The role of humic substances in the acidification response of soil and water—Results of the Humic Lake Acidification Experiment (HUMEX)

Major results of the Humic Lake Acidification Experiment (HUMEX) are summarized, based on 2 y of pretreatment and 2.5 y of posttreatment data. The major objectives of the HUMEX project are to quantify the role of acid deposition on the properties of humic substances (HS) and the role of humic substances (HS) in the acidification processes that occur in soil and water. The project involves artificial acidification of one half of a divided dystrophic lake and the corresponding catchment. A combination of sulphuric acid and ammonium nitrate has been applied via sprinkler systems, mounted on trees, during precipitation events since 1990. The treatment has resulted in small changes in water quality, including an increase in SO 4, NO 3, and H + concentrations in the lake water and in the soil water of some of the upper soil horizons, and small changes in the nature of the HS. The results of biological studies show increased toxicity in fish, increase in the phytoplankton primary production, and disappearance of some of the dominating species of zooplankton. Epiphytic growth increased in the treated basin, whereas a group of macrophytes was reduced. Present knowledge of the relationships between chemical changes and biological response is not sufficient to explain the observed changes in biota.