Nitrogen dynamics in an Austrian alpine forest ecosystem on calcareous soils: a scenario-based risk assessment under changing environmental conditions

Wien

2007

Artikel aus einer ZeitschriftUnselbständiges Werk

200138955

The behaviour of an Austrian Alpine forest ecosystem on calca reous soils under changing climate and atmospheric nitrogen deposition scenarios was modelled. The change of nitrate leaching, emission rates of nitrogen compounds and forest productivity were calculated using four process-oriented models for the periods 1998 to 2002 and 2048 to 2052. Each model reflects with high detail a segment of the ecosystem: PnET-N-DNDC (photosynthesis-evapotranspiration-nitrification-denitrification-decomposition; short-term nitrogen cycling), BROOK90 (water balance for small and homogenous forest watersheds), HYDRUS (water flux in complex and heterogenous soils) and PICUS v1.3 (forest productivity). The nitrogen balance model (NBM) combines the individual results into a comprehensive picture and extends the specific values beyond the limits of the individual models. The evaluation of the findings was outlined with TRACE, a model enabling a long term prognosis of nitrogen cycling in annual time steps. Temperature increase and nitrogen input are influenced by various components and processes of the forest ecosystem. An increase of the temperature of 2.5 °C led to an enhancement of the N2O emission rates and affected the mineralization and the nitrification rates with the consequence of increased nitrate leaching into the subsoil. Enhanced nitrogen input also showed notable effects on nitrate leaching. Increasing temperature led to enhanced N20-emissions, a significant increase of the mineralization and nitrification rates (nitrogen deposition caused these processes to a minor degree), more nitrate formation in the soil and therefore an enhanced risk of nitrate leaching into the subsoil and the aquifer, increased nitrogen in the woody and root biomass. The Simulation of the increment of the stemwood biomass over the next 50 years gave a mean annual increment of 6.2 m3 p.a. under present climatic conditions. Elevated temperature led to a decrease to 5.8 m3 p.a. due to an elevated increase in tree mortality. Increasing nitrogen input led only to a slightly increased formation of N2O, but to an escalated nitrate leaching already under the current nitrogen deposition, especially if the nitrogen deposition was doubled. Conclusions: The combination of different models yielded improved predictions in comparison to the individual models. All relevant processes of the nitrogen balance were captured. The results of the individual models were combined and validated with the results of the open field measurements and literature references. Increasing temperature led to enhanced nitrogen mineralization rates. This is in accordance with field experiments over 18 months at the study site showing strong temperature dependency of net nitrogen mineralization and net nitrification. The model scenarios reflecting a temperature increase also displayed a distinct rise of the gross minerahzation rate. This supports the importance of mineralization and nitrification as the key nitrogen mobilization processes contributing to nitrate leaching, the cardinal symptom of nitrogen saturation. Enhanced nitrate leaching into the groundwater under changing environmental conditions has been verified. A combination of the risk factors "elevated temperature and nitrogen inputs" has a distinct negative effect on forest ecosystems.