Unveiling the greenhouse gas mitigation potential for Danish farmers: the SmartField project

The successful implementation of greenhouse gas mitigation measures requires the accurate quantification of emission fluxes in space and time. The SmartfField project comprises a unique infrastructure to measure complete N and C balances in combination with year-round measurements of N₂O and other environmentally important GHG and trace gases (NH₃, NO/NO₂, O₃, CO₂, CH₄). The measurement infrastructure comprises chamber and micrometeorological measurements on two experimental sites (Supersite A and B) on plot and field scale for typical Danish crop rotations, and a mobile eddy-flux and chamber system to deploy elsewhere. The overall aim is to identify greenhouse gas mitigation options which (1) can be easily integrated into existing crop rotations (2) avoid pollution swapping (nitrate leaching and ammonia emissions), (3) do not compromise crop yields, and (4) can be scaled from plot to field. In this framework, the emission factors for different amendments will be calculated, including synthetic and biological nitrification inhibitors, biochar, and rock flour, and their combined effects. The experiments will commence in 2025, and baseline measurements started in March 2024. This included soil mapping using electromagnetic induction (DUALEM-21H) and gamma ray (Medusa 2000) sensors, sensor-guided soil sampling, and soil profile descriptions. The N₂O flux was measured using the LI-7820 trace gas analyser and survey chamber on field scale, and continuously with automated chambers inside and outside tractor tracks (n=5). The crop was spring barley (Hordeum vulgare), undersown with grass-clover (Lollium perenne, Trifolium pratense, Trifolium repens). These initial measurements revealed large spatial and temporal variations of soil parameters and greenhouse gas emissions. The EC_a varied from 1.8-14.8 mS m^-1, indicating substantial soil textural changes. The depth of the A-horizon varied between 22-30 cm, and average topsoil bulk density was higher in the tractor tracks (1.51 vs. 1.36 g cm^-3). The N₂O flux varied substantially within the field with a daily CV of 51%-138%. The mean daily N₂O flux outside the tractor tracks was 64.3 µg N₂O-N m^-2 d^-1, while it was 157.3 µg N₂O-N m^-2 d^-1 inside the tracks. There is a need to account for the observed spatiotemporal variation to correctly assess mitigation measures.