Carbon dioxide and nitrous oxide fluxes from a legume-based grassland during contrasting seasons in eastern Finland

Legumes facilitate soil carbon (C) sequestration and mitigation of nitrous oxide (N₂O) emissions. They have an important role in improving model predictions of future feedbacks from the high-latitude carbon dioxide (CO₂) and N₂O fluxes and climate driving the response of northern ecosystems to warming. Legume based grasslands are an important part of the economy as high protein fodder for the cattle and thus, they are crucial for meat and dairy industries in Europe. However, there is a lack of regionally based, ecosystem scale field experimental data on legume based grasslands. Therefore, using the eddy covariance technique, we measured CO₂ and N₂O fluxes from a grassland site, growing timothy (Phleum pratense L.) and red clover (Trifolium pratense L.), treated with mineral nitrogen (MinN) or with digestate residue (OrgN) in eastern Finland during two growing seasons (May – Sep 2017 and 2018). Results showed that higher mean seasonal temperature (2018) increased net ecosystem CO₂ exchange (NEE) and total dry matter (DM) and decreased N₂O emissions. Specifically, NEE was 8.3 and 12.1 Mg ha^-1 in 2017 and 2018, respectively with no differences between treatments over the two years. The DM yield was 5.9 and 4.9 Mg ha^-1 for MinN and OrgN, respectively, in 2017, while it was 6.3 and 6.8 Mg ha^-1 in 2018. Cumulative N₂O fluxes were 0.01 (100-year global warming potential CO₂-equivalent) and -0.6 Mg CO₂ ha^-1 in 2017 and 2018, respectively. Summing up the seasonal NEE, N₂O flux and DM yield, the seasonal C balance was 2.1 and -1.3 Mg ha^-1 for MinN and OrgN treatments in 2017, and it was -2.5 and -3.5 Mg ha^-1, respectively, in 2018. Our observations from two climatically contrasting seasons suggest that the legume based grasslands in the boreal region have a strong C sequestration potential and the addition of organic fertilizer turns the systems to a larger sink in in the warmer year.