Microbial residues and necromass comprise the majority of soil organic matter (SOM), yet the mechanisms controlling the formation and stability of microbial C are still poorly understood. We designed a series of experiments to determine how mineralogy influenced the initial association and subsequent destabilization of microbial C. We used a novel application of Raman spectroscopy to incubations with 13C enriched microbial necromass and pure minerals (amorphous aluminum hydroxide and feldspar). Raman spectroscopy allows for the simultaneous quantification and identification of living microbes, minerals, carbon, and highly 13C enriched substrates in one sample through time. This technique also allowed us to determine the mechanism of microbial C association with minerals of different C stabilization capacity. In a second experiment we used 13C-glucose to produce 13C enriched microbial residues at three soil depths (A, A/B, and B horizons) in soil columns. We then used water or dissolved organic matter (DOM) to assess the stability of those formed microbial residues during five wet-dry cycles over seven months.
Results/Conclusions
Using Raman spectroscopy we found that microbial C was more effectively stabilized by assimilation than sorption over the course of our incubation. We also found that mineralogy altered the pathway of microbe-mineral association, where microbial C sorbed to the amorphous Al mineral but required anabolism prior to association with feldspar. Because anabolism was an effective mechanism for microbial C retention, both minerals retained the same quantity of C after extraction despite the low C stabilization potential of feldspar. Therefore, mineral-associated microbial C formed through anabolism should be resistant to destabilization regardless of mineralogy. We used our column experiment to further test this hypothesis, and found that microbial C formed through anabolism was relatively resistant to destabilization by wetting and drying cycles with both water and DOM, with > 70% remaining in the A horizon and > 80% remaining in the A/B and B horizons. Moreover, microbial C retention did not scale directly with the quantity of clay minerals but rather resulted from both biotic transformation and abiotic stabilization. Taken together, these results provide new insight into our understanding of the mechanisms of microbe-mineral interactions in soils.