Sorption of organic matter (OM) to mineral surfaces is a primary process governing its persistence by rendering it unavailable for microbial degradation. How climate change-induced shifts in temperature and moisture influence the bioavailability of previously-bound OM remains highly uncertain. We used a short-term laboratory incubation to test the combined effects of temperature (4˚ vs 20˚C) and moisture (50 vs 100% water-filled pore space, WFPS) on microbial respiration, biomass and community composition in capillary fringe-sediments (Nisqually River, Mt. Rainier, WA) maintained under different mineralogical conditions (illite amended vs un-amended). We also measured changes in the molecular composition of soluble-OM using high-resolution, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). We hypothesized that OM in clay-amended sediments would be less vulnerable to changes in moisture and temperature than OM in un-amended, natural sediments as the OM would be protected from decomposition via mineral sorption. We also hypothesized that increased temperature, more than a change in moisture content, would alter mineral-OM interactions in clay-amended sediments resulting in more transformations of OM at 20˚C than at 4˚C.
At 4˚C, CO2-C was more sensitive to changes in moisture content in clay-amended sediments, with the highest CO2-C emitted from clay-amended sediments held at 50% WFPS compared to sediments under saturated conditions at 4˚C. At 20˚C, unexpectedly more CO2-C was emitted from un-amended, saturated sediments (100% WFPS), compared to clay-amended sediments maintained at 50 and 100% WFPS. This may be due to an increased abundance of microaerophilic organisms. Cumulative CO2-C was positively correlated with many FT-ICR-MS metrics, such as the nominal oxidation state of carbon (NOSC, p = 0.0004), aromaticity (AI, p = 0.0004) and the double-bond equivalent (DBE, p < 0.0001), suggesting that complex forms of OM are thermodynamically available for microbial degradation. We also observed increases in NOSC, AI and DBE, but only in clay-amended sediments maintained at 50% WFPS. In addition, decreases in lipid-, lignin- and tannin-like compounds were mostly observed in clay-amended sediments maintained at 50% WFPS, compared to un-amended sediments under saturation. Our results, contrary to our hypotheses, show that OM transformations were most vulnerable in clay-amended sediments compared to un-amended, natural sediments and that moisture content, rather than temperature, was more important for transformations in OM composition.