Effects of long-term nitrogen fertilization on N₂O, N₂ and their yield-scaled emissions in a temperate semi-arid agro-ecosystem

Purpose: Nitrous oxide (N₂O) measured simultaneously with di-nitrogen (N₂) emissions from soils are greatly uncertain due to large temporal and spatial variations. This study aims to report N₂O, N₂, N₂/N₂O, ¹⁵N-N₂O, wheat-maize annual grain yields, and yield-scaled N₂O and N₂O plus N₂ emissions on the responses to different nitrogen (N) fertilizer rates in a winter wheat-summer maize cropping system. Furthermore, this study also seeks to determine controlling factors for N₂O, N₂, and N₂/N₂O emissions and significantly investigate the relationship between the soil-climate measured factors and ¹⁵N-N₂O. Materials and method: Three N inputs and control treatments, 0, CK; 200, LN; 400, MN; and 600, HN kg N ha⁻¹ year⁻¹ were set since 1998. Direct measurement method has been used to quantify N₂O and N₂ emissions. Results: Our results indicated that the effects of long-term N fertilization significantly increased N₂O and N₂ and also reduced N₂/N₂O emission ratios as described by exponential functions. Using structural equation modeling (SEM), NH₄⁺, WFPS, NO₃^-, and DOC were revealed to be main controlling factors for N₂O, while N₂ by DOC, NO₃^-, WFPS, and temperature finally N₂/N₂O was positively related to temperature. Furthermore, the ¹⁵N-N₂O was positively related to N₂/N₂O ratios, indicating that denitrification is the dominant process at the study site. The yield-scaled N₂O emissions followed the order HM>MN>LN>CK, and they were 1.56, 1.47, and 1.07 times greater than CK, respectively. Total yield-scaled N₂O plus N₂ were in the order of CK>HN>MN>LN. Conclusion: N fertilization has shown strong impact not only on N₂O, N₂, and N₂/N₂O emissions but also on yield-scaled N₂O and N₂O plus N₂ emissions. High agronomic nitrogen use efficiency (NUE), low yield-scaled N₂O emissions, and low cumulative N₂O plus N₂ emissions were observed at 200-LN treatment, suggesting this rate to be an optimum and sustainable agricultural management practice with no significant crop yield reduction as compared to the current farmers’ practice of 400 kg N ha⁻¹ year⁻¹.