Accumulation of phosphorus fractions in contrasting soils under long‐term phosphate fertilization

Understanding phosphorus (P) dynamics in soils under conservation agriculture remains challenging because the long‐term effects of fertilization rates and soil texture on P accumulation, availability, and environmental risk are not being fully understood. This study evaluated P fraction accumulation and saturation indices across soil layers in response to increasing phosphate fertilizer rates in two long‐term experiments in North Carolina. The trials were conducted on Portsmouth soil (fine‐loamy over sandy or sandy‐skeletal, mixed, semiactive, thermic Typic Umbraquults) at Tidewater, managed under minimum tillage, and Lloyd soil (fine, kaolinitic, thermic Rhodic Kanhapludults) at Piedmont, under no‐tillage. Soil samples from 0‐ to 5‐cm, 5‐ to 10‐cm, 10‐ to 20‐cm, and 20‐ to 30‐cm depths were analyzed in 2022 using sequential chemical fractionation and P‐related indices, including P sorption and degree of P saturation (DPS). Most P fractions were significantly influenced by P rates and depth. In clayey Piedmont soil, occluded P reached 58% of total P and increased linearly with rates (up to 30 cm). Sandy Tidewater soil showed higher soluble P (up to 4 mg kg −1 at 0–5 cm) and DPS values reaching 40%, signaling environmental risk. The DPS index proved sensitive to increasing P fertilization, outperforming the P sorption index. Mehlich‐3 P exceeding 169 mg kg −1 in sandy soil indicates a contamination risk threshold due to elevated soluble P. Different behaviors of P fractions, especially occluded P, highlighted the importance of soil‐specific fertilization strategies and considering P saturation as essential for optimizing P use and mitigating environmental impacts. The DPS index emerges as a valuable tool for assessing fertilization history and guiding P management strategies.