The mechanisms of organic carbon protection and dynamics of C-saturation in Oxisols vary with particle-size distribution

摘要

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; C-13-labelled litter inputs equivalent to 0, 4.5, 9.0 and 18.0mgC g(-1) soil were applied to samples of six Brazilian Oxisols, taken from depths of 0-10, 10-20, 20-40 and 60-100cm. The effect of PSD on SOC protection and C-saturation was assessed by diluting' the mass of the clay+silt fraction (53 mu m) by adding fine sand (150-250 mu m) in increments of 0, 20, 40 and 80% relative to the fine earth fraction (2mm). 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 61.6g Ckg(-1) clay+silt. Conversely, when the proportion of the fraction 53 mu m exceeded 60%, CSL occurred at 33.4g Ckg(-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.