From Fertilizer to Flux: Investigating N2O Emissions in Compacted Cultivation Systems

Agricultural soils are a significant source of nitrous oxide (N₂O) emissions, primarily driven by denitrification and nitrification pathways. Recently, emissions appeared to be strictly related to soil structure characteristics, which may also play a substantial role in the emission pathways. Among these characteristics, the extent to which soil compaction impacts N₂O emissions is still debated. To investigate this, a three-year lysimeter experiment was conducted to assess N₂O emissions under five cultivation systems with four replicates each: bare soil (BS), conventional (CV), conventional + cover crop (CC), conservation with shallow soil compaction (0-25 cm, CA1), and conservation with deep soil compaction (25-45 cm, CA2). Maize and grain sorghum were grown as main crops, fertilized using solid digestate (300 kg N ha^-1). Continuous automatic measurements of N₂O emissions were collected using a non-steady state through-flow chamber system and an FTIR gas analyzer, capturing up to seven flux measurements for each chamber per day. Daily emissions were split into four periods per year. The relative importance of nitrification and denitrification to the flux of N₂O was hinted at by concurrently measuring NOx emissions and the water-filled pore space (WFPS) and soil temperature measured in the 0-30 cm profile. Additionally, 280 soil samples per year were collected in the 30-days post-fertilization from 0-5 cm and 5-15 cm depths for pH analysis and monitoring ammonia and nitrate pool dynamics. A mixed-effects model was used to test sub-daily emissions. The most pronounced N₂O emissions were observed during the initial two weeks following fertilization, with maximum observed emissions highest in CC (208 g ha^-1 d^-1) and lowest in CA2 (53 g ha^-1 d^-1) for 2023. Notably, CA2 consistently exhibited lower cumulative N₂O emissions, suggesting a complex interaction between management practices and soil conditions. These findings highlight the importance of soil structure and cultivation system in managing N₂O emissions.