Nitrogen leaching from N limited forest ecosystems: the MERLIN model applied to G?rdsj?n, Sweden

摘要

Chronic deposition of inorganic nitrogen (N) compounds fromthe atmosphere to forested ecosystems can alter the status of a forest ecosystem fromN-limited towards N-rich, which may cause, among other things, increased leaching ofinorganic N below the rooting zone. To assess the time aspects of excess N leaching, aprocess-oriented dynamic model, MERLIN (Model of Ecosystem Retention and Loss of InorganicNitrogen), was tested on an N-manipulated catchment at G?rdsj?n, Sweden (NITREXproject). Naturally generated mature Norway spruce dominates the catchment with Scots pinein drier areas. Since 1991, ammonium nitrate (NH₄NO₃) solution at arate of about 35 kg N ha-1 yr-1 (250 mmol m-2 yr-1)has been sprinkled weekly, to simulate increased atmospheric N deposition. MERLINdescribes C and N cycles, where rates of uptake and cycling between pools are governed bythe C/N ratios of plant and soil pools. The model is calibrated through a hindcast periodand then used to predict future trends. A major source of uncertainty in model calibrationand prediction is the paucity of good historical information on the specific site andstand history over the hindcast period 1930 to 1990. The model is constrained poorly in anN-limited system. The final calibration, therefore, made use of the results from the6-year N addition experiment. No independent data set was available to provide a test forthe model calibration. The model suggests that most N deposition goes to the labile (LOM)and refractory (ROM) organic matter pools. Significant leaching is predicted to start asthe C/N ratio in LOM is reduced from the 1990 value of 35 to <28. At ambient depositionlevels, the system is capable of retaining virtually all incoming N over the next 50years. Increased decomposition rates, however, could simulate nitrate leaching losses. Therate and capacity of N assimilation as well as the change in carbon dynamics are keys toecosystem changes. Because the knowledge of these parameters is currently inadequate, themodel has a limited ability to predict N leaching from currently N-limited coniferousforest ecosystems in Scandinavia. The model is a useful tool for bookkeeping of N poolsand fluxes, and it is an important contribution to further development of qualitativeunderstanding of forest N cycles.