Nitrous oxide flux dynamics on agricultural grasslands in Western Finland

Agricultural grasslands may act as carbon sinks, counteracting emissions caused by agricultural practices. Contrastingly, grasslands may also act as sources of Nitrous oxide (N₂O). For example, in Finland, N₂O emissions from grasslands total up to 30 % of the climate impact of milk production. The N₂O emissions from grasslands are induced by the use of nitrogen fertilizers, crop management and various environmental factors. However, the dynamics of N₂O production are complex and not yet fully understood. Successful mitigation of N₂O fluxes from agricultural soils calls for better understanding of these dynamics and the finding of practical solutions for farmers.

We measured greenhouse gas fluxes (N₂O, carbon dioxide (CO₂) and methane (CH₄)) in May-September on five Finnish grasslands with varying soil types (organic and mineral) and vegetation (perennial grasses) in 2022. The grasslands were harvested and fertilized twice during the measurement period. Each grassland had a fertilized and nonfertilized plot. The fluxes were measured every second week, with a dark chamber method by using online gas analysers, from both plots and adjacent soil samples were collected each time for chemical analysis. Soil parameters (temperature, moisture and conductivity) were measured continuously on each plot.

The measured N₂O fluxes varied from -22.6 to 928 µg N₂O m^-2 h^-1. All measured grasslands were N₂O sources over the measurement period, with organic soils showing higher emissions. Statistical analysis showed that N₂O fluxes were affected by soil moisture, soil type and pH, whereas the effect of fertilizers and harvests was not as clear. However, the harvests and fertilizers seemed to have a more profound effect on CO₂ respiration. The results indicate, that environmental factors govern the changes of N₂O fluxes in these agricultural grasslands, but there is a need for long term, continuous measurements.