Potential use of Sr isotope ratio to evaluate trace metal uptake in moss

Abstract

Moss biomonitoring is widely applied to evaluate trace metals in atmospheric deposition and can be especially useful in mountainous areas where the installation and maintenance of physical equipment are often challenging. Nonetheless, the accuracy of moss biomonitoring is debatable because trace metals in moss are also affected by local soil. The Sr isotope ratio (87Sr/86Sr) is useful in the examination of the influence of soil because it allows characterization of the sources of trace metals in moss. Accordingly, this study determined the trace metal uptake in moss from local soil using the 87Sr/86Sr ratio in mountainous areas. Moss samples were collected in study plots located at elevations of 1800–2800 m in the Yatsugatake Mountains, central Japan. The changes in their Sr concentrations (moss Sr) and Sr isotope ratios (moss 87Sr/86Sr) with elevation were analyzed in relation to the Sr isotope ratios in soil (soil 87Sr/86Sr) and moss biomass. Moss 87Sr/86Sr was positively correlated with soil 87Sr/86Sr, indicating that moss mainly took up soil-derived Sr across all elevation ranges. This strong influence of soil-derived Sr on moss may be attributed to the higher amount of Sr in soil than in atmospheric deposition. Moss 87Sr/86Sr had higher values than soil 87Sr/86Sr at higher elevations. Given that atmospherically deposited 87Sr/86Sr exhibits higher values than soil 87Sr/86Sr, Sr input from atmospheric deposition was increased at higher elevations. The moss biomass had negative effects on moss Sr and positive effects on moss 87Sr/86Sr. These correlations can be explained by the reduced uptake of soil-derived Sr in moss as the moss biomass increased. Therefore, patterns of trace metal uptake in moss changed with elevation; however, moss mainly took up Sr from soil across all elevations.