Long-term Conservation Agriculture increases sulfur pools in soils together with increased soil organic carbon compared to conventional practices

Long term depletion of sulfur (S) in soils is common in many cropping systems globally, and especially in intensive, rice-based rotations in Asia. We hypothesized that Conservation Agriculture (CA) practices (minimal soil disturbance and crop residue retention) will increase S in soils primarily through changes in soil organic carbon (SOC). A long term experiment was sampled after 24 crops of continuous practice of (A) contrasting soil disturbance (strip planting = SP and conventional tillage = CT) and (B) two levels of crop residue retention (low residue = LR and high residue = HR). Sulfur fractions and dynamics in soil at five depths (0-5, 5-15, 15-30, 30-45 and 45-60 cm) along with bulk density (BD), pH, SOC, total nitrogen (TN) and extractable nutrients were determined. Levels of total S (584 - 668 mg kg(-1)), organic S (50.0- 153 mg kg(-1)), available S (7.9 - 21.6 mg kg(-1)) and water soluble S (6.0 - 17.0 mg kg(-1)) were the highest in SP with HR (P < 0.05), while inorganic S (505 - 533 mg kg(-1)) and adsorbed S (2.3 - 4.4 mg kg(-1)) were significantly higher (P < 0.05) in HR plots irrespective of tillage systems at 0-30 cm soil depth. All the S fractions were positively and strongly correlated with SOC and except for inorganic and adsorbed S, all other S fractions were positively correlated with TN, extractable P, Zn, B and Fe. All S fractions were negatively correlated with BD. Inorganic S was the dominant S fraction but highly significant and positive correlations of available S with organic S (r = 0.92) and water-soluble S (r = 0.92) suggests these two fractions were the main sources of plant available S. This study suggests that both minimal soil disturbance and increased crop residue retention, core components of CA, increased S pools in soils primarily due to increased SOC sequestration.