DNDC modelling of greenhouse gas exchange from a boreal legume grassland under organic and mineral nitrogen management

While grasslands are the cornerstone of Finnish livestock industries, practices for mitigating greenhouse gas (GHG) emissions are not well elucidated. Here, we used eddy covariance flux data measured on clover grassland in eastern Finland to calibrate and validate the Heat Exchange DNDC (HE-DNDC) model in response to synthetic mineral (Nmin) and organic manure (Norg) fertilizer. The study covered an entire grass/crop rotational cycle, and the study years were represented as R1 (May 20, 2017–May 19, 2018), R2 (May 20, 2018–May 19, 2019), and R3 (May 20, 2019–May 19, 2020). Simulated carbon flux was good/fair, evidenced by relatively high Mean Absolute Error (MAE) (11.1–20.9 kg C ha−1) for both the treatments. For over 60 % of the observations, simulated biomass yield values were within one standard deviation of the measured values, indicating reasonable simulation accuracy. Cumulative measured N2O emissions were 7.5 kg N ha−1 (Nmin) and 10.9 kg N ha−1 (Norg), with simulations closely matching Nmin (6.9 kg N ha−1), but underestimating Norg (7.3 kg N ha−1). Simulated net GHG balance (NGB) indicated that grasslands were a carbon sink for Norg in R1. During R2 and R3 (crop re-establishment) grasslands became a carbon source regardless of fertilizer treatment. We conclude that DNDC provides a reasonable estimation of the magnitude and timing of GHG fluxes associated with organic and mineral fertilizer application in boreal grasslands, although significant improvements are needed in simulating processes governing N2O emissions.