Unraveling N2O production pathways in agricultural and forest soils using stable isotope analysis

Nitrous oxide (N₂O) is a potent greenhouse gas and a significant contributor to global warming and ozone layer depletion. It is primarily emitted from soils through microbial processes such as nitrification and denitrification and shows spatial and temporal variations driven by environmental factors such as the availability of nitrogen (e.g., in the form of fertilizers), organic carbon, soil moisture, temperature and oxygen levels. However, estimates on the relative contribution of different N₂O producing pathways are frequently uncertain and knowledge on how environmental factors influence N₂O emissions dynamics is still limited. Therefore, closing these knowledge gaps is crucial for improving mitigation strategies.

This study aims to analyze the patterns of N₂O emissions across diverse forest and agricultural soils, taking geographic variations into account, and to determine the relative contributions of the primary N₂O producing and consuming pathways specific to each soil type.

Batch experiments were conducted using four agricultural soils and four forest soils from sites of the ICOS (https://www.icos-cp.eu) and FLUXNET (https://fluxnet.org/about/) networks. Agricultural soils were obtained in France, Belgium, Italy and Switzerland, while forest soils were obtained in Finland, Sweden, Belgium and Italy. These soils exhibited a range of intrinsic characteristics, such as texture, organic matter content and type, and nitrogen sources. The incubations took place in complete darkness at a constant temperature of 22 ºC for approximately 30 hours after rewetting dry soil. Each soil type was tested with five replicates across five time points (i.e., 25 reactors for soil type). For each reactor we measured the production of N₂O and its isotopic composition including the δ¹⁵N-N₂O_bulk, δ¹⁸O-N₂O_bulk, and site preference δ¹⁵N-N₂O_SP (i.e., the intramolecular distribution of N isotopes, since the N₂O molecule has an asymmetric linear structure [N-N-O]). Additionally, the isotopic compositions of nitrate and ammonium from soil KCl extracts are being analyzed (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ¹⁵N-NH₄⁺) and microbiological characterization is also being performed.

Preliminary results revealed significantly higher N₂O production in agricultural soils compared to forest soils during the 30-hour incubation period, with rates reaching up to 130 μg N-N₂O/kg/h in agricultural soils and only 0.3 μg N-N₂O/kg/h in forest soils. Notable differences were also observed among the four tested soils within each category (agricultural or forest). These differences might be mainly attributed to differences in the nitrogen and organic carbon content as well as the texture. The isotopic analysis of N₂O suggests that denitrification is the primary process driving N₂O emissions in the studied soils, with nitrification also contributing to varying extents depending on the soil type.

Ongoing isotopic analyses of nitrate and ammonium in soil KCl extracts alongside microbial characterization, will provide deeper insights into the dominant processes driving N₂O emissions in each soil type and the key environmental factors influencing them.