NUTRIENT CYCLING AND ACIDIFICATION OF A NORTHWEST GERMAN FOREST SITE WITH HIGH ATMOSPHERIC NITROGEN DEPOSITION

Input measurements of major elements were carried out from 1983 to 199 0, at the Wingst (WI) and Westerberg (WE) forests, located close to th e northwestern coast of Germany, and stocked with 90-year-old (WE) and 120-year-old (WE) Norway spruce (Picea abies L. Karst.). The sites ex hibit different degrees of needle browning and needle loss and differe nt exposure to the prevailing winds (WE exposed, WI protected and less needle loss). Soil types on glacial/aeolic sands (WI, Spodic-Dystric Cambisol; WE, Podzol) and composition of cation exchange capacity (CEC ) indicate strong acidification at both sites. Total atmospheric input (throughfall) is dominated by sea salts (at WE (kmol, ha-1 year-1): C l- 3. 1, SO42- 3.1, Na+ 2.5) and by the input of NH4+ (mean 2.9 kmol(c ) ha-1 year-1; maximum 3.7 kmol(c) ha-1 year-1). The NH4+ input is hig her at WE. Estimated interception deposition (ID) indicated an uptake of NO3- by the canopy at both sites (0.1 7 kmol(c) ha-1 year-1) and an enhanced ID of NH3 at the WE site (1.1 kmol, ha-1 year-1). The actual acid load, based on the flux balance and calculated for 1988 and 1989 for the WE site only is 5.9 kmol(c) ha-1 year-1. Up to 56% is caused by the accumulation of deposited acids (80% NH4+) and up to 44% by int emal proton production processes (47% desorption of S, 45% nitrificati on, 8% dissociation of organic acids). The main buffer mechanism (86%) is the release of M(a) cations, primarily as Al (95%). There is a het erogenic but considerable output of NH4+ at the WE site, indicating a disturbed microorganism activity. A sufficient supply of nutrient cati ons for tree growth is only guaranteed by sea salt input at the sites. Net excess N at the WE site is about 2 kmol, ha-1 year-1. Observed da mage symptoms were related to a combination of high particle impact on the needles (Cl-and (NH4)2SO4), soil drought and nutrient imbalances.