Quantifying nitric oxide emissions under rice-wheat cropping systems

Abstract

Urea deep placement (UDP) increases nitrogen use efficiency (NUE) and crop yields while reducing nitrogen (N) losses to the environment. However, studies on its environmental impacts on nitric oxide (NO) emissions are still limited. Therefore, we conducted a greenhouse experiment to quantify the NO emissions from a rice-wheat system. NO emissions were measured from three N fertilizer treatments – control (no N), UDP, and broadcast prilled urea (PU) – using an automated gas sampling and analysis system continuously for a rice-wheat cropping cycle. In rice, UDP was tested under two water regimes – continuous flooding (CF) and alternate wetting and drying (AWD). Fertilizer treatments had significant effects (p < 0.05) on NO emissions. UDP with AWD irrigation increased NO emissions (3.41 g N ha−1) (p < 0.05) by 2.5-times compared to UDP with CF (1.35 g N ha−1). But emissions were similar between UDP and broadcast PU under the CF water regime. In wheat, the application of N fertilizer – regardless of application methods – increased NO emissions (615 g N ha−1, average across application methods) by 10-times over control (62.52 g N ha−1). However, emissions were not significantly (P > 0.05) different among the treatments. Fertilizer induced emission factors (EFs) were not affected by N placement methods in either rice or wheat. On average, EFs in the rice were very low (<0.002%) compared to the wheat (0.5%). This study reveals that (regardless of treatments), the contribution of rice (<4 g N ha−1) on total annual NO emissions (433 g N ha−1) was very small (<0.5%) compared to emissions from wheat.