Alkali and alkaline earth metals in decomposing macrophytes in a wetland system

The background concentration of selected alkali and alkaline earth metals (sodium, potassium, calcium and magnesium) in some macrophytes was explored in the wetland system of Keoladeo National Park, Bharatpur, India. Changes in the concentration of these elements in the course of macrophyte decomposition were also studied. The species selected were Paspalum distichum, Paspalidium punctatum, Cyperus alopecuroides, Pseudoraphis spinescens, Ipomoea aquatica, Neptunia oleracea and Hydrilla verticillata, which dominate the aquatic vegetation of the Park. Litterbag decomposition experiments were carried out with nylon bags of two different mesh sizes (0.14 and 0.375 mm) in the laboratory and in the field. Among the macrophytes, Hydrilla was the fastest decaying species with the lowest half-life (12.65 days) and Paspalidium the slowest with the highest half-life (385.08 days). Overall, the grasses had low decay rate and high half-life. Background concentration of Na, K, Ca and Mg varied among the plant species. During decomposition and towards the end of the experiment, Na, K and Ca gradually declined whereas Mg increased. The variation was significant (ANOVA, P <0.05) among metals and macrophytes. Na showed no correlation with the weight loss of decomposing macrophytes in the field or in the laboratory tanks. In the field, the K concentration, in contrast with the observations in the tanks, was negatively correlated with the biomass and positively correlated with Ca content. Ca concentration in the biomass was negatively correlated with the weight of the remaining biomass, while Mg was positively correlated with the biomass in the litterbags.