Self-organization is a process of internal feedbacks and local interaction among agents giving rise to spatial patterns on an otherwise homogeneous template. In this study, we explore mechanisms that generate the observed regular stripe patterning in soil profiles. In upland and wetland soils, we found strikingly regular banding in a soil profile at a certain depth. This banding consists of two types of patches that are different in color, soil texture, and biogeochemical processes: Fe-oxide-rich orange bands and organic-carbon and clay-rich gray bands. Bands of these two contrasting colors alternate with each other, with each band about 2-5 cm thick and horizontal to the soil surface. The questions we investigate are: (Q1) what are the underlying mechanisms that give rise to the regular banding, (Q2) what determines the starting depth and extent of the spatial banding, and (Q3) what variables control the emergence of such phenomena—as such banding does not occur in all soil profiles. We used a diffusion-reaction redox model, combined with empirical data collected from several soil pits under different hydrological conditions to address these questions.
Results/Conclusions
Model results show that the regular banding is self-organized—as a result of soil redox feedback between organic carbon (from roots), O2 concentration, Fe2+ concentration, and Fe3+ deposition, as opposed to a response to the pre-existing heterogeneity in soils. We found that such patterning could occur in soils of both imperfectly drained or even well drained upland ecosystems and in periodically inundated wetlands. For both types of systems, soils are periodically O2-limited: well-drained upland soils are O2-limited during and immediately after precipitation events. For wetlands, periodicity is imposed by the seasonality of wetland plant growth: during the growth season, a substantial amount of O2 is released from the roots of wetland plants, which intermittently reduces O2 limitation in wetland soils. Lastly, the depth at which the regular banding starts to appear, the depth at which it disappears, and the spatial characteristics of the pattern (e.g., color contrast between two patches, width of bands) are determined by the degree of O2 limitation and soil texture (which determines hydraulic conductivity). O2 limitation is further affected by precipitation regime, groundwater table, and plant physiology (ability for wetland species to transport O2 to roots and release of O2 to soil by roots).