Oj. Kjonaas et Rf. Wright, Nitrogen leaching from N limited forest ecosystems: the MERLIN model applied to Gardsjon, Sweden, HYDROL E S, 2(4), 1998, pp. 415-429
Chronic deposition of inorganic nitrogen (N) compounds from the atmosphere
to forested ecosystems can alter the status of a forest ecosystem from N-li
mited towards N-rich, which may cause, among other things, increased leachi
ng of inorganic N below the rooting zone. To assess the time aspects of exc
ess N leaching, a process-oriented dynamic model, MERLIN (Model of Ecosyste
m Retention and Loss of Inorganic Nitrogen), was tested on an N-manipulated
catchment at Gardsjon, Sweden (NITREX project). Naturally generated mature
Norway spruce dominates the catchment with Scots pine in drier areas. Sinc
e 1991, ammonium nitrate (NH4NO3) solution at a rate of about 35 kg N ha(-1
) yr(-1) (250 mmol m(-2) yr(-1)) has been sprinkled weekly, to simulate inc
reased atmospheric N deposition.
MERLIN describes C and N cycles, where rates of uptake and cycling between
pools are governed by the C/N ratios of plant and soil pools. The model is
calibrated through a hindcast period and then used to predict future trends
. A major source of uncertainty in model calibration and prediction is the
paucity of good historical information on the specific site and stand histo
ry over the hindcast period 1930 to 1990. The model is constrained poorly i
n an N-limited system. The final calibration, therefore, made use of the re
sults from the 6-year N addition experiment. No independent data set was av
ailable to provide a test for the model calibration.
The model suggests that most N deposition goes to the labile (LOM) and refr
actory (ROM) organic matter pools. Significant leaching is predicted to sta
rt as the C/N ratio in LOM is reduced from the 1990 value of 35 to <28. At
ambient deposition levels, the system is capable of retaining virtually all
incoming N over the next 50 years. Increased decomposition rates, however,
could stimulate nitrate leaching losses. The rate and capacity of N assimi
lation as well as the change in carbon dynamics are keys to ecosystem chang
es. Because the knowledge of these parameters is currently inadequate, the
model has a limited ability to predict N leaching from currently N-limited
coniferous forest ecosystems in Scandinavia. The model is a useful tool for
bookkeeping of N pools and fluxes, and it is an important contribution to
further development of qualitative understanding of forest N cycles.