Fire Reduces Soil Nitrate Retention While Increasing Soil Nitrogen Production and Loss Globally

Elucidating the response of soil gross nitrogen (N) transformations to fires could improve our understanding of how fire affects N availability and loss. Yet, how internal soil gross N transformation rates respond to fires remains unexplored globally. Here, we investigate the general response of gross soil N transformations to fire and its consequences for N availability and loss. The results showed that fire increased gross N mineralization rate (GNM; +38%) and ammonium concentration (+47%) as a result of decreased soil C/N ratio but decreased microbial nitrate immobilization (I_NO3; −56%), resulting in increased nitrous oxide (N₂O; +50%) and nitric oxide (+121%) emissions and N leaching (+308%). Time since fire affected soil N cycling and loss. Fire increased GNM, ammonium concentration, and N₂O emission, and decreased I_NO3 only when time since fire was less than one year, while increased N leaching in the short (<one year) and long (>one year) terms. Thus, the consequences of fire were a short-lived increase in N availability and N₂O emissions (lasting less than one year) but with persistent risks of N loss by leaching over time. Overall, fire increased the potential risks of N loss by stimulating N production and inhibiting nitrate retention.