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.