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

Hem Raj Bhattarai,Wolfgang Wanek,Maarit Liimatainen,Jussi G Ronkainen,Hannu Nykänen,Christina Biasi,Marja Maljanen,Yuntao Hu,Henri M P Siljanen

doi:10.1038/s43247-021-00125-7

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.

Highlights

Nitrous acid (HONO) photolysis produces hydroxyl radicals—a key atmospheric oxidant
We examined the 15N atom percent excess (APE) of emitted HONO (HO15NO APE) and of soil NO2− (15NO2− APE) in live and sterile soil samples from time zero to 240 minutes, after the addition of labeled 15NO2− (LNi) and unlabeled NO2− (Fig. 1)
Both soils showed a steady decrease in HO15NO APE under live conditions, with the high organic matter (HOM) soils exhibiting a stronger decrease than the low organic matter (LOM) soils

Summary

Introduction

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. Only one systematic study has addressed the pathways of soil HONO formation using a tracer approach (labeled ammonium (15NH4+)), highlighting the sole importance of NH3 oxidizers in HONO production and excluding the role of NO3− reduction in aerobic soils[11]. We performed a series of 15N tracer experiments to understand HONO production pathways in two distinct boreal agricultural soils in Finland; one with a low organic matter (LOM) content and the other with a high organic matter (HOM) content (Supplementary Table 1). To determine the contribution of the two main biotic HONO production pathways (microbial NH3 oxidation and NO3− reduction), we performed separate experiments using 15N-labeled ammonium (NH4+, LA) and NO3− (LNa) in live (non-sterilized) soils. Gross NO2− production rates across soil pH levels, and the contribution of nitrification and denitrification to the soil NO2− pool were compared with published data

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