Boosting soil carbon and nitrogen stocks by increasing soil test phosphorus

The phosphorus (P) availability's role in carbon (C) and nitrogen (N) accumulation in long‐term systems remains unclear. This study evaluated the P fertilization's influence on C and N storage, C:N ratio, humic matter, and the C:clay ratio in two long‐term corn ( Zea mays L.)/soybean [ Glycine max (L.) Merr.] rotation trials under conservation tillage in North Carolina. Soil samples were collected at 0–5, 5–10, 10–20, and 20–30 cm. A linear‐plateau model evaluated the effect of soil test phosphorus (STP), from long‐term fertilization, on C and N stocks at 0–10, 0–20, and 0–30 cm. Both sites exhibited depth‐based STP gradients, although P rates significantly affected C stocks only in the 0–10 cm layer at Tidewater. P availability influenced C stocks at both sites, with greater P content and a higher critical soil test phosphorus value (CSTV) in Tidewater. CSTVs derived from C and N stocks were strongly correlated with those based on relative crop yield ( R 2 = 0.99). On average, the sandy soil at Tidewater accumulated more C than the clayey soil at Piedmont, reflecting differences in C stabilization. Maintaining soil test phosphorus near the CSTV increased C stocks by 2.1–2.7 Mg ha − 1 and N stocks by 0.2–0.3 Mg ha − 1 across the evaluated depths, contributing to improved soil fertility and agroecosystem resilience. Piedmont soils, despite lower total C stocks, showed greater C storage potential due to higher clay content, reinforcing the need for site‐specific P management adapted to soil texture and C stabilization capacity.