Soil nitrification rises under ozone and manure in 2-year wheat field trial, revealing hidden nitrogen risk

Ground-level ozone and partial replacement of synthetic nitrogen with manure both speed up soil nitrification under wheat, but on different timescales and through different mechanisms, according to a two-year open-air field experiment published in Plant and Soil. The result matters for nitrogen use efficiency because faster nitrification means quicker nitrate supply to the crop and a higher risk of nitrate leaching and nitrous oxide emissions.

The study, led by Bo Shang and Ziyi Guo with corresponding author Zhaozhong Feng of Nanjing University of Information Science and Technology and Chao Xiong of the University of Western Australia, used a Free-Air Ozone Concentration Enrichment (O3-FACE) platform. The system releases ozone into open plots through a pipe network, so the wheat grows under real wind, rain and soil conditions rather than in chambers.

How the soil nitrification trial was set up

On top of the ozone treatment, the researchers compared two fertilizer regimes: conventional inorganic fertilizer and a combined organic-inorganic treatment (COM) in which part of the synthetic nitrogen was replaced with manure.

The headline measurement was soil potential nitrification rate (PNR), a lab assay of how fast soil microbes can convert ammonium into nitrate. The team also tracked the two microbial groups that carry out the first, rate-limiting step of nitrification, ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), using qPCR of the amoA gene and 16S rRNA sequencing. Soil pH, ammonium, dissolved organic carbon and dissolved organic nitrogen were measured alongside.

Ozone lifted soil nitrification only in a high-ozone year

Elevated ozone significantly raised potential nitrification in the first year of the trial, which had higher ambient ozone, but not in the second. The first-year increase came with more AOA amoA gene copies, greater AOB diversity, and shifts in soil pH and ammonium.

The mechanism is indirect. Ozone reacts in the air and on leaves, and barely reaches the soil. It stresses the wheat canopy instead, changing photosynthesis, root exudation and the flow of carbon and nitrogen from plant to soil, and those rhizosphere changes cascade down to the ammonia oxidizers.

Manure substitution took a season to change nitrogen cycling

The combined organic-inorganic regime did nothing to nitrification in year one. In year two it raised PNR at every measured growth stage. The authors attribute the lag to the slow, cumulative effect of manure on soil chemistry: by the second season, dissolved organic carbon had risen and soil pH had shifted.

Notably, COM increased nitrification without changing the abundance or diversity of AOA or AOB. The existing microbial community became more active as its chemical environment improved, rather than being replaced.

No statistically significant ozone-by-fertilizer interaction appeared in the headline nitrification rate, which the authors attribute to timing: ozone dominated year one, fertilizer year two. At a finer scale, combined fertilization amplified ozone-driven changes in dissolved organic nitrogen and ammonium at grain filling in the first year.

What it means for nitrogen use efficiency and inhibitors

For agronomists and input suppliers, three practical points stand out:

• Single-season trials can mislead. A low-ozone year could suggest ozone has no effect on rhizosphere nitrogen, while a high-ozone year could overstate how consistent the effect is.
• Manure benefits are cumulative. Short trials may underestimate how organic amendments change nitrification, nitrate leaching and nitrous oxide emissions over time.
• Inhibitor strategies may need adjusting. Because COM boosted activity without restructuring the ammonia-oxidizer community, nitrification inhibitors in manured soils may need to target microbial activity rather than community composition.

The study adds to a growing body of evidence that environmental stress reshapes how much applied nitrogen reaches the crop. Earlier this year, a Nature Food analysis found that nitrogen losses from saline soils run 61% above normal.

Ozone is one of the most damaging air pollutants for staple crops in East Asia, and governments in the region are actively promoting partial substitution of synthetic fertilizer with manure. Both pressures now act on the same soil, and the authors argue that nitrogen models and fertilizer recommendations for wheat will need to account for year-to-year swings in air quality as well as the slow build-up of organic matter.

Source: Bioengineer.org