Abstract
Increasing exports of Fe and DOC from soils, causing browning of freshwaters, have been reported in recent decades in many regions of the northern hemisphere. Afforestation, and in particular an increase of Norway spruce forest in certain regions, is suggested as a driver behind these trends in water chemistry. In this study, we tested the hypothesis that the gradual accumulation of organic soil layers in spruce forests, and subsequent increase in organic acid concentrations and acidity enhances mobilization of Fe. First generation Norway spruce stands of different ages (35, 61, 90 years) and adjacent arable control plots were selected to represent the effects of aging forest. Soil solutions were sampled from suction lysimeters at two depths (below organic soil layer and in mineral soil) during two years, and analyzed for Fe concentration, Fe speciation (XAS analysis), DOC, metals, major anions and cations. Solution Fe concentrations were significantly higher in shallow soils under older spruce stands (by 5- and 6-fold) than in control plots and the youngest forest. Variation in Fe concentration was best explained by variation in DOC concentration and pH. Moreover, Fe in all soil solutions was present as mononuclear Fe(III)-OM complexes, showing that this phase is dominating Fe translocation. Fe speciation in the soil was also analyzed, and found to be dominated by Fe oxides with minor differences between plots. These results confirmed that Fe mobilization, by Fe(III)-OM complexes, was higher from mature spruce stands, which supports that afforestation with spruce may contribute to rising concentrations of Fe in surface waters.
Highlights
Exports of iron (Fe) from catchment soils to surface waters are increasing in many northern regions and are observed as rising Fe concentrations in surface waters (Bjorneras et al 2017; Neal et al 2008; Sarkkola et al 2013)
The higher soil Fe mobilization under older stands of Norway spruce, which we show here, is in line with previous suggestions that trends of increasing Fe concentrations and water color in surface waters are at least partly linked to afforestation (Bjorneras 2019; Skerlep et al 2020)
Hydrology exerts a primary control on site specific fluxes of Fe and DOM and on which parts of the catchment that are hydrologically connected, with forested surface soil layers being the dominant source of Fe and DOM at high flow and wetlands dominating at low flow in boreal catchments (Bjorkvald et al 2008; Laudon et al 2011)
Summary
Exports of iron (Fe) from catchment soils to surface waters are increasing in many northern regions and are observed as rising Fe concentrations in surface waters (Bjorneras et al 2017; Neal et al 2008; Sarkkola et al 2013). The increase in Fe concentrations is generally synchronous with that of terrestrially derived dissolved organic carbon (DOC). These trends have received much attention in recent years, as they lead to an increase in water color, known as browning or brownification (Kritzberg and Ekstrom 2012). Wetlands and sediments, Fe mineral surfaces bind organic carbon and protect it from microbial mineralization, thereby acting as an important carbon (C) sink (Lalonde et al 2012; Wagai and Mayer 2007). Understanding the processes behind the increasing Fe mobilization and subsequent export to surface waters is important to predict the fate of C in the environment, and the potential effects on the global C cycle. Fe influences the mobility and transport of environmentally relevant elements, such as arsenic (Bauer and Blodau 2006; Cummings et al 1999; Mudroch and Clair 1986) and phosphorous (Johnson and Loeppert 2006; Jones et al 1993) in both terrestrial and aquatic ecosystems
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