Deposition of base-cations in Europe and its role in acid neutralization and forest nutrition

In considering acidification problems, emphasis is seldom placed on the role of deposition of base-cations such as Na+, Mg2+, Ca2+ and K+. Besides their ability to neutralize acid input, base-cations are found to be important nutrient elements for ecosystems. Up to now no accurate estimates of base-cation deposition are available on a European scale, despite their indispensability for the estimation of critical loads as well as exceedances. In this report, maps of total (i.e. wet and dry) base-cation deposition in Europe, based on the so-called inferential modeling technique are presented. Wet deposition is mapped on the basis of field measurements made at approximately 600 sites scattered over Europe. Generally, deposition fields found resemble the geographic variability of sources, land use and climate. In large parts of southern Europe more than 50% of the potential acid deposition is found to be counteracted by deposition of the sum of non-sea salt Mg2+, Ca2+ and K+. In central and northwestern Europe base-cation deposition usually amounts to less than 25% of the acid input. The lowest base-cation deposition relative to potential acid deposition is found in southern Scandinavia, Denmark, northern Germany and the Netherlands. A similar spatial pattern is found for the neutralization of acid anions in precipitation. Modeled deposition estimates compare reasonably well with deposition estimates derived from throughfall and bulk-precipitation measurements made at 174 sites scattered over Europe, taking into account the relatively large uncertainty in both estimates. On average, no significant differences are found, but the model seems to underestimate dry deposition of base-cations near local sources. Using error propagation, the random and systematic errors in total deposition for an average grid cell of 10x20 km were estimated to equal 35-50% and 25-40%, respectively