MODELING AMMONIUM AND NITRATE UPTAKE BY A MATURE DOUGLAS-FIR STAND FROM A SOIL WITH HIGH ATMOSPHERIC NHX INPUT

Model calculations were performed on N uptake by 40-year-old Douglas-fir trees from a sandy soil situated in an area with a high NH(x) input level. Two model approaches were applied to distribute N uptake over the root system and over the two N sources, ammonium and nitrate. In the first approach, priority was given to maximizing the relative ammonium contribution to N uptake; the other approach aimed at a uniform N uptake rate for all roots. Total N uptake was the same with both approaches, but the distribution of N uptake over roots in various layers was different, as was also the contribution of ammonium to total-N uptake. During a large part of the growing season the amount of N that could be taken up by the roots was lower than the N demand of the trees. Ammonium accounted for at least one quarter and at most about half of the amount of N taken up by the trees, which uncertainty resulted from the different model approaches used and from variability in input data. Large differences in relative ammonium contribution to N uptake existed among roots in various soil layers. In the upper soil layers, the ammonium contribution was considerable during a large part of the growing season, so acidification of the rhizosphere may be expected. In deeper layers, mainly nitrate was taken up, so the rhizosphere will be alkalized. Possible effects of changes in NH(x) input level, nitrification rate and tree growth on N uptake are discussed.