Determining N₂O and N₂ fluxes in relation to winter wheat and sugar beet growth and development using the improved ¹⁵N gas flux method on the field scale

The objectives of this field trial were to collect reliable measurement data on N₂ emissions and N₂O/(N₂O + N₂) ratios in typical German crops in relation to crop development and to provide a dataset to test and improve biogeochemical models. N₂O and N₂ emissions in winter wheat (WW, Triticum aestivum L.) and sugar beet (SB, Beta vulgaris subsp. vulgaris) were measured using the improved ¹⁵N gas flux method with helium–oxygen flushing (80:20) to reduce the atmospheric N₂ background to < 2%. To estimate total N₂O and N₂ production in soil, production-diffusion modelling was applied. Soil samples were taken in regular intervals and analyzed for mineral N (NO₃⁻ and NH₄⁺) and water-extractable Corg content. In addition, we monitored soil moisture, crop development, plant N uptake, N transformation processes in soil, and N translocation to deeper soil layers. Our best estimates for cumulative N₂O + N₂ losses were 860.4 ± 220.9 mg N m⁻² and 553.1 ± 96.3 mg N m⁻² over the experimental period of 189 and 161 days with total N₂O/(N₂O + N₂) ratios of 0.12 and 0.15 for WW and SB, respectively. Growing plants affected all controlling factors of denitrification, and dynamics clearly differed between crop species. Overall, N₂O and N₂ emissions were highest when plant N and water uptake were low, i.e., during early growth stages, ripening, and after harvest. We present the first dataset of a plot-scale field study employing the improved ¹⁵N gas flux method over a growing season showing that drivers for N₂O and N₂O + N₂ fluxes differ between crop species and change throughout the growing season.