Interactions Between Dissolved Organic Carbon and Aluminium Activity in Tropical and Temperate No‐Till Agroecosystems

Soil acidity and aluminium (Al) toxicity constrain crop production in both tropical and temperate agroecosystems, particularly under long‐term no‐till (NT) management where subsurface amelioration is limited. While soil pH amendments are widely used to mitigate acidity, the role of dissolved organic carbon (DOC) in regulating aluminium activity across soil depths remains insufficiently understood. This study evaluated DOC dynamics, aluminium activity (Al³⁺) and aluminium saturation within reserve acidity (PAl%) in long‐term NT experiments conducted in contrasting tropical (Brazil) and temperate (USA) soils. Across both systems, DOC concentrations declined with depth but exhibited greater stability at 5–10 cm compared with surface soil. Aluminium activity was suppressed with increasing DOC and converged towards sub‐phytotoxic levels at subsurface depths, even where soil pH varied minimally. These patterns indicate that DOC contributes to regulating Al³⁺ activity independently of pH in deeper soil layers. In the tropical Oxisol, PAl% values were relatively low and exhibited clear depth dependence. Soybean yield responded to PAl% in a segmented manner, with lower aluminium tolerance in surface soil and progressively higher thresholds at depth. When PAl% remained below critical depth‐specific thresholds, yield losses were minimized and positively associated with organic carbon variables. Collectively, these results demonstrate that aluminium toxicity under long‐term NT is governed by depth‐dependent interactions between pH and DOC. Integrating organic carbon management with conventional pH correction offers a more sustainable strategy for mitigating aluminium constraints and improving crop productivity in acid‐prone soils.