Investigating Nitrous Oxide Pathways and Soil Carbon-Nitrogen Interactions Using Isotopic Techniques to Mitigate Greenhouse Gas Emission

Effective management of carbon (C) and nitrogen (N) in agricultural soils is crucial for mitigating greenhouse gas (GHG) emissions, particularly nitrous oxide (N₂O) and carbon dioxide (CO₂). This study investigates innovative dual C and N isotope-based methods to explore the mechanisms driving N₂O and CO₂ production and their potential mitigation, while maintaining soil fertility. By applying selectively labelled fertilizers with labelled in both fractions (¹⁵NH₄NO₃ or NH₄¹⁵NO₃) the microbial transformations of N in soil are traced, allowing for the identification of conditions that promote N₂O production or its reduction to the environmentally benign N gas (N₂).

The impact of different labile and recalcitrant C sources on N cycling and GHG emissions is investigated by applying ¹³C-labelled maize-derived plant litter and biochar. The interaction between labile-C (e.g., plant litter) and recalcitrant C (e.g., biochar) with N in soils plays a critical role in regulating microbial processes and, consequently, GHG emissions. Plant litter, as a labile C source, stimulates microbial activity, (i) enhancing N-cycling and potentially increasing N₂O emissions or, alternatively, (ii) stimulating microbial inorganic N immobilization thereby reducing N availability to gaseous and hydrological N loss processes. In contrast, recalcitrant C, such as biochar, provides a stable C form with long term C storage potential in soils. Biochar with its large specific surface area is recognized for its ability to sorb inorganic N such as ammonium and nitrate, reducing its availability for microbial processes that produce N₂O and thereby may mitigate soil N₂O emissions. However, how C inputs and N availability influence each other and affect microbial processes linked to GHG emissions remains poorly understood.

To address these challenges, a large-scale incubation study was initiated using soils sampled from a field experiment in Grabenegg, Austria, conducted by, The University of Natural Resources and Life Sciences, Vienna (BOKU) and Austrian Agency for Health and Food Safety, Vienna (AGES). One experimental soil was amended with NPK fertilizer, while the other received both NPK and hardwood- derived biochar since 2022. Soil samples were collected from the upper 10 cm of the root zone in October 2024 and used in a laboratory mesocosm experiment to trace litter-C and biochar-C processing and their effects on soil inorganic N cycling using ¹⁵N and ¹³C isotope tracing and isotope pool dilution measurements. Key measurements, including emissionsof ¹⁵N₂O, ¹⁵N₂, and ¹³CO₂, ¹³C tracing into particulate organic ¹³C, mineral-associated organic ¹³C, and microbial biomass ¹³C and, ¹⁵N tracing in, mineral-N (¹⁵NH₄, ¹⁵NO₃) and microbial ¹⁵N will be performed at various intervals over one month, and data evaluated using numerical modelling. Findings from this study will greatly contribute to optimizing climate-smart soil management practices aimed at reducing GHG emissions from soil while maintaining its fertility.