Floating duckweed mitigated ammonia volatilization and increased grain yield and nitrogen use efficiency of rice in biochar amended paddy soils

Biochar (BC) potentially accelerates ammonia (NH₃) volatilization from rice paddy soils. In this regard, however, application the floating duckweed (FDW) to biochar-amended soil to control the NH₃ volatilization is not studied up-to-date. Therefore, the impacts of BC application with and without FDW on the NH₃ and nitrous oxide (N₂O) emissions, NUE and rice grain yield were evaluated in a soil columns experiment. We repacked soil columns with Hydragric Anthrosol and Haplic Acrisol treated in triplicates with Urea, Urea + BC and Urea + BC + FDW. Total NH₃ losses from Hydragric Anthrosol and Haplic Acrisol were 15.2–33.2 kg N ha⁻¹ and 19.6–39.7 kg N ha⁻¹, respectively. Urea + BC treatment recorded 25.6–43.7% higher (p < 0.05) NH₃ losses than Urea treatment, attributing to higher pH value of floodwater. Floating duckweed decreased soil pH and therefore significantly reduced (p < 0.05) the NH₃ volatilizations from the two soils by 50.6–54.2% over Urea + BC and by 34.2–38.0% over Urea treatment. Total N₂O emissions from Hydragric Anthrosol and Haplic Acrisol were 1.19–3.42 kg N ha⁻¹ and 0.67–2.08 kg N ha⁻¹, respectively. Urea + BC treatment increased N₂O emissions by 58.8–68.7% and Urea + BC + FDW treatment further increased N₂O emission by 187.4–210.4% over Urea treatment. Higher ammonium content of the topsoil, explained the N₂O increases in the Urea + BC and Urea + BC + FDW treatments. Urea + BC slightly reduced the rice grain yield and NUE, while the Urea + BC + FDW promoted both rice yield and NUE. Our data indicate that co-application of FDW along with BC in paddy soil could mitigate the NH₃ volatilization and enhance the rice grain yield and NUE.