Long-term stabilization of crop residues and soil organic carbon affected by residue quality and initial soil pH

Residues differing in quality and carbon (C) chemistry are presumed to contribute differently to soil pH change and long-term soil organic carbon (SOC) pools. This study examined the liming effect of different crop residues (canola, chickpea and wheat) down the soil profile (0–30 cm) in two sandy soils differing in initial pH as well as the long-term stability of SOC at the amended layer (0–10 cm) using mid-infrared (MIR) and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. A field column experiment was conducted for 48 months. Chickpea- and canola-residue amendments increased soil pH at 0–10 cm in the Podzol by up to 0.47 and 0.36 units, and in the Cambisol by 0.31 and 0.18 units, respectively, at 48 months when compared with the non-residue-amended control. The decomposition of crop residues was greatly retarded in the Podzol with lower initial soil pH during the first 9 months. The MIR-predicted particulate organic C (POC) acted as the major C sink for residue-derived C in the Podzol. In contrast, depletion of POC and recovery of residue C in MIR-predicted humic organic C (HOC) were detected in the Cambisol within 3 months. Residue types showed little impact on total SOC and its chemical composition in the Cambisol at 48 months, in contrast to the Podzol. The final HOC and resistant organic C (ROC) pools in the Podzol amended with canola and chickpea residues were about 25% lower than the control. This apparent priming effect might be related to the greater liming effect of these two residues in the Podzol.