Depth for time substitutions within the critical zone: The biogeochemistry and microbial ecology of soil depth
The critical zone can be defined as a constantly evolving region of the Earth’s surface where rock, soil, water, air, and living organisms interact. This region within the soil subsystem evolves vertically due to differential rates of mineral weathering and organic matter inputs as a function of depth. For instance, weathering intensity and rates of organic matter input are greatest near the surface, while they are both lowest deep in the soil profile. Such spatial patterns in soil processes are also observed during soil and terrestrial ecosystem development through time, where young land surfaces are sparsely weathered and contain little organic matter, while older land surfaces are relatively strongly weathered with substantial accumulations of organic matter due to detrital inputs from vegetation over long time periods. Because of these potential similarities in critical zone structure and function across space and time, we applied conceptual models of how biogeochemistry and microbial ecology of entire soil profiles evolve over time to individual soil profiles in space as a function of depth.
Our analyses suggest that biogeochemical and microbial changes across time are similar to those that occur vertically across space. For example, the Walker and Syers (1976) model of changes in soil phosphorus pools and availability during soil development across time was applicable to individual soil profiles at a given point in time. Additionally, we hypothesize that the metabolic development of the soil microbial community from autotrophic-dominated metabolisms on organic matter-poor, young land surfaces to primarily heterotrophic metabolisms on older, organic matter-rich land surfaces will be similar to metabolic changes that occur from deeper to shallower soils. We argue that these similarities in biogeochemical and microbial patterns across vertical space and time provide a powerful framework for elucidating and modeling changes in critical zone structures and function in a changing world.