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The mechanisms of organic carbon protection and dynamics of C ‐saturation in O xisols vary with particle‐size distribution
Author(s) -
Souza I. F.,
Almeida L. F. J.,
Jesus G. L.,
Kleber M.,
Silva I. R.
Publication year - 2017
Publication title -
european journal of soil science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.244
H-Index - 111
eISSN - 1365-2389
pISSN - 1351-0754
DOI - 10.1111/ejss.12463
Subject(s) - silt , saturation (graph theory) , chemistry , litter , total organic carbon , soil science , mineralogy , soil carbon , environmental chemistry , soil water , environmental science , geology , agronomy , mathematics , geomorphology , combinatorics , biology
Summary 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 O xisols. To address this issue, we carried out a 12‐month incubation experiment; 13 C ‐labelled litter inputs equivalent to 0, 4.5, 9.0 and 18.0 mg C g −1 soil were applied to samples of six B razilian O xisols, 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 O xisols 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 O xisols can be inferred.