Walker and Syers in the critical zone: Integrating successional dynamics and soil development

Abstract:

Walker and Syers developed the widely accepted concept that most nutrients, especially P, transition from primarily mineral-bound to organic matter (OM)-bound as soils develop. This concept addresses the role of soil development in plant nutrition but does not fully incorporate the role of decadal scale root system development in subsequent longer-term soil development. Here, we reorient Walker and Syers’ soil developmental framework toward the whole critical zone (CZ), hypothesizing that temporally-varying rooting system nutrient economies mediate the role of OM in forest nutrition as weathering proceeds. We selected CZ Observatory Network sites representing a continuum of soil development and forest stands in different stages of rooting system development. We performed leaf litter decay experiments to determine the potential nutritional role of OM-bound P as it is released during decay across these CZs. In young, relatively shallowly rooted forests, mineral-bound P comprised a greater proportion of forests’ nutrient demands as originally proposed, but the original hypothesis did not fully capture the nutrient dynamics of middle aged and older forests. In middle aged forests, OM decomposition could match most if not all of the vegetation’s P demands regardless of the developmental stage of the underlying substrate, while very old stands exhibited a mineral-based P economy, likely owing to their comparatively deep rooting systems. These forest age-related dynamics produced a relationship between soil development and potential OM-bound nutrient provision that was best represented by a non-linear functional form (P =0.07; r² = 0.41). Further, we observed that rooting system economies produced rooting-zone soils that appear comparatively less P deficient than less weathered soils as a consequence of nutrient uplift, producing a nonlinear, positive relationship between stand age and soil P enrichment (P = 0.008, r² = 0.87). These patterns are accentuated by human activity, which alters soil developmental patterns and the rooting systems that encounter them. Thus, we demonstrate how root system growth, and associated land use history, interact with soil development to generate geochemical signals in Earth’s CZ.