, and NO production (Oswald et al 2013, Scharko et al 2015, Ermel et al 2018, Wu et al 2019, Bhattarai et al 2021, Han et al 2021 (figure 1). In addition to reports from agricultural soils, a few studies have reported HONO and NO emissions from soils under natural vegetation, such as biocrust (Weber et al 2015) and forest soils (Mushinski et al 2019).…”

“…In addition to reports from agricultural soils, a few studies have reported HONO and NO emissions from soils under natural vegetation, such as biocrust (Weber et al 2015) and forest soils (Mushinski et al 2019). However, pristine Arctic soils have so far remained entirely unexplored concerning HONO (Bhattarai et al 2021) and NO (Davidson and Kingerlee 1997) emissions, despite their well-documented capability to emit N 2 O and extensive soil N stocks, which are becoming increasingly vulnerable to mobilization (Voigt et al 2020) as a consequence of warming and permafrost thaw (Meredith et al 2019).…”