Note on the unparallel vertical distribution of nitrate and sulfate in Mollisols

Soil fertility diagnosis often omits subsoil measurements, impacting precision. Our objective was to compare the vertical distribution of nitrate and sulfate in agricultural Mollisols. Both anions were measured in 34 Mollisols of the Pampean region (Argentina) sampled to 160-cm depth at 20 cm intervals. Nitrate exhibited a continuous downward trajectory, with maximum values at 0–20 cm (12.7 mg N kg⁻¹) and minimum values at 140–160 cm (3.3 mg N kg⁻¹). Sulfate displayed a sinuous pattern, with a minimum at 60–80 cm (3 mg S kg⁻¹). The 60–160/0–160 cm concentration ratio was 42% for nitrate and 60% for sulfate, indicating greater topsoil stratification for nitrate. Predicting deep-layer nitrate concentrations from topsoil data was more accurate than for sulfate. This poses a challenge for assessing soil S bioavailability, as subsoil sulfates go undetected in conventional sampling.