Ammonia fluxes and emission factors under an intensively managed wetland rice ecosystem

Abstract

Nitrogen (N) loss from rice production systems in the form of ammonia (NH₃) can be a significant N loss pathway causing significant economic and environmental costs. Yet, data on NH₃ fluxes in wetland rice ecosystems are still very scarce which limits the accuracy of national and global NH₃ budgets. We measured the NH₃ fluxes in situ in a wetland rice field and estimated emission factors (EF) under two soil management systems (i.e. conventional tillage, CT and strip tillage, ST); two residue retention levels (i.e. 15%, LR and 40% crop residue by height, HR); and three N fertilization rates (i.e. 108, 144 and 180 kg N ha⁻¹) in two consecutive years (2019 and 2020). The highest NH₃ peaks were observed within the first 3 days after urea application. The mean and cumulative NH₃ fluxes significantly increased with the increases in N fertilization rates and were 18.5% and 18.6% higher in ST than in CT in 2020 but not in 2019. Overall, the highest mean NH₃ fluxes were in 180 kg N ha⁻¹ coupled with either HR or LR and ST or CT. In 2019, the NH₃ EF was unchanged by any treatments. In 2020, the lower EF was in CT coupled with LR (15%) than all other treatment combinations, where ST with HR showed the highest EF (20%). Likewise, the lowest N rate (108 kg N ha⁻¹) in ST had the highest NH₃ EF (20%) that was similar to higher N rates (144 and 180 kg N ha⁻¹) in the same tillage treatment and to 180 kg N ha⁻¹ in CT. Our results highlight that NH₃ fluxes in rice field particularly the effects of ST correlated with higher soil pH and NH₄⁺ content and lower redox potential. Our results highlight that NH₃ fluxes are a potentially large N loss pathway in wetland rice under conventional and decreased soil disturbance regimes.