How do microbial communities influence the formation rate, stability, and chemistry of soil organic matter?
Soil organic matter (SOM) comprises the largest terrestrial sink of carbon and its concentration is directly related to soil fertility. SOM sink size is determined by the balance between SOM formation and loss, requiring understanding of both processes to predict SOM stocks under global change. SOM loss processes have received most attention, given concerns that microbial-mediated decomposition will be stimulated by global changes such as climate warming. An emerging thesis, however, is that soil microbial activity also affects SOM formation rates because microbial-derived compounds are the primary constituents of stable, long-term SOM stores. Under this thesis, microbes degrade, assimilate and biosynthesize plant inputs to soils prior to SOM stabilization. Multiple hypotheses suggest that variation in environmental factors – such as temperature and nutrient availability – affect microbial community composition and physiology and that these properties, in turn, affect the formation rate, stability and chemical composition of new SOM. I evaluate these hypotheses with empirical data from a set of lab and field studies where stable isotope tracers are used to track the fate of low molecular weight carbon (glucose, glycine and sucrose) inputs to soils under varying temperature and availabilities of nitrogen, phosphorus and carbon.
Microbial community properties are not thought to influence the decomposition rate of stable, mineral-associated SOM stocks. These stocks may, however, respond to microbial community properties that affect the formation rate and stability of new SOM. Higher carbon amendment rates result in distinct shifts in microbial communities from dominance by oligotrophic to copiotrophic phyla. These shifts correlate with greater absolute amounts of SOM formation, but the fraction of added carbon that forms SOM is greater under low carbon amendments. This may suggest that oligotrophic microbes have higher growth efficiencies than copiotrophs, which is consistent with a K vs. r life history strategy. Elevated temperatures theoretically lower the growth efficiencies of microbial communities and hence should also cause lower SOM formation rates. Individual heterotrophic soil microbes display unimodal growth efficiencies across temperature, but SOM formation rates at 15 and 30oC do not differ. Nitrogen addition does stimulate growth efficiency, leading to higher SOM formation. None of the environmental variables influence the stability of new SOM. Microbial biosynthesis creates SOM with chemical signatures distinct from plant material, but there is uncertainty as to how differences in microbial properties might translate to SOM chemistry. Microbial properties influence SOM formation but their effects on stability and chemistry are uncertain.