Human activity is affecting the composition of the atmosphere and the climate, in turn altering the frequency and severity of ecological disturbances. The combined effects of global environmental changes and wildfires on ecosystem functioning have not been investigated yet, even though climate change is expected to increase wildfire risk.

We investigated the impacts of a wildfire on soil emissions of nitrous oxide (N₂O), a highly active greenhouse gas, from a grassland ecosystem exposed to simulated global changes as part of the Jasper Ridge Global Change Experiment. This field experiment gave us a unique opportunity to study the interactive effects of an accidental wildfire and three main global change factors - atmospheric CO₂ concentration, precipitation, and atmospheric nitrogen deposition - manipulated at ambient and realistic levels for the second half of the 21^st century. We measured soil N₂O emission rates across the three years following the fire event, along with measurements of the two main processes associated with N₂O emissions – nitrification and denitrification – and with their environmental drivers.

Results/Conclusions

Over the three years following the disturbance, we found that wildfire increased soil N₂O emission rates by a factor of three. Fire effects were still large 33 months after the disturbance, and strongest when associated with simulated increased atmospheric nitrogen deposition and elevated CO₂ concentration. Soil N₂O emission rates increased by a factor of two in the burned plots and by a factor of six under the combined influence of elevated CO₂, nitrogen and fire, thus resulting respectively in cumulative extra releases of 31 and 134 g equivalent CO₂ m^-2 in the atmosphere over the three years after fire. Measurements of nitrification and denitrification support the idea that higher soil N₂O emission rates were due to higher denitrification rates in the burned plots.

In conclusion, our results show that wildfires and global environmental changes can lead to massive increases in emissions of greenhouse gases. Positive feedbacks from ecosystems to climate may thus be underestimated without considering simultaneous changes in disturbance frequency and severity.