Effects of long-term tillage regimes on the vertical distribution of soil iron/aluminum oxides and carbon decomposition in rice paddies

Tillage regimes play an important role in soil organic carbon (SOC) accumulation and atmospheric carbon mitigation. However, the mechanism of SOC decomposition induced by tillage regimes is not clearly understood, particularly related to SOC protection by soil iron (Fe) and aluminum (Al) oxides. Therefore, we assessed three types of soil Fe/Al oxides (Non-crystalline oxides, Feo/Alo; organic bound oxides, Fep/Alp; total free Fe/Al, Fed/Ald), and their relationship with SOC mineralization at the 0–50 cm soil depth after long-term tillage practices in a double rice cropping system in southern China. Four tillage practices were investigated: conservation tillage (no tillage + rice residue mulch, NTS), traditional tillage including rotary tillage (RTS), and plow tillage with or without rice residue incorporation (CTS and CT, respectively). The Feo and Fep/Alp had lower concentrations in deeper soil layers compared with the topsoil. At the 0–50 cm soil depth, the Feo content in the NTS treatment was 1.55 mg g−1, which was 7.5%, 9.3%, and 14.3% higher than that under RTS, CTS, and CT (P < 0.05), respectively. Additionally, the Feo and Fep/Alp were correlated (P < 0.05) with SOC content and SOC mineralization. At the 0–50 cm soil depth, CT decreased almost all soil Fe/Al oxides as compared with the three residue retaining treatments. Higher SOC content was found with the application of residue retention particularly under NTS and RTS, possibly due to SOC and Fe/Al oxides bonding/protection. Furthermore, we discovered a potential source of SOC loss induced by tillage operation (particularly plow tillage), which inverts the deep soil (5–20 cm) to the surface, as deep soil lacks protection from Fe/Al oxides and has significant SOC mineralizability.