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 (R² = 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.