Imaging organo-mineral associations of creek sediments

Interactions of organic matter with mineral surfaces are seen as one of the important mechanisms to increase carbon preservation in soils. Often, the mineral associated organic matter is assumed to consist of microbial derived material, because of its small C/N ratio and its isotopic signature.

We sampled sediments and surface water flocs from a small creek (pH 6.4) to obtain natural samples with a much higher microbial versus plant derived organic matter input than expected for soils. The bulk material was investigated by CNS analysis, X-ray diffraction (XRD), and infrared spectroscopy (FTIR). Organo-mineral associations were imaged by a combination of atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray fluorescence spectroscopy (STXM-XRF) at 2550 eV (S, P, Si, Al, Fe) and 320 eV (C) at a spatial resolution of 50 nm. The speciation of C and P was addressed by near edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS). Synchrotron measurements were performed at the PO4 beamline at PETRA III using the Animax STXM endstation with a 4-channel fluorescence detector with a solid angle of detection of up to 1.1 sr.

Organic matter was mainly found on Fe oxides (ferrihydrite). However, the C concentration on the Fe oxides varied and some Fe oxides were not covered by organic matter. Clay minerals (mainly illite) were either free of organic matter or showed a lower concentration of organic matter than the Fe oxides. Phosphorus was only observed on some of the Fe oxides surfaces and its P K-edge NEXAFS spectrum usually showed a small pre-edge peak at ~2150 eV, which can be taken as evidence for inner-sphere Fe-O-P bonds. Although Fe oxides were often found in close proximity of bacterial cells, the Fe oxide-associated organic matter was rich in carbonyl C and O-alkyl C, but showed higher contributions of aryl C and/or alkyl C than pure extracellular polymeric substances (EPS) or bacterial cells.

Our observations confirm a high reactivity of Fe oxides towards organic matter and phosphate. However, the Fe oxides were not fully coated, i.e. saturated with organic matter. The mineral associated organic matter was not similar to EPS or bacterial cells.