Decomposition processes in soil of a healthy and a declining Phragmites australis stand

Decomposition processes were investigated in the soil of a declining, more eutrophic and a healthy, less eutrophic freshwater reed (Phragmites austral is (Cav.) Trin. ex Steudel) stand in the Littoral zone of Rozmberk fishpond , Czech Republic. Soil and pore water were sampled five times from April to October 1998. Chemical properties, CO2 production in oxic and anoxic condi tions, CH4 production, denitrifying enzyme activity (DEA) and bacterial bio mass were measured under laboratory conditions in suspensions prepared from homogenised soil samples. The more eutrophic West stand was more anaerobic than the East stand, with lower redox potential, lower pH and with a highe r amount of organic acids, mainly acetic and lactic acid. Mean seasonal con centrations of total nitrogen in pore water, nitrogen of amino acids and pr oteins, and reducing sugars were all higher in the soil at the more eutroph ic stand. Higher nutrient status and more reduced conditions at the more eu trophic stand were accompanied by (i) a limitation of aerobic microbial act ivities (CO2 production in oxic conditions: 0.35 versus 0.54 mu mol CO2 cm( -3) h(-1)); lower DEA (4.0 versus 20.2 nmol N2O cm(-3) h(-1)) and a lower p roportion of bacteria that were active in aerobic conditions; (ii) by a pre valence of anaerobic over aerobic microbial processes; (iii) by a higher ra te of methanogenesis (15.0 versus 11.5 nmol CH4 cm(-3) h(-1)) and (iv) by a n overall lower rate of microbial processes as compared to less eutrophied stand. The shift from aerobic to anaerobic microbial metabolism, and a coin ciding restriction of metabolic activities at the more eutrophic stand are indicative of an elevated oxygen stress in the soil, associated with accumu lation of metabolites toxic to both the micro-organisms rind the reed. Poss ible links between eutrophication, decomposition processes in the soil and reed decline are discussed. (C) 2001 Elsevier Science B.V. All rights reser ved.