How does N deposition affect belowground heathland recovery following wildfire?

Climate change is predicted to increase the frequency and severity of summer fires in the UK. However, the effects of fire on ecosystem processes are likely to depend on pre-burn vegetation characteristics, soil chemistry and microbiology, all of which are known to be affected by changes in nitrogen (N) deposition. The occurrence of a severe summer fire after eight years of experimental N additions at Thursley Common National Nature Reserve (UK) thus provided a unique opportunity to evaluate the impacts of a major perturbation on nutrient and microbial dynamics in soils of contrasting chemistry. Soil N and P concentrations were substantially reduced by fire, although significant pre-fire treatment effects on soil N were still apparent immediately after the fire. Soil nutrient availability, turnover rates and leaching losses were also increased by fire. Concentrations of extractable soil N and leaching losses of Ca2+, K+, Cl− and SO42− immediately after the fire were significantly higher in plots which had previously received N additions, demonstrating effects of N loading on post-fire nutrient dynamics and economy. Effects of N addition on the activity of soil microbial populations were influenced by soil moisture levels, with significantly higher activity in +N plots compared to controls when soil moisture levels were relatively high (>15%). Microbial community composition varied considerably between sampling dates, although clear N treatment differences were apparent, particularly for bacterial communities, both immediately after and one year on from the fire. Persistent effects of wildfire and N deposition on the stocks and availability of N, and microbial community dynamics, are likely to have longer-term consequences for the functioning of heathlands and other similar semi-natural ecosystems.