The potential importance of soil denitrification as a major N loss pathway in intensive greenhouse vegetable production systems

Background
About 30 % of vegetables in China are produced in intensively managed greenhouses comprising flood irrigation and extreme rates of nitrogen fertilizers. Little is known about denitrification N losses.
Methods
Soil denitrification rates were measured by the acetylene inhibition technique applied to anaerobically incubated soil samples. Four different greenhouse management systems were differentiated: Conventional flood irrigation and over-fertilization (CIF, 800 kg N ha$^{-1Zahl}$, 460 mm); CIF plus straw incorporation (CIF+S, 889 kg N ha$^{-1}$, 460 mm); Drip fertigation with reduced fertilizer application rates (DIF, 314 kg N ha$^{-1}$, 190 mm); DIF plus straw incorporation (DIF+S, 403 kg N ha$^{-1}$, 190 mm). Soil denitrification was measured on nine sampling dates during the growing season (Feb 2019-May 2019) for the top-/ subsoil (0 – 20/ 20- 40 cm) and on three sampling dates for deep soils (40-60/ 80-100 cm). Data was used to constrain N-input-output balances of the different vegetable production systems.
Results
Rates of denitrification were at least one magnitude higher in topsoil than in sub- and deep soils. Total seasonal denitrification N losses for the 0 – 40 cm soil layer ranged from 76 (DIF) to 422 kg N ha$^{-1}$ (CIF+S). ... mehrStraw addition stimulated soil denitrification in top- and subsoil, but not in deep soil layers. Integrating our denitrification data (0-100 cm) with additional data on N leaching, N2O emissions, plant N uptake, and NH3 volatilization showed, that on average 50 % of added N fertilizers are lost due to denitrification.
Conclusions
Denitrification is likely the dominant environmental N loss pathway in greenhouse vegetable production systems. Reducing irrigation and fertilizer application rates while incorporating straw in soils allows the reduction of accumulated nitrate.