Divergent roles of iron and aluminum in sediment organic matter association at the terrestrial–aquatic interface

Organic matter (OM) can be protected from abiotic and biotic breakdown via its association with iron (Fe) and aluminum (Al) in both terrestrial and aquatic ecosystems. We hypothesized that the natural variation in sediment redox and pH regime govern how the two metals interact with OM in near-surface mineral sediments of 40 kettle holes of varying hydroperiod in NE Germany. Sediments were separated by density, and Fe and Al present in the high-density fractions (> 1.6 g cm−3, HF) were quantified by pyrophosphate (PP) and dithionite-citrate (DC) extractions. The OM in HF was analyzed for δ13C and δ15N values before and after the extractions to assess the possible origin and its degradation state. While only < 20% of OM in HF were dissolved by PP and DC extractions, OC in HF showed significant positive correlation with the extractable Fe and Al. DC-extractable Fe was predominantly present as low-crystallinity phase, and mostly overlapped with PP-extractable and, for the selected samples examined, with acid-oxalate and ascorbic acid extractable Fe pool. Across the HF samples, high levels of AlPP and AlDC contents corresponded to sediments having lower pH and OM with lower δ15N and higher C:N ratio. In contrast, the sediments with higher FeDC contents corresponded to shorter hydroperiod and showed OM with higher δ15N and lower C:N ratio. These results support the idea that reactive monomeric Al preferentially binds with organic ligands derived from less-decomposed OM under more acidic and anoxic conditions, whereas low-crystallinity Fe formed under more oxic conditions tend to bind with more microbially-processed OM.