Iron reservoirs in a forested headwater catchment: stocks and isotopy

Abstract

An increasing number of studies demonstrate the potential of Fe isotope ratios as a tool to identify soil formation processes, to pinpoint Fe translocation processes within soils and to determine the geologic source of Fe (1, 2). Fe isotope ratios of the suspended river load have recently been used to determine the geologic source of Fe in the Amazon river system (dos Santos Pinheiro et al., 2013) and the correlation of dissolved organic carbon (DOC)/[Fe] with δ56Fe of the dissolved load was identified as an indicator for authigenic Fe input (3).The Wüstebach catchment is a small forested subcatchment (~38.5 ha) located within the National Park Eifel at the southernmost outreach of the Rur catchment. It is part of the TERENO Eifel/Lower Rhine Valley Observatory (4) which aims at monitoring the long-term impacts of environmental change at a regional scale. The site is equipped with in situ sensors monitoring constantly a variety of parameters (5). At this catchment we aim to identify the magnitudes of different Fe stocks as well as sources and sinks of the Fe cycle. Evaluation of the Fe concentration data in the dataset paper of Gottselig et al. (6) revealed a stock of 33.6 t of plant available Fe (extracted with CaCl2/pentatic acid) in the top 30 cm of soil. The total stock was estimated to be approx. 2770 t (extracted by pressurized microwave digestion) in the top 30 cm of soil. The vegetation consisted mainly of Norway spruce (90%). The Fe stock contained in the vegetation was estimated to be approx. 1.6 t. Monitoring of the Fe concentration in the stream water over 5 years revealed that, depending on hydrological conditions, the catchment loses between 18 and 57 kg dissolved Fe per year. First Fe isotope measurements of environmental samples (stream water, soil horizons, spruce needles and beech leaves) revealed a range of δ56Fe values from -1.82 to 0.78 ‰ (values relative to IRMM-14). Vegetation samples represent the lightest reservoir with values of -0.61 ‰ (spruce needles) and -1.82 ‰ (beech leaves). In contrast, soil samples ranged from -1.63 to 0.61 ‰ showing the effects of litter recycling in the top soil and redox variations in the subsoil, respectively. The stream water therewith represents a sink of Fe from different sources and liberation processes.References:(1) Fekiacova Z., Pichat S., Cornu S., Balesdent J., 2013. Geoderma 209-210, 110-118.(2) Dos Santos Pinheiro, Poitrasson F., Sondag F., Cruz Vierira L., Martins Pimentel M., 2013. Journal of South American Earth Sciences, 44, 35-44.(3) Chen J-B., Busigny V., Gaillardet F., Louvat P., Wang Y-N., 2014. Geochimica et Cosmochimica Acta 128, 128-143.(4) Bogena, H., Kunkel, R., Pütz, T. et al., 2012. TERENO - Long-term monitoring network for terrestrial environmental research. Hydrol. Wasserbewirts. 56, 138-143.(5) Bogena, H.R., Bol, R., Borchard, N. et al., 2015. A terrestrial observatory approach to the integrated investigation of the effects of deforestation on water, energy, and matter fluxes. Sci China Earth Sci 58, 61-75.(6) Gottselig, N., Bol, R., Nischwitz, V., Vereecken, H., Amelung, W., Klumpp, E., 2014. Distribution of phosphorus-containing fine colloids and nanoparticles in stream water of a forest catchment. Vadose Zone J. 13