The chemical nature of soil organic phosphorus: A critical review and global compilation of quantitative data

Abstract

Historically, the chemical nature of organic phosphorus (P) in soil has largely been considered to comprise of recognizable biomolecules that predominantly include inositol phosphates, nucleic acids and phospholipids. However, these forms alone do not explain the existence of, or account for the processes responsible for, a larger pool of “unresolved” organic P that exists in soils. We critically reviewed the historic literature and carried out a global compilation of quantitative data to understand the chemical nature of soil organic P, including insight on what might constitute unresolved forms. We identified five key eras spanning the mid-1800s to current. Understanding of the chemical nature of organic P has largely reflected the predominant analytical technique in use, which generally involved focusing on a particular class of organic P. While inositol phosphates have been a focus throughout most eras, quantitative data reveal that the composition of the majority of organic P (typically>50%) in soil remains unresolved. Insight on its chemical nature has revealed that it is largely comprised of phosphomonoesters (P–O–C) and is associated with large molecular weight fractions, including the soil humic acid fraction. Furthermore, there is strong evidence that this is concomitant with the existence of a broad spectral feature that appears along with sharp peaks attributable to specific compounds in the phosphomonoester region of solution ³¹P nuclear magnetic resonance spectra. Here, we highlight the need to improve our understanding of the chemical nature and cycling of diverse forms of organic P in soil, including that of “unresolved” pools. This will necessitate the use of multiple techniques and approaches in soil biogeochemistry that require a holistic approach to understanding soil organic matter dynamics, and the association of organic P with soil organic carbon.