Can prescribed burning compensate for atmospheric nutrient loads in wet heathlands?
Härdtle, Werner; Niemeyer, Thomas; Assmann, Thorsten; Meyer, Hartmut; Von Oheimb, Goddert
The increased deposition of nutrients from the atmosphere has contributed to widespread changes in wet heathland ecosystems throughout Europe. As a result, management measures are nowadays considered a tool with which to mitigate impacts of atmospheric nutrient loads by reducing nutrient stores in the above-ground biomass and soils. A burning experiment was carried out in order to evaluate the effects of prescribed winter burning on the nutrient budgets of an extensive wet heathland complex in NW Germany (with particular reference to growth limiting N and P, but also Ca, K, and Mg). Changes in nutrient budgets were calculated by analysing the current input (atmospheric deposition), the output due to biomass combustion, and N losses due to denitrification. In the study area, atmospheric inputs amount to 3.31 g m−2 yr−1 for N, and < 0.05 g m−2 yr−1 for P. Mean N stores in the above-ground biomass are 9.34 and 9.66 g m−2 (in Calluna-Erica- and Calluna-dominated wet heaths, respectively), and biomass P contents in both vegetation types amount to 0.43 g m−2. From these stores, 47-68% (for N) and 2-16% (for P) were removed by burning (taking into consideration the return of nutrients as a result of ash deposition). Nitrogen losses owing to denitrification range between 0.5 and 1.6 g m−2 yr−1 and may, thus, compensate for about 15-48% of the atmospheric input. However, calculations of input/output-ratios show that prescribed burning is insufficient to counterbalance present-day atmospheric nutrient loads since management cycles usually exceed a 10-year interval. Our results suggest that wet heaths managed by means of prescribed burning will accumulate nutrients, particularly N, in the long term. As a means of preserving a balanced nutrient budget on a long-term basis, high-intensity measures (e.g. high intensity mow), applied in combination with low-intensity measures, will be an indispensable instrument in wet heathland preservation.