Constraining the denitrification process in conventional and regenerative agriculture

In this study, we aimed to constrain and characterize the dynamics of denitrification in three different fields: one conventional arable and two types of pasture (“leys”). During a one-year field campaign, denitrification was measured using our newly developed method combining the application of ¹⁵N tracer and artificial atmosphere for the incubation of soil cores under field conditions (Micucci, 2022), while total N₂O emissions were measured using static flux chambers during parallel incubations. Our objectives were to determine the best way to upscale soil core denitrification measurements and trace the fate of applied synthetic nitrogen fertilizer via denitrification in conventional agriculture in comparison to pastures under regenerative agriculture practices.

We determined that the best way to derive field-scale fluxes of denitrification was to use the core method to calculate the source partitioning coefficient (SPC) and product ratio (PR) and use these metrics in combination with static chamber data. The SPC is defined as the proportion of total N₂O emissions that originates from denitrification while the product ratio measures the proportion of denitrification product emitted as N₂O rather than N₂.

During the field campaign, we estimated that 22 kgN ha^-1 were lost via denitrification in the arable field, amongst which 15.17 were attributed to fertilizer application, representing around 8% of the 200 kgN ha^-1 applied. Furthermore, 9 % of the denitrified fertilizer was emitted as N₂O rather than N₂. On the other hand, the unfertilized ley emitted only 2.6 kgN ha^-1 via denitrification annually. Overall, the total N₂O emissions in the fertilizer arable field were responsible for around 2 t eqCO2 ha^-1 year^-1 compared to 0.15 in the unfertilized ley, highlighting the importance of land management in strategies of greenhouse gas emission reduction.