Impacts of burning and increased nitrogen deposition on nitrogen pools and leaching in an upland moor

1. Upland moorlands are an extensive semi-natural resource, frequently burned either through management or uncontrolled outbreaks of fire. These systems are often situated in areas receiving high levels of atmospheric nitrogen (N) deposition, yet the effects of burning combined with high N deposition on ecosystem N pools and N leaching to surface waters are unknown. 2. A management burn was applied to an upland Calluna vulgaris moor which contained a set of long-term experimental plots treated with simulated increased N deposition at rates of +0, +40, +80 and +120 kg ha−1 year−1. Leaching losses of total dissolved inorganic N (TDNin) and dissolved organic N (DON) from organic and mineral soil horizons and the N pools in these horizons, as well as in litter and vegetation, were compared before and after the burn. 3. The results showed that leaching of TDNin and DON from both soil horizons increased in a 6-month period after the burn, with leaching of TDNin remaining elevated 2–3 years later. N pools in the deeper mineral layer of the soil also increased after the burn. Increasing long-term N additions magnified the burn effect on leaching losses but lessened the burn effect on the N pools in the mineral layer. In the +40 N addition plots, the amount of N removed in burning vegetation was of an equivalent size to the amount of additional N retained within the system. 4. Synthesis: These results suggest that burning approximately every 10 years may be effective in removing N retained in the system at N deposition rates up to 56 kg N ha−1 year−1. However, extensive burning of moorland or uncontrolled outbreaks of fire over wide areas may considerably exacerbate the threat of N loading to groundwater in areas where moors are more heavily N polluted, increasing the potential for acidification, eutrophication and brown water colouration. The data suggest that this is because the mineral horizon of upland moors receiving high N inputs has already been saturated with N such that increased downward percolation rates of N caused by the burn have risen above a threshold for immobilization (hence leading to more substantial post-burn increases in leaching of N)