High C accumulation potential of deep soil via mineral association in Andisols, Japan.

Soil organic carbon (SOC) sequestration in deep soil (below 30 cm depth) is considered a promising strategy for climate change mitigation and has recently attracted increasing global attention. However, studies on the accumulation potential and mechanisms of newly added residue-derived C in deep soil remain limited, particularly using ¹³C-labeled residue under in-situ conditions. Residue-derived C accumulates through two pathways: as mineral-associated organic matter (MAOM) through chemical protection, or as particulate organic matter (POM) through physical protection. These accumulation mechanisms are influenced by soil depth-dependent properties such as carbon saturation of clay minerals, as well as by residue quality. This study aims to evaluate the C accumulation potential of newly added residue across soil depth via MAOM accumulation and POM persistence in Andosols, and to assess how different residue quality affects these pathways. To compare the residue-derived C accumulation potential across soil depths, we conducted a 1-year in-situ incubation experiment on Andisols cropland in Japan. Soil of each depth (10, 50, and 90 cm depth) was mixed with ¹³C-labeled maize leaf, stem, and root residue (C/N ratio = 21–39) at 2 g C kg⁻¹, and buried at the corresponding depth. After 3, 6, and 12 months, buried samples were collected and fractionated into free particulate organic matter (fPOM; < 1.7 g cm⁻³), occluded POM (oPOM; < 1.7 g cm⁻³,> 53 μm), and mineral-associated organic matter (MAOM; > 1.7 g cm⁻³, < 53 μm), and residue-derived C in each fraction was quantified. After 12 months, the remaining proportion of residue-derived C in bulk soil was significantly highest in 90 cm (36–43 %), followed by 50 cm (32–36 %) and 10 cm depth (22–25 %), indicating that the deep soil of Andisols has a higher accumulation potential of newly added residue-derived C across all residue quality than topsoil. Residue-derived C in MAOM was highest in 90 cm depth (0.58–0.71 g C kg⁻¹), followed by 50 cm (0.52–0.64 g C kg⁻¹) and 10 cm depth (0.37–0.46 g C kg⁻¹) after 12 months, and negatively correlated with carbon saturation statuses of soil. These results indicate the high accumulation of residue-derived C via mineral association in deep soil which has lower C saturation. Furthermore, root residue-derived C in oPOM remained higher in deep soil than in topsoil, while it was not observed for leaf- and stem-derived C in oPOM after 12 months. It should indicate that oPOM persistence can provide an additional accumulation pathway for root residue in deep soil. Overall, we found that the deep soil of Andisols has a higher C accumulation potential than the topsoil, through enhanced MOAM formation and POM persistence.