Combined application of organic manure with urea does not alter the dominant biochemical pathway producing N₂O from urea treated soil

Combined application of organic amendment with synthetic fertilizer is an emerging management technique for maximized agronomic benefits without drastic soil health effects. However, little attention has been paid to the environmental costs of such management as the primary focus has remained on agronomic outcomes. In present study, we investigated the impacts of three organic amendments including dairy cattle manure (20 t/ha), poultry manure (20 t/ha), and a biochar (30 t/ha) applied along with urea (55 kg-N/ha) on soil N2O fluxes and explored the mechanisms behind N₂O emissions. Plots with urea-only application were used as a control. For source partitioning of soil-emitted N₂O, we used isotopocule mapping approach. The results showed significantly higher N₂O emission rates from manure-treated plots compared with biochar or control treatments plots. The cumulative N₂O emissions rates for 125 days from control, biochar, dairy cattle, and poultry treatments were 0.76, 0.70, 0.96,and1.05kgN₂O-N/ha, respectively. Isotopocule mapping approach revealed that bacterial denitrification was the dominant pathway for temporal high N₂O emission events as the fractions of bacterial denitrification ranged between 0.5 and 0.9 among treatments. Soil DNA-based Q-PCR assays showed significant increase in abundance of NO₂⁻ reducing denitrifiers in dairy cattle and poultry treated plots suggesting acceleration in N₂O emissions was due to shift in the molecular ratios of (nirS+nirK)/nosZdenitrifying bacterial community. In contrast to manure treatments, the combined application of biochar along fertilizer signif-icantly improved soil C contents with a slight decrease in N₂O emission rates. Therefore, biochar appeared to be the best option to minimize soil quality loss without additional environmental cost.