Integrating decadal and century-scale root development with longer-term soil development to understand terrestrial nutrient cycling

Nearly 50 y ago, Walker and Syers hypothesized that sources of most terrestrial nutrients shift in dominance from mineral- to organic matter-derived over millennia as soils weather. We investigated how overlaying this soil development framework with vegetation dynamics that can feed back to soil development on relatively short timescales offers insight into ecosystem functioning. To test the hypothesis that forest nutrient economies mediate the nutritional importance of organic matter as mineral weathering proceeds, we paired litterfall decay experiments with soil mineralogical data from diverse forests across the Critical Zone (CZ) Observatory Network, USA. Our findings suggest that dominant sources of tree P may shift from organic matter-bound stocks to minerals as roots expand during the transition from mid to late stages of forest growth and encounter deeper soils that have experienced a lesser degree of weathering. Thus, plants may develop nutritional strategies that do not necessarily rely most heavily on the dominant P form present in an ecosystem, typically driven by stage of soil development, but rather on root proliferation over time, which governs the ability of plants to mine soil volumes at a diversity of depths. Ecosystem P nutrition therefore depends strongly on the interaction between dominant P form and root system growth, particularly as it reflects past land use for both plants and soils. We use these findings to produce a novel framework of vegetative nutrient economics that highlights how root system growth and land use change can influence nutrient transformations and bioavailability, and soil development, across Earth's critical zones.