Photocatalysis in agricultural soils: Mineralogy and soil properties control the fixation and emission of NOx and other trace gases of N

Trace gases of nitrogen (N), such as NO_x (nitric oxide, NO + nitrogen dioxide, NO₂) have a negative impact on human health and the environment. Although NO_x are naturally produced in volcanic eruptions, forest fires and biotic nitrification and denitrification in soils, human activity is a major source of these contaminants via e.g. the combustion of fossil fuels. Additionally, N fertilization in agricultural soils is also an important source of NO_x emissions. These emissions involve a loss of soil N to the atmosphere and have a negative impact in air quality. The abiotic part of the N cycle in terrestrial ecosystems has not received as much attention as the biotic part and certain abiotic reactions could play a key role in regulating NO_x emissions. Photocatalysis is an example as this is used to abate NO_x gases in urban and industrial areas. This reaction requires the presence of a catalyst (e.g. titanium oxide), oxygen, water, and energy from the sun (UV-visible light) to transform NO from the atmosphere into innocuous inorganic N forms (mainly nitrate, NO₃^-). There is a continuous investment in the production of catalysts by the industry. However, a variety of soil minerals such as anatase or rutile (titanium oxides), hematite and goethite (iron oxides), are found in soils and they could act as catalysts; however, the occurrence of photocatalysis in soils has not been evaluated so far. In this study, we assess (i) the potential of a selection of soils with different mineralogy and a wide variety of soil properties to fix or emit NO_x through photocatalysis, and (ii) the possible alterations in the fixation or emission of other N gases from the soil, i.e., nitrous oxide (N₂O) and ammonia (NH₃), when photocatalysis is induced. Around thirty agricultural soils were selected to meet the first objective and irradiated for 1 hour with UV-visible light under a constant flux of air and NO (100 ppm). Similar experiments were carried out with a selection of soils, whose potential to fix NO was different and tested in the previous experiment, to satisfy the second objective. However, only air (without NO) was pumped within the soil chamber in this case and the soils were previously fertilized with different N fertilisers (urea or KNO₃^-) and rates (0 to 250 mg N kg^-1 soil). Our experiments show that weathered soils (with a high content in titanium and iron oxides) were able to fix more atmospheric NO through photocatalysis (objective i), and that NO and NH₃ fixation and emissions after N fertilization depended not only on the N fertilizer and rate but also on soil properties, mainly soil pH and N content (objective ii). Soil mineralogy and properties play a key role in soil photocatalysis, and this abiotic reaction should be considered in order to design more sustainable strategies for agriculture.