Effects of water acidification on the decomposition of Juncus bulbosus L

Several field and laboratory experiments were performed to study the impact of acidification on the decomposition of Juncus bulbosus L. The breakdown of coarse detritus of this aquatic macrophyte appeared to be very slow. After 7 weeks of incubation in an acidified and a circumneutral moorland pool the remaining fraction of Juncus leaves varied between 46 and 75% of the original organic weight. Differences in decay patterns were observed among both systems. Furthermore, the breakdown rates were highly dependent on the season. Highest weight loss rates were recorded at high pH during the summer period. In acidified water and during the winter period the breakdown rates were reduced. Nutrient dynamics of detritus in the circumneutral soft water and the acidified system differed remarkably. In litter bags laid down on the bottom surface of the acidified moorland pool the density and diversity of macroinvertebrate assemblages was generally lower than in the bags immersed in the non-acidified body of water. In particular, the abundance of detritivores and herbivores at low pH was strongly reduced. However, in winter qualitative and quantitative differences in the macrofauna in the litter bags did not significantly affect decay rates of detritus. Laboratory studies revealed that the pH, bicarbonate concentration and aluminium content of the water particularly influenced the rate of decomposition of the pre-dried Juncus material. The extent of leaching and autolysis and the relative importance of fungi and bacteria in the decomposition of fresh Juncus leaves were studied using antifungal and antibacterial agents. In the laboratory leaching and autolysis accounted for 28–32% of the organic weight loss of coarse detritus. Leaching and autolysis caused the highest weight loss in the strongly acid medium. After 37 days of incubation in control media with pH 3.5 and 5.6 total organic weight loss was 55 and 59%, respectively. At pH 3.5 and 5.6 microbial decomposition accounted for organic weight losses of 23 and 31%, respectively. At low pH microbial decomposition was dominated by fungi, whereas, at pH 5.6 both fungi and bacteria played an important role in the breakdown processes. The contribution of macroinvertebrates to the breakdown of Juncus detritus seemed to be of minor importance.