Hot water extractable carbon in whole soil and particle-size fractions isolated from soils under contrasting land-use treatments

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Context Understanding the dynamics of soil organic matter (SOM) requires that it be divided into fractions with contrasting behaviour. Aim Using soils from a field trial with a diverse set of treatments (pasture to arable conversion, ex-pasture soil maintained fallow for 13 years and arable cropping with contrasting tillage treatments), we evaluated two approaches to characterise SOM: particle-size fractionation and hot water extraction. Methods Soils were separated into four size fractions, including the >50 μm fraction (particulate organic matter, considered the most labile fraction) and the <5 μm fraction (stabilised C associated with clay particles). Hot water extractable C (HWC; 80°C for 16 h) was determined on whole soil and on the isolated size fractions. Key results Whereas total soil C stocks (0–25 cm) declined by an average of 14% under arable cropping and by 23% under continuous fallow, the decline in HWC was much greater (31% under arable and 49% under bare fallow). A large part (>50%) of the C lost under cropping and bare fallow was derived from the clay fraction, which was also the source of 59–77% of HWC. Conclusions and Implications Our results indicate that hot water is specific for the labile component of SOM while SOM in size fractions is a composite of labile and stable components, the proportions of which may vary depending on land use and other factors. Ideally, data on quantity of SOM in size fractions should be complemented by information on its lability.