IS THE CHARACTERISTIC ELEMENTAL, ISOTOPIC AND BIOCHEMICAL COMPOSITION OF MINERAL-ASSOCIATED ORGANIC MATTER IN SOILS AND SEDIMENTS THE CONSEQUENCE OF SORPTION?

    The importance of organo-mineral associations in stabilizing organic carbon and nitrogen in soils, sediments and the marine water column have become increasingly appreciated over the last decade.  However, relatively little attention is given to how sorption processes might affect commonly used biochemical and isotopic compositional signatures relative to organic matter sources.  In an earlier study (Aufdenkampe et al. 2001, Limnol. Ocean. 46, 1921), we show that sorption processes are responsible for the characteristic low carbon-to-nitrogen (C/N) ratios (6-12), high amino acid contents (5-40% of OC and 10-70% N) and amino acid compositions (elevated basic and hydrophobic functional groups) of mineral-associated organic matter (OM) found in both sediments and soils.

    To follow up on these results, we conducted a set of experiments to examine whether other compositional signatures, such as d¹³C and d ¹⁵N, are affected by sorption under both sterile and microbially active conditions.  To adequately characterize kinetic and equilibrium processes, more than 150 incubations were performed over a variety of dissolved OM concentrations and a range of time scales.  In addition to confirming previous results, these experiments showed that sorbed OM exhibited isotopic signatures that were 1.5‰ to 2.4‰ enriched in ¹³C and 2‰ to 4‰ enriched in ¹⁵N relative to the initial dissolved OM.  These patterns likely result from preferential sorption of proteins, which in general are isotopically enriched relative to the bulk OM and biomass of sources.

    These findings are of substantial consequence to a wide range of biogeochemical applications.  For example, a very common observation in pristine soils is a 2-3 ‰ increase in d¹³C with soil depth, yet possible explanations continue to be hotly debated in the literature.  Given that these trends covary with increasing proportions with depth of mineral-associated OM versus low-density detrital material, our results could help resolve the mechanisms for observed d¹³C profiles, which is critical to modeling carbon turnover rates in soils.  In sum, our results suggest that the characteristic elemental, isotopic and biochemical composition of mineral-associated OM in soils and sediments are indeed largely a consequence of patterns of preferential sorption.