Contrasting catchment soil pH and Fe concentrations influence DOM distribution and nutrient dynamics in freshwater systems

Organic matter (OM) quantity, quality, and nutrient dynamics within twelve shallow lakes in the Czech Republic were assessed in the context of catchment soil pH and iron (Fe) concentration. The catchments of the lakes were classified into two categories: (i) slightly acidic (soil pH = 5.1–6.3) with Fe-rich soils (H_Fe; Fe = 315–344 mg kg−1 in Mehlich 3 extract); and (ii) neutral (soil pH = 6.8–7.6) with Fe-poor soils (L_Fe; Fe = 126–259 mg kg−1 in Mehlich 3 extract). The quality of OM in the two lake types was characterized using a combination of spectroscopic techniques (UV–Vis, fluorescence, and Fourier Transform Infrared spectroscopy). We show that dissolved nutrient and dissolved organic carbon (DOC) concentrations, as well as the amount of aromatic and protein-like compounds in the water column and sediment porewater were significantly (p < 0.01) lower in the H_Fe lakes compared to the waterbodies located within L_Fe catchments. The FTIR analyses of the H_Fe sediments contained higher relative concentrations of aromatic compounds with hydroxyl-containing functional groups and carbohydrates, while more aliphatic and oxidised OM was found in the L_Fe lake sediments. These results suggest that the pH value of catchment soils and, particularly, their Fe content have profound geochemical effects on the mobility of OM and nutrients in the sediments of recipient waters. Because the OM-Fe association stabilises OM in sediments, waterbodies within L_Fe catchments are likely more vulnerable to increasing eutrophication and oxygen depletion compared to those in H_Fe catchments and this has important implications for water quality management, risk assessment, and predictions of aquatic ecosystem vulnerability under conditions of accelerating climate change. [Display omitted] •Soil Fe content and pH in catchment is critical to the fate of OM in recipient lakes.•Higher stability of organic matter against biodegradation in Fe rich sediments.•Fe rich sediments have lower mobility of phosphorus and ammonia‑nitrogen.•Sediments in acidic and Fe rich areas are enriched in carbohydrates and aromatics.•Sediments in neutral and Fe poor areas are enriched in aliphatics and carboxylics.