Seasonal and Temporal Variability and drivers of Nitrous Oxide Emissions from Northern Agriculture Soil

Agricultural soils are the largest anthropogenic source of nitrous oxide (N₂O), a potent greenhouse gas and an ozone-depleting substance. We investigated the seasonal and diurnal variability of N₂O fluxes and their controlling factors in an agricultural ecosystem at the SMEAR-Agri Viikki site (Helsinki, Finland) over three years. The field was cultivated with timothy (Phleum pratense) in 2022 and was renewed in spring 2023 with barley (Hordeum vulgare) undersown with red clover (Trifolium pratense) and grasses; in 2024 the site was managed for silage production. N₂O emissions in 2022 showed no consistent seasonal pattern but a high early-summer emission peak, whereas 2023 and 2024 were characterised by multiple emission events with smaller magnitudes. Minimum fluxes occurred in autumn 2022 (0.003 µg m⁻² s⁻¹), winter 2023 (0.008 µg m⁻² s⁻¹) and summer 2024 (0.006 µg m⁻² s⁻¹). The highest fluxes were observed in 2022 summer (0.089 µg m⁻² s⁻¹) and spring (0.088 µg m⁻² s⁻¹), while peak emissions in 2023 (0.028 µg m⁻² s⁻¹) and 2024 (0.032 µg m⁻² s⁻¹) occurred during autumn. Our results highlight strong interannual variability in both the timing and magnitude of N₂O emissions, likely linked to changes in crop N utilization and management, soil conditions and meteorological drivers. Seasonal variations in N₂O emissions during spring, summer and autumn were primarily driven by soil and meteorological factors, including air and soil temperature, soil moisture, water-filled pore space, electrical conductivity and redox potential. During winter, however, N₂O fluxes showed little association with these variables, suggesting a shift in controlling processes under cold conditions. Overall, the findings reveal substantial seasonal and interannual complexity in N₂O dynamics and underscore the importance of integrating soil conditions, management practices and seasonal context when assessing and mitigating N₂O emissions from managed agricultural systems.

Keywords: Nitrous oxide flux, greenhouse gas, seasonal variability, agricultural ecosystems, soil moisture, temperature.