Natural C-13 abundance: A tool to trace the incorporation of dung-derived carbon into soil particle-size fractions

During the decay of C-13 enriched dung patches, the ;delta(13)C signal of s urface soil (1-5 cm) increased with a temporary maximum after 42 d, To unde rstand the underlying processes, we investigated the incorporation of dung- derived C into soil particle-size fractions. Dung, collected from beef stee rs fed on maize (delta(13)C = -15.36 parts per thousand) or ryegrass (delta (13)C = -25.67 parts per thousand), was applied in circular patches to a C3 pasture at North Wyke, UK. Triplicates were sampled from surface soil (1-5 cm) at 14, 28, 42, and 70 d after application, pooled, separated into fine (<0.2 mu m) and coarse clay (0.2-2 mu m), silt plus fine sand (2-250 mu m) , and coarse sand (250-2000 mu m), and analyzed for total C, N, and delta(1 3)C. As particle-size diameter decreased, the C/N ratios decreased and delt a(13)C values increased at all plots due to increasing microbial alteration of soil organic matter. After dung application, ca, 60% of dung-derived C in soil was recovered in the 0.2-250 mu m fractions during the whole experi ment, The proportion of dung-derived C in the fine clay peaked 42 d after d ung application, coinciding with the delta(13)C maximum in the bulk soil an d the maximum leaching rate measured in lysimeters at this time in another study at the same sites, The percentage of dung-derived C as particulate C in the coarse sand fraction increased until the end of the experiment, We c onclude that incorporation of C into soil from decomposing dung patches inv olved both temporary sorption of leached dung C to <0.2 mu m fractions and continuous accumulation of particulate C (>250 mu m). Copyright (C) 1999 Jo hn Wiley & Sons, Ltd.