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During open-cast lignite mining in Lusatia (NE Germany), iron sulfide minerals associated with lignite, are exposed to air and weather. The weathering products iron (Fe), sulfate and acids subsequently enter the fluvial-lacustrine system Spree and its tributaries.
The aim of this work was to identify and explain the evolving biogeochemical signatures (spatio-temporal patterns) in River Spree. These signatures are significantly influenced by biogeochemical turnover processes triggered by iron input. To identify the signatures, a regional field study was conducted from the inland delta Spreewald to the mouth. This study was followed by two laboratory experiments under controlled redox conditions to investigate the effect of Fe on the long-term binding of phosphorus and the degradation of particulate organic material in the iron-enriched sediments.
Characteristic elements introduced by lignite mining (iron, sulfur, aluminum, cobalt, nickel) and by urban areas (zinc, lead, copper, chromium) could be determined by means of a principal component analysis. Based on these findings, the impact range of the open-cast mining was determined to be at least ~ 90 km downstream of the Spreewald. Within the urban-influenced systems Fürstenwalde and Berlin, the urban signature is more dominant than the mining signature. By means of sequential extractions and XRD analyses of the sediments, iron binding forms and characteristic iron minerals have been additionally identified (e.g., Fe(III) oxyhydroxides, pyrite, vivianite).
Sediment analyses and column experiments have shown that iron increases the phosphorus sorption and decreases the phosphorus release in the sediments of three studied lakes of the fluvial-lacustrine system. However, the application of different phosphorus retention models for Lake Neuendorfer See showed that Fe cannot reduce the eutrophic state of the lakes along the river, since the water retention times are too short and the external phosphorus loads are too high. Additionally, lake sediments with different Fe contents have been incubated with particulate organic material. Higher Fe contents resulted in reduced emissions of the greenhouse gases CO₂ and CH₄, which can be attributed to a reduced degradation of organic matter by Fe. Possible reasons include sorption processes of (dissolved) organic matter onto Fe oxyhydroxides and toxic effects of iron on microorganisms. Nevertheless, Fe (and sulfate) are available as electron acceptors during the decomposition of organic matter in River Spree.
The findings of this work deepen the understanding of the impact of iron from mining on sulfur, phosphorus and carbon turnover processes in fluvial-lacustrine systems. As a consequence of the Fe input from the Lusatian area, the signature will persist for decades in River Spree.
With regard to the depletion of global phosphorus reserves and with the aim of ensuring sustainable soil fertility on agricultural soils, a fundamental understanding of mechanisms of fixation and mobilization of inorganic phosphorus in soils is required. Amongst others, phosphorus availability is affected by ad- and desorption reactions on pedogenic Fe- and Al-hydroxide surfaces. The characterization of phosphate binding on those contrasting mineral surfaces can help to find solutions for enhancing the mobilization of fertilized but not available soil inorganic phosphate. Fourier-transform infrared spectroscopic experiments were carried out during phosphorus adsorption on crystalline gibbsite, poorly crystalline 2-line-ferrihydrite and amorphous Fe:Al-hydroxide mixtures. Desorption experiments with CaCl₂, CaSO₄, citric acid (C₆H₈O₇), and humic acid (C₉H₉NO₆) were conducted to determine the capacity of phosphate fixation and mobilization in short- and long-term. Additionally, phosphorus release from the Fe- and Ca-phosphates vivianite and hydroxyapatite were analyzed.
For gibbsite, the formation of AlHPO₄ and Al₂HPO₄ can be assumed, while for ferrihydrite, a FeHPO₄ or Fe₂PO₄ complex and the precipitation of FePO₄ with longer equilibration time were observed. Fe₂HPO₄ or a Fe₂PO₄ surface complex was deduced for amorphous Fe-hydroxides, an AlH₂PO₄ surface complex was identified for Al-hydroxides. The weakly associated amorphous FeO(OH) molecules enhance the precipitation of FePO₄. With high Al content, a weaker phosphate binding of both inner- and outer-sphere complexes and either no or minor quantities of precipitate were formed. Ferrihydrite showed a more rigid structure and a lower extent of precipitation compared to amorphous Fe-hydroxide. The cumulative phosphorus desorption followed the order CaCl₂ < CaSO₄ < humic acid < citric acid for crystalline and amorphous Fe- and Al-hydroxides as well for vivianite and hydroxyapatite. While inorganic anion exchange took part at easily available binding sites and fast exchangeable phosphorus, organic acids additionally affect the more heavily available binding sites and slow exchangeable phosphorus. For humic acid, the accumulation of metal-organic complexes in the desorption solution was suggested, whereas for citric acid the dissolution of the minerals was maintained. The cumulative release rates of the Flow-Through-Reactor setup were higher compared to batch due to a short residence time and a continuous concentration gradient. This could lead either to an over- or underestimation of the available phosphorus pools and influenced the comparability of both methods.
In NITROLIMIT wurde die bisher größte Datenbank zu Gewässergüteparametern aus 373 natürlichen Seen der norddeutschen Tiefebene zusammengestellt.
Die Auswertung dieser Daten zeigte:
• Der größte Teil der Seen befindet sich in einem mäßigen bis schlechten ökologischen Zustand.
• Sowohl Stickstoff als auch Phosphor sind eng mit der Phytoplanktonbiomasse korreliert. Als Prädiktor für N- bzw. P-Limitation wurde ein DIN:TP-Massenverhältnis von 1,6 ermittelt.
• N-Limitation tritt fast genauso häufig auf wie P-Limitation, wobei seentypspezifische und saisonale Limitationsmuster auftreten: N-Limitation tritt häufiger in flachen durchmischten Seen und Flussseen auf, und P-Limitation häufiger in tiefen geschichteten Seen. Im Verlauf der Vegetationsperiode findet häufig ein Wechsel von P-zu N-Limitation statt und zudem nimmt die Häufigkeit von anderen Limitationszuständen (beispielsweise Licht und Temperatur) zu.
• Die ermittelten TN- und TP-Zielwerte zum Erreichen der oberen Grenze der ökologischen Zustandsklasse „gut“ reichen für die verschiedenen Seentypen von 480-800 μg L-1 TN und 22-66 μg L-1 TP.
• Die derzeitigen TN- und TP-Konzentrationen in den Seentypen überschreiten die oben genannten Zielwerte in den meisten Fällen deutlich. Dies trifft insbesondere auf die TN-Konzentrationen zu. Der große Anteil von Seen mit einem mäßigen bis schlechtem ökologischem Zustand zeigt, dass zur Umsetzung der WRRL weitere Anstrengungen unternommen werden müssen.
Dabei bekräftigen unsere Ergebnisse, nach denen P-Limitation etwas häufiger als N-Limitation auftritt, die derzeitige Strategie, durch Maßnahmen zur Reduktion der Phosphoreinträge eine Verbesserung des ökologischen Zustandes herbeizuführen. Unserer Ansicht nach sollte diese Strategie fortgeführt werden, wobei auch strengere Reduktionsziele für Phosphoreinträge zu erwägen sind.