Leaching characteristics of anions and cations from evergreen leaves supplied to the stream bed and influences on stream water composition in the southern Kyusyu [Japan] mountains

The amount of allochthonous material that enters lotic ecosystems annually from terrestrial riparian forests is estimated to be considerable, and substances derived from the decomposition of leaf litter are thought to have an important impact on stream water quality. The purpose of the present study is to clarify the leaching characteristics of leaf litter of the evergreen species that are dominant in the riparian zone of the investigated stream and to examine the influence of leachates on mountain stream water ionic composition by laboratory leaching experiments and field measurements. Five fresh fallen leaves and fallen leaves of ten species of evergreen trees dominant in the riparian zone of the investigated reach were placed into a beaker filled with 500 cc of distilled water and allowed to stand for 30 days at ambient temperature between 25-30degC after well washing with distilled water, air-dried for one week and oven-dried at 80degC for 48 hours and the electric conductivity (EC), hydrogen ion concentration (pH), and the concentrations of cations (Lisup(+), Nasup(+), NH4sup(+), Ksup(+), Casup(2+) and Mgsup(2+)) and anions (Fsup(-), Clsup(-), NOsup(2-), Brsup(-), NOsup(3-), PO4sup(3-) and SO4sup(2-)) were measured at 1 and 30 days after submersion. On the field measurements, stream water samples were collected from four different environments (springs, riffles, pools and side-pools) and were transported to our laboratory and EC, pH and the concentrations of the cations and anions were measured. The field measurements were conducted monthly from April 2006 to March 2007. EC, pH, and cation and anion concentrations, continued to increase in all samples for 30 days following the immersion of evergreen tree leaves in distilled water for being attributed to the leaching of some components from the leaves. The EC values of water samples containing submerged green Symplocos theophrastifolia Sieb. et Zucc. leaves exceeded 150 S/cm, and pH values increased following the rapid decrease immediately after submergence, from 6.0 to 7.0. The most dominant cation in the water samples was Ksup(+), accounting for more than 70% of all cations in most sample waters. On the 30th day after submergence, the order of average cation release rates from green leaves was Ksup(+) Mgsup(2+) Casup(2+) Nasup(+), which coincided to the experimental results in previous studies. The order of average anion release was Clsup(-) PO4sup(3-) SO4sup(2-). In contrast to the higer Ksup(+) ratios in the water of the leaching experiments, the monthly Ksup(+) concentrations in the stream water were relatively lower than those of the other cations and anions. The lower concentration of Ksup(+) in the stream water during the growing season would suggest that there is a differential utilization of Ksup(+) by the biota and stream sediment acts as an important agent for removal of leachate from water. Therefore, the clay component of the sediment may serve as a reservoir of chemically bound Ksup(+) and biota may also provide a reservoir for Ksup(+) if there is some net increase in biomass. The EC value is lower in the spring than in the other environments and that the Ksup(+), Mgsup(2+) and Casup(2+) concentrations are considerably higher in the riffle, pool and side-pool, in which litter had been supplied, than in the spring where leaf litter is not deposited. These results indicate that the primarily source of ions is rock weathering in the spring and simultaneous ion loading from multiple sources occurs in the riffle, pool and side-pool and that leaf litter is one of the primary sources of Ksup(+), Mgsup(2+) and Casup(2+) in the stream water. Based on these results, it is demonstrated that anion and cation concentrations throughout the stream reach are not always uniform due to heterogeneous distribution of ionic material caused by leachate from leaves, although stream water has been regarded as providing a uniform continuous body for the ionic materials, and changes in water composition due to the natural input and subsequent leaching from leaf litter were predicted.