Assessing Nitrous Oxide Emissions from Agricultural Soil: A Comparison of Two Grass-clover Proportions

The inclusion of grass-clover (GC) leys in crop rotations on dairy farms may contribute to climate change mitigation by facilitating carbon sequestration in soils. A long-term experiment in Denmark found that soil organic carbon (SOC) and soil total nitrogen (STN) increased with the proportion of GC (i.e., 2 and 4 years of GC in a six-year rotation) from 2006 to 2020. However, the incorporation of GC residues may potentially increase nitrous oxide (N₂O) emissions due to the increased SOC and STN stocks. Yet, limited information exists regarding N₂O emissions with different proportions of GC. We hypothesized that N₂O emission will increase with more GC years in the rotation. This study aimed to quantify the emissions of N₂O in two long-term crop rotations with different proportions of GC years (1/3 or 2/3), and all crops present each year. A one-year experiment is currently conducted including the rotation year preceding, and the year following GC cultivation where spring barley is cultivated, in both crop rotations (n=2, total: 8 plots). Emissions of N₂O were quantified starting from April 2023 (day of year, DOY 111). Surface N₂O fluxes were measured with the LI-7820 N₂O/H₂O trace gas analyzer connected to the 8200-01S Smart Chamber (LI-COR Biosciences, Lincoln, NE, USA). Linear mixed models were used to analyze N₂O with crop rotation (1/3 or 2/3 GC) and rotation year (pre- or post-GC) as fixed effects and sampling date and block as random effects. Preliminary results showed elevated N₂O fluxes (up to 443 ug N₂O-N m^-2 h^-1) with a longer high-flux period in post-GC rotation years (DOY 111-151), than pre-GC years (DOY 111-139). The highest cumulative emission was 420 mg N₂O-N m^-2 in the post-GC year of DOY 320 in 1/3 GC. For pre-GC in 1/3 GC, pre-GC and post-GC in 2/3 GC, emissions were 249, 341 and 252 mg N₂O-N m^-2, respectively. For both N₂O fluxes and cumulative emissions, the 1/3 GC rotation was significantly higher (p<0.01) than the 2/3 rotation. In addition, the N₂O fluxes and cumulative emissions in the post-GC year were significantly (p<0.01) higher than the pre-GC year in the 1/3 GC crop rotation, while the years pre- and post-GC showed no difference in rotation with 2/3 GC. In contrast to our initial hypothesis, the preliminary results did not show higher N₂O emissions with increased GC years. This currently suggests that the 2/3 GC inclusion in crop rotations has a greater potential for climate mitigation as compared to 1/3 GC. Further investigations will focus on the drivers of N₂O emissions and the climate mitigation potential, considering both C sequestration in soil and N₂O emissions in the long term.