Mutually transformative interactions: The relationship between soil microbial function, aggregate longevity, and organic matter stabilization

Background/Question/Methods

Microaggregates (MAs) are small (< 250 µm) transient structures formed by the coalescing of mineral particles and soil organic matter (SOM). SOM within these structures typically exhibits longer transit times than SOC in bulk soil, suggesting that occlusion within MAs protects SOC from microbial decomposition and subsequent mineralization to CO2. Despite the apparent importance of MAs to long-term SOC stabilization, relatively little is known about their lifecycle or material structure (i.e., the amount, composition, and spatial distribution of their contents). Recent experiments have demonstrated that a meaningful fraction of persistent SOM is composed of microbial necromass. Evidence also suggests that necromass forms an important structural component of MAs by acting as a binding agent. As both contributors to and consumers of the SOM pool within aggregates, microbes remodel MA material structure in ways that may be significant enough to alter MA longevity. Here we investigate: 1) How do microbial community characteristics and MA material structure influence each other? 2) What structural characteristics promote MA longevity? 3) What is the relationship between MA lifespan and the transit time of occluded SOM?

Results/Conclusions

We define a conceptual framework that places soil MAs at the confluence of multiple processes influencing the dynamics of C flows in the soil. In this model, MAs can be understood as discrete microecosystems interconnected via fluid flows through surrounding pores. MA-associated microbial communities experience diachronous selective pressures as intra-MA environmental conditions vary over time in ways dependent on MA structure. Thus, community composition is temporally variable and driven by the degree to which an MA represents a closed system, the composition of occluded SOM, soil mineralogy, O2 depletion over time, moisture availability, and other MA characteristics. Collectively, the functional characteristics of MA-associated microbial communities sculpt the physical distribution, amount, and composition of SOM within an MA. We present a series of idealized community “snapshots” that demonstrate the mutually transformative interactions between microbial communities and MA material structure. Our model represents a novel framework for understanding biotic and abiotic controls on soil structure, and thus offers insight into the influence of microbial processes and soil aggregates and pore networks as they interact and feedback to Earth’s climate.