Residual phosphorus is a significant pool in soil phosphorus cycling

Authors

• Markku Yli-Halla Department of Agricultural Sciences, Viikinkaari 9, FI-00014 University of Helsinki, Finland https://orcid.org/0000-0002-8062-7019
• Risto Uusitalo Natural Resources Institute Finland (Luke), FI-31600 Jokioinen, Finland https://orcid.org/0000-0002-4427-9053

DOI:

https://doi.org/10.23986/afsci.180120

Keywords:

agricultural soils, inorganic P, P fractionation, residual P, total P

Abstract

Sequential phosphorus (P) fractionation schemes are widely used to characterize soil P pools, yet the most chemically resistant forms are often omitted in modern applications of fractionations. We quantified inorganic and organic P in ten predominantly clayic or fine sandy acidic agricultural soils from SW Finland, paying special attention to the residual P not recovered in the four common extractions of Chang-Jackson scheme (NH₄Cl-, NH₄F-, NaOH-, and H₂SO₄-extractable P). Total P (H₂SO₄-H₂O₂-HF) ranged from 1148 to 2467 mg kg⁻¹, with organic P constituting 15–49%. The four inorganic fractions represented on average 43% of total P. A substantial residual P pool (304–474 mg kg⁻¹) accounted for 25% of total P and 32–42% of inorganic P, exceeding the size of any individual P fraction. Our results suggest that routine fractionation may substantially underestimate soil inorganic P reserves and oversimplify our view on long-term P dynamics. This has implications for long-term P budgets and P availability under fluctuating redox conditions, including erosion-mediated P losses.