Soil organic matter fractions as early indicators for carbon stock changes under different land-use?

Abstract With respect to carbon sequestration in soil, attempts have been made to identify soil organic matter (SOM) fractions that respond more rapidly to changes in land-use than bulk SOM, which could thus serve as early indicators for the overall stock change. We used a combination of physical fractionation (size and density separation) and chemical characterisation (C-to-N ratios, CuO lignin signature, 13C NMR spectroscopy) to identify sensitive SOM fractions in an agricultural system with sandy dystric cambisols in Bavaria, Germany, 7 years after a land-use change. Land-use types included long-term arable land and grassland, and conversion from one system to the other. Soil carbon and nitrogen contents in 0–3 cm increased from 14 to 39 mg organic carbon g−1 soil, and from 1.7 to 3.9 mg nitrogen g−1 soil in the following order: permanent arable, conversion grassland to arable, conversion arable to grassland, and permanent grassland. Wet sieving and ultrasonic dispersion with 22 J ml−1 released 20 μm relative to complete dispersion. The most sensitive fraction, with respect to land-use, was SOM in the fraction >20 μm not released after sequential wet sieving and ultrasonic dispersion. In contrast, the proportion of free light (wet sieving, density 20 μm were more sensitive.