Effect of plant development and N uptake on denitrification in two contrasting crop species

Denitrification is the main source of the greenhouse gas N₂O emitted from agricultural soils. While N₂O emissions and influencing factors have been very well studied in field experiments, there are hardly any reliable data for N₂ emissions on the field scale. However, these are essential to understand under which conditions complete denitrification occurs leading to N₂ formation and when N₂O is the main end product. Whether NO₃^- is reduced to N₂O or N₂ depends on several factors: the availability of NO₃^- and available organic C, as well as pH, oxygen availability, soil moisture, denitrifier community structure, and temperature. All of these parameters are highly dependent on crop development, as growing plants take up NO₃^- and water while increasing organic C availability via root exudates and dying roots, and alter soil pH as well as microbial communities by rhizosphere dynamics.

The objective of this field trial was to collect reliable measurement data on N₂ and N₂O emissions in typical German crops. Two crops were chosen that differ greatly in their temporal development: Winter wheat (Triticum aestivum L.) and sugar beet (Beta vulgaris subsp. vulgaris). Both crops were grown site-typically according to the rules of good agricultural practice. To measure N₂O and N₂ emissions, the improved ¹⁵N gas flux method including high enrichment ¹⁵N-labeled fertilizer was applied. Prior to gas sampling, chambers were purged with a mixture of helium and oxygen (80:20) to reduce the atmospheric N₂ background to < 2%. Soil samples were taken at regular intervals and analyzed for mineral N (NO₃^- and NH₄⁺) and water-soluble Corg content. In addition, we monitored crop development, plant N uptake, N transformation processes in soil, and N translocation to deeper soil layers.