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.