Weathering Intensity and Presence of Vegetation Are Key Controls on Soil Phosphorus Concentrations: Implications for Past and Future Terrestrial Ecosystems

Rebecca M Dzombak,Nathan D Sheldon

doi:10.3390/soilsystems4040073

Abstract

Phosphorus (P) is an essential limiting nutrient in marine and terrestrial ecosystems. Understanding the natural and anthropogenic influence on P concentration in soils is critical for predicting how its distribution in soils may shift as climate changes. While it is known that P is sourced from bedrock weathering, relationships between weathering, P, and other soil-forming factors have not been quantified at continental scales, limiting our ability to predict large-scale changes in P concentrations. Additionally, while we know that Fe oxide-associated P is an important P phase in terrestrial environments, the range in and controls on soil Fe concentrations and species (e.g., Fe in oxides, labile Fe) are poorly constrained. Here, we explore the relationships between soil P and Fe concentrations, soil order, climate, and vegetation in over 5000 soils, and Fe speciation in ca. 400 soils. Weathering intensity has a nuanced control on P concentrations in soils, with P concentrations peaking at intermediate weathering intensities (Chemical Index of Alteration, CIA~60). The presence of vegetation (but not plant functional types) affected soils’ ability to accumulate P. Contrary to expectations, P was not more strongly associated with Fe in oxides than other Fe phases. These results are useful both for predicting changes in potential P fluxes from soils to rivers under climate change and for reconstructing changes in terrestrial nutrient limitations in Earth’s past. In particular, soils’ tendency to accumulate more P with the presence of vegetation suggests that biogeochemical models invoking the evolution and spread of land plants as a driver for increased P fluxes in the geological record may need to be revisited.

Highlights

Phosphorus (P) is an essential, often limiting nutrient in ecosystems across the globe [1,2]
While the Principle Components Analysis (PCA) results (Supplemental Figure S10) do not support a relationship between P and weathering, we interpret this discrepancy as being due to the complexities within the P-weathering relationship rather than negating the observations made between latitude, weathering, and P concentrations because the PCA eigenvalues are so low, and essentially, are clustered on the origin, which indicates a not-predictive value
P’s relationship with weathering is complex and due to a variety of factors, and by looking only at the end product—which is what is left in the geologic record for analysis—we must inherently work around those limitations and draw the most robust conclusions possible from limited data on these large scales

Summary

Introduction

Phosphorus (P) is an essential, often limiting nutrient in ecosystems across the globe [1,2]. P has been studied in plants, soils, rivers, lakes, and oceans—both modern and ancient—and is at the center of critical questions about Earth’s past and future. Vegetation, and weathering and erosion rates are inextricably linked, so in order to improve quantitative constraints on potential P fluxes from soils, each of these factors must be considered. The redox-sensitive metal iron (Fe) is often associated with both P and organic matter/carbon (C) in soils (e.g., [6,7,8,9]). While much research linking P, weathering, and soil age has been done, such work has typically done on relatively small scales (e.g., chronosequences in Hawaii) relative to the global scale of the questions

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Conclusion