Lignin is preserved in the fine silt fraction of an arable Luvisol

Knowledge about the fate of individual biomolecules during the decomposition process in soil is limited. We used the natural isotopic label introduced by 23 years of continuous maize cropping, together with compound specific 13C isotope analysis, to study lignin monomers in particle size fractions of a Luvisol. Isotope data indicated apparent decadal turnover times for lignin. A kinetic model suggests the existence of a fast and a slow decomposing lignin pool in the soil, reconciling a low stock-to-input ratio with decadal turnover times. We found new, maize-derived lignin primarily in the 63-2000 μm fraction, whereas old, C3-derived lignin from the pre-maize vegetation had accumulated mainly in the silt (2-20 μm) fraction. This distribution of lignin differed from that of total organic carbon, which was concentrated in the <2 μm fraction. Old, C3-derived carbon in all the soil fractions was depleted in lignin compared to new, maize-derived carbon. The observation that the 2-20 μm fraction was less depleted than the others indicates that lignin preservation is particle size specific, but the underlying mechanism controlling its preservation is not clear.