Changes in soil physical structure and microbial physiology in a warming world: Consequences for ecosystem carbon dynamics
A large and poorly understood component of global warming is terrestrial carbon-cycle feedback to the climate system. Simulation experiments with fully coupled, three- dimensional carbon-climate models suggest that carbon-cycle feedbacks could either accelerate or slow climate change over the 21st century, but gaps in our basic understanding of terrestrial ecosystem processes mean that both the sign and magnitude of these feedbacks in the real Earth system are highly uncertain. The chemical conformation (quality) of soil organic matter (SOM) and its physico-chemical protection are both implicated in its resistance to decomposition. The former is embodied in the depolymerization process, the latter by adsorption/desorption and aggregate turnover, and both factors may vary in their temperature sensitivity. We hypothesize that changes in microbial community functional capacity as well as changes in growth strategy are associated with decreased physical protection of soil organic matter. This work examines the effect of 23 years of warming in temperate terrestrial forest soils at the Harvard Forest Long-Term Ecological Research site in central Massachusetts. We evaluated the warming effect on soil aggregate size distribution as well as enzyme activities and markers of changing substrate availability in response to long-term warming.
Long-term warming resulted in an increase in enzyme activity on a per total organic nitrogen basis, but not on a per gram soil basis, suggesting increased enzyme efficiency with warming. Changing substrate availability may also feed back to altered community function, as evidenced by an altered capacity for soil microbial communities to metabolize certain carbon substrates and by changes in soil aggregate size structure. Changes in soil aggregate sizes may reflect altered physical protection, and it is known that soil aggregate size varies with enzyme activity, carbon chemistry, and microbial community profiles. Communities exposed to long-term warming may adjust their trophic strategy towards one that is more oligotrophic. This trend is reflected in the microbial physiological characteristics of trophic strategy including reduced ribosomal RNA operon copy number and average genome size, both of which are significantly reduced in oligotrophic compared to copiotrophic microorganisms. The mechanism of soil microbial community acclimation to reduced and lower quality soil substrates is not well understood but under investigation. This microbial acclimation would be consistent with the soil microbial community enacting a self-reinforcing feedback to the climate system due to warming. Together, these data should improve incorporation of microbial parameters into climate models.