Phosphorus fractions status of representative agricultural fields in North Carolina

Most agricultural soils in North Carolina have high levels of available phosphorus (P), yet little is known about other accumulated P fractions and their associated environmental risks. This study characterized soil P fractions through sequential chemical fractionation of 391 samples (0-10 cm and 10-20 cm) collected from 57 counties across three regions (Tidewater, Coastal Plain, and Piedmont + Mountains). Metadata on soil management practices and crop history were also collected. Soil samples were chemically characterized and subjected to sequential fractionation using CaCl₂ (P_CaCl₂), Mehlich-3 (P_M3), NaOH (Po_OH, Pi_OH), and HCl (P_HCl) extracts. Occluded P (P_OCL) was determined by subtracting the sum of the previously extracted P fractions from the total P (P_T), which was obtained through soil digestion. On average, P_OCL was the predominant fraction (39%), followed by Pi_OH (26%), P_M3 (17%), Po_OH (11%), P_HCl (6%), and P_CaCl₂ (0.3%). Organic P forms accounted for only 11% of the total extracted P. The highest P_M3 content was observed in Coastal Plain soils (79 ± 9 mg kg^-1), which-like all regions-exceeded the critical soil test value for P in North Carolina (52 mg kg^-1). Canonical discriminant analysis distinguished soil regions, with Piedmont + Mountains showing higher clay content and P_OCL, Coastal Plain linked to higher Fe, sand content, and P_M3, and Tidewater linked to higher Al, acidity, humic matter, and C. Coastal Plain soils also exhibited the highest degree of P saturation (29%), followed by Piedmont + Mountains (24%) and Tidewater (15%), with 75% of the evaluated counties presenting a high risk of P contamination. Minimal differences in P fractions between tillage systems suggest excessive P fertilization and limited adoption of conservation practices, highlighting the urgent need for improved P management.