CONTINENT-SCALE TRENDS IN SOIL PHOSPHORUS, WEATHERING, AND IRON OXIDES: IMPLICATIONS FOR SOIL FERTILITY UNDER CHANGING CLIMATES

Phosphorus (P) is an essential, limiting nutrient in terrestrial environments and is unique in that it is sourced solely from bedrock weathering. Understanding climatic, environmental, and chemical controls on its concentration and bioavailability in soils is critical for predicting changes in soil fertility and ecosystem composition as climate changes. Soil/surface age is typically considered a primary control on P concentration, and the presence of Fe oxides can limit P bioavailability due to effective sorption. To test these patterns and to quantify landscape-scale variability of P, Fe, weathering intensity, and other soil-forming factors, we compiled geochemical and environmental data from over 5000 soils from the Northern Hemisphere, including those collected in the field, gathered from the USDA KSSL soil repository, and from a large USGS soil geochemistry database (Smith et al., 2014). Additionally, we performed a sequential Fe extraction on a subset of soils to characterize Fe phases and test the strength of P-Fe_oxide relationships on large scales.

Our results conform several key, basic assumptions about soil P and weathering. (1) Soil P concentrations peak at moderately-weathered soils (CIA ~60; Entisols-Inceptisols); (2) Soil weathering intensity generally decreases with latitude; therefore (3) lower mid-latitude soils have the highest P concentrations. Additionally, our work revealed that the mere presence of vegetation (as opposed to dominant vegetation type) is critical for the accumulation of P in subsurface horizons. Finally, the expected positive correlation between P and Fe oxides was not present, suggesting that smaller-scale chemical interactions may be important to consider for that relationship. These patterns should improve predictions for soil P availability as regional precipitation regimes shift and gain/lose vegetation. Constraints on these relationships could also improve biogeochemical models of ancient terrestrial nutrient cycles.