A Quantitative Look at Sources and Fates of Inorganic Nutrients at Skorradalur, Iceland

Bedrock chemistry plays an important role in shaping plant communities in areas where roots contact bedrock or water that contains nutrients derived from the bedrock. Where bedrock and soil lack certain nutrients plants must rely more on atmospheric inputs or efficient recycling-mechanisms. At Skorradalur, Iceland, the bedrock contains little K, and plant growth is particularly sensitive to fluctuations in atmospheric inputs of K. This study examines the sources and fates of K, Mg, and Ca at 5 small catchments underlain by tholeiitic basalt at Skorradalur, Iceland. The bedrock is overlain by up to 500 cm of basaltic till which was deposited when glaciers retreated from this area between 7000 and 10000 years ago. Catchment vegetation varies from mosses and lichens to birches to mixed conifers. Samples of stream water, throughfall, precipitation, soil, and rock were collected at varying intervals between 1995 and 1998 (Moulton, West, and Berner, 2000). All samples were analyzed for major element chemistry. Sources of nutrients were calculated as percentage of nutrients derived from atmospheric inputs and bedrock for the summer and winter seasons. Nearly all of the K originates from atmospheric input throughout the year. Relatively more Mg and Ca are supplied from the atmosphere during the summer (about 60 and 50 percent, respectively) than during the winter (about 50 and 25 percent); the remainder is supplied from bedrock. The fate of K, Mg, and Ca within the catchments was traced using calculations of nutrient fluxes from basalt weathering, atmospheric inputs, biomass storage, and soil storage in secondary minerals. Biomass storage in living and dead tissue accounts for about 60 percent of the total K input, less than 2 percent is stored in soil, and the remainder exits in stream water. Biomass and soil storage account for about 6 and 13 percent, respectively, of the Mg and Ca inputs. These results show that plants at Skorradalur rely almost exclusively on atmospheric inputs and biomass recycling to provide K for growth. Moulton, K.L., West, J., and Berner, R.A., 2000, Solute Flux and Mineral Mass Balance Approaches to the Quantification of Plant Effects on Silicate Weathering, American Journal of Science, v. 300, p. 539-570.