Particle‐size distribution (PSD) determines soil C‐saturation; that is, the capacity of the mineral matrix to protect soil organic carbon (SOC) against decomposition. However, the mechanistic connection between PSD and C‐saturation is not entirely clear, especially for Oxisols. To address this issue, we carried out a 12‐month incubation experiment; 13C‐labelled litter inputs equivalent to 0, 4.5, 9.0 and 18.0 mg C g−1 soil were applied to samples of six Brazilian Oxisols, taken from depths of 0–10, 10–20, 20–40 and 60–100 cm. The effect of PSD on SOC protection and C‐saturation was assessed by ‘diluting’ the mass of the clay + silt fraction (< 53 µm) by adding fine sand (150–250 µm) in increments of 0, 20, 40 and 80% relative to the fine earth fraction (< 2 mm). Carbon‐saturation level (CSL) was assumed to be a linear function of clay + silt contents, whereas C‐saturation deficit (CSD) was the difference between the CSL and original SOC content in the samples. After the incubation, litter‐derived C within the clay + silt fraction increased exponentially with CSD. Carbon saturation was indicated by an asymptotic relation between the litter‐derived C in the clay + silt fraction and the additions of litter‐C. For clay + silt contents as small as 15%, CSL was achieved at 61.6 g C kg−1 clay + silt. Conversely, when the proportion of the fraction < 53 µm exceeded 60%, CSL occurred at 33.4 g C kg−1 clay + silt. Thus, a PSD‐dependent hierarchy of SOC protection and C‐saturation in Oxisols can be inferred. Our observations support a conceptual model of C‐saturation where surface interactions provide the dominant mechanism of SOC protection at small clay + silt contents. At large clay + silt contents, physical protection of SOC resulting from the spatial arrangement of fine‐sized minerals defines C‐saturation.
Highlights
- The extent to which particle‐size distribution affects mechanisms that define C‐saturation are unclear.
- For a clay + silt content of 15%, C‐saturation occurred at 61.6 g C kg−1 clay + silt.
- When the proportion of the fraction < 53 µm exceeded 60%, CSL occurred at 33.4 g C kg−1 clay + silt.
- A PSD‐dependent hierarchy of SOC protection and C‐saturation in Oxisols can be inferred.