SOIL ACIDIFICATION AND ELEMENT FLUXES OF A FAGUS-SYLVATICA FOREST AS INFLUENCED BY SIMULATED NITROGEN DEPOSITION

The influence of simulated 3- and 9-fold increases in the current N deposition on element fluxes and soil acidification was investigated in a beech forest in S Sweden. Treatments, in three replicates, were 66 and 198 kg N ha-1 yr-1 as NH4NO3 during 5.5 yr. All the supplied N could not be accounted for by leaching or soil accumulation, and possible pathways are discussed. Leaching of NO3- increased greatly upon treatment. Nitrification, also occurring in the control soil, was considerably enhanced by the low-N but not by the high-N treatment. Leaching of NO3- (added or resulting from nitrification) had a strongly acidifying influence. Soil-solution (cup lysimeters at 30 cm depth) pH was 4.51, 4.07 and 3.81 in the control, low- and high-N plots, respectively. The buffering of protons shifted from the base-cation to the Al buffer range. The base-cation/Al ratio (equivalents) of cation mobilization was 2.7 in the control, 1.1 in low-N and 0.15 in high-N plots. Also Mn and Rb fluxes were drastically increased. The C:N ratio of beech litterfall decreased with increasing N supply but fluxes of elements in litterfall and in throughfall were not influenced. The bulk deposition of N was 18 kg ha-1 yr-1. A moderate further increase in the N deposition to this N-saturated forest would considerably increase NO3- leaching and soil acidity and cause depletion of base cations and mobilization of Al and Mn.