How timing of fertilization affects N2O emissions from a legume grassland on northern mineral soil

Agricultural soils are significant sources of N₂O, and they have been estimated to be responsible for about 60% of global anthropogenic N₂O emissions. These emissions mainly originate from the application of synthetic fertilizers. Globally, 120 Tg N of new N as synthetic fertilizer is introduced to soil every year to sustain crop and grass production. Agricultural sector is responsible for 14% of the total anthropogenic greenhouse gas (GHG) emissions in Finland, from which 54% are N₂O emissions. Growing season in Finland is relatively short (May-September) and all the grazing and feed production happens during those months. About 35% of Finland's cultivated arable land is cultivated with forage. With optimized management practices, such as crop species selection and timing of fertilization, N₂O emissions from agricultural fields and per units of feed produced could be reduced. Since N₂O is the most potent GHG, even small reductions in its emissions can yield significant climate benefits.

Introducing legumes, such as red clover (Trifolium pratense), to crop rotations reduces the need of synthetic fertilizers, due to the ability of the legumes to fix their own N via a symbiosis with rhizobia bacteria. Previous studies have shown lower N₂O fluxes in red clover grass mixtures compared to monocultures and grass mixtures with other grass species. Lower levels of synthetic N fertilization also reduce indirect N₂O and CO₂ emissions which are generated during the fertilizer manufacturing process. In mineral soils, highest N₂O emission peaks are often measured after fertilization events. The fertilizer induced emission peak can be reduced by shifting the timing of fertilization, e.g. week after harvest, when plants are in active growth phase and can utilize nutrients more efficiently.

In this research we tried to answer to two research questions: 1) Do annual N₂O emissions from the agricultural field to the atmosphere decrease with increasing red clover coverage? 2) How does the timing of post-harvest fertilizer application influence subsequent N₂O emission peaks? The research was conducted in a 6.3 ha agricultural field on a mineral soil, near Maaninka, eastern Finland. N₂O exchange of the field was studied using the eddy covariance technique from four years of grass rotation cycle (2022–2025). Crop, soil and environmental variables were also measured to help explain the N₂O exchange patterns and N dynamics. We hypothesized that delaying the fertilizer application by approx. one week after harvest decreases the resulting N₂O emission peaks and that annual N₂O emissions from the agricultural field to the atmosphere decreases with increasing red clover coverage. In this presentation, we highlight the changes in red clover coverage, total yield, N₂O emissions originating from fertilization and the annual N₂O dynamics.