How does nitrogen deposition affect heathland recovery following a major fire?

Background/Question/Methods 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. Long term N addition experiments at a lowland heathland in the UK (Thursley Common National Nature Reserve) have established the mechanisms by which nutrient loading drives above- and belowground ecosystem change, and resulted in many treatment-related differences in plant and soil characteristics. The occurrence of a severe summer fire at the site in 2006, therefore, provided the opportunity to evaluate the impact of a major perturbation on nutrient and microbial dynamics in soils of contrasting chemistry and microbiology. The aims of the study were to determine the impacts of fire on: (1) soil nutrient dynamics and nutrient economy; (2) the dynamics of soil microbial community composition and activity, in relation to nutrient availability; and (3) the relationship between belowground chemistry, microbial community characteristics and post-fire regeneration of aboveground vegetation. Regular measurements were made of soil nutrient availability, microbial activity and microbial community composition, and plant re-growth for a 12 month period following the fire.

Results/Conclusions Results indicate that the fire significantly reduced the size of soil nutrient stores, although pre-existing differences in N and P availability between control and N-treated plots remained. Temporal variation in nutrient availability and leaching was high, reflecting patterns in microbial activity and climate. Microbial community composition was dynamic, with clear differences apparent between both unburnt and burnt areas, and control and N-treated plots. The persistence of clear N treatment-related differences in community structure following a high impact, high temperature fire was surprising, and suggests that below-ground effects of N deposition are likely to be highly persistent. Vegetation recovery following fire showed clear effects of N treatment, reflecting patterns in both nutrient availability and microbial community composition. Evidence from the study to date indicates that, although severe summer fires have the potential to remove a proportion of accumulated soil nutrient stores, deposition-driven differences in nutrient availability, microbial community composition and vegetation performance remain, at least in the short term.