Effects of artificial acid rain on decomposition of spruce needles and on mobilisation and leaching of elements

Abstract

Dry matter and chemical changes in decomposing spruce needles were investigated after 16 and 38 weeks in laboratory lysimeters treated with distilled water or distilled water acidified to pH 3 or 2 with sulphuric acid. The water was added twice weekly in quantities equal to 100 or 200 mm month−1. The CO2 evolution and leaching of P, K, Mg, Mn, and Ca was followed together with pH measurements of the leachate. The loss of dry matter was approximately 25% during the first 16 weeks and approximately 37% after 38 weeks. At the first samling, 16 weeks, the amount of material decomposed was greater from the lysimeters given 100 mm month−1 of water. At this water quantity dilute sulphuric acid increased the decomposition. After 38 weeks sulphuric acid at pH 3 and 2 had decreased the decomposition at 200 mm month−1. However, the effects of acid application were small. The effect of treatment using acidified water on the content of monosaccharides was not consistent, whereas there was an indication of reduced decomposition of lignin when treated with 200 mm water month−1 at pH 3 and 2. Nitrogen was conserved in the lysimeters with small differences between the various treatments. The order of mobility of metal elements was K>Mg>Mn>Ca. Increasing the quantity of water increased the leaching of K especially, whereas addition of dilute sulphuric acid increased the leaching of Mg, Mn and particularly Ca. During the first 16 weeks of the experiment, sulphuric acid reduced the leaching of P while later on this treatment increased the leaching. The pH of the leachate from the lysimeters treated with distilled water was initially 4.0–4.6 increasing to approximately 6.6 after 22 weeks. The pH of the decomposed needle material was 4.6 and approximately 5.2 after 16 and 38 weeks respectively. When treated with water at pH 3 the pH of the leachate was between 4 and 5, and the pH of the needles 4.2–5.1. Treatment with water at pH 2 gave a leachate with pH just above 2 and decreased the pH of the needles that had received 200 mm ‘rain’ month−1 to 2.9. The effect of the artificial acid rain appears to be more pronounced on the leaching of metal elements than on the biological activity and the dynamics of N and P. The treatments must be considered extreme when compared with the acidity of natural rain.