Nutrient cycling and acidification of a northwest German forest site with high atmospheric nitrogen deposition

Abstract

Input measurements of major elements were carried out from 1983 to 1990, at the Wingst (WI) and Westerberg (WE) forests, located close to the northwestern coast of Germany, and stocked with 90-year-old (WE) and 120-year-old (WE) Norway spruce ( Picea abies L. Karst.). The sites exhibit different degrees of needle browning and needle loss and different 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 c ha −1): Cl − 3.1, So 4 2− 3.1, Na + 2.5) and by the input of NH 4 + (mean 2.9 kmol c ha −1 year −1; maximum 3.7 kmol c ha −1 year −1). The NH 4 + input is higher at WE. Estimated interception deposition (ID) indicated an uptake of NO 3 − by the canopy at both sites (0.17 kmol c ha −1 year −1) and an enhanced ID of NH 3 at the WE site (1.1 kmol c 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. Up to 56% is caused by accumulation of deposited acids (80%) NH 4 +) and up to 44% by internal proton production processes (47% desorption of S, 45% nitrification, 8% dissociation of organic acids). The main buffer mechanism (86%) is the release of M a cations, primarily as Al (95%). There is a hetorogenic but considerable output of NH 4 + at the WE site, indicating a disturbed microorganism activity. A sufficient supply of nutrient cations for tree growth is only guaranteed by sea salt input at the sites. Net excess N at the WE site is about 2 kmol c ha −1 year −1. Observed damage symptoms were related to a combination of high particle impact on the needles (Cl − and (NH 4) 2SO 4, soil drought and nutrient imbalances.