Characteristics of annual NH3 emissions from a conventional vegetable field under various nitrogen management strategies

Abstract

High N-fertilizer applications to conventional vegetable production systems are associated with substantial emissions of NH₃, a key substance that triggers haze pollution and ecosystem eutrophication and thus, causing considerable damage to human and ecosystem health. While N fertilization effects on NH₃ volatilization from cereal crops have been relatively well studied, little is known about the magnitude and yield-scaled emissions of NH₃ from vegetable systems. Here we report on a 2-year field study investigating the effect of various types and rates of fertilizer application on NH₃ emissions and crop yields for a pepper-lettuce-cabbage rotation system in southwest China. Our results show that both NH₃ emissions and direct emission factors of applied N varied largely across seasons over the 2-year period, highlighting the importance of measurements spanning entire cropping years. Across all treatments varying from solely applying urea fertilizers to only using organic manures, annual NH₃ emissions ranged from 0.64 to 92.4 kg N ha^-1 yr^-1 (or 0.07-6.84 g N kg^-1 dry matter), equivalent to 0.05-5.99% of the applied N. At annual scale, NH₃ emissions correlated positively with soil δ¹⁵N values, indicating that soil δ¹⁵N may be used as an indicator for NH₃ losses. NH₃ emissions from treatments fertilized partially or fully with manure were significantly lower compared with the urea fertilized treatment, while vegetable yields remained unaffected. Moreover, full substitution of urea by manure as compared to the partial substitution further reduced the yield-scaled annual NH₃ emissions by 79.0-92.4%. Across all vegetable seasons, there is a significant negative relationship between yield-scaled NH₃ emissions and crop N use efficiency. Overall, our results suggest that substituting urea by manure and reducing total N inputs by 30-50% allows to reduce NH₃ emissions without jeopardizing yields. Such a change in management provides a feasible option to achieve environmental sustainability and food security in conventional vegetable systems.

Keywords: Ammonia; Conventional vegetable production; Emission factor; Manure; Yield-scaled emission.