Soil organic matter pools and carbon-13 natural abundances in particle-size fractions of a long-term agricultural field experiment receiving organic amendments

The present study combined a physical fractionation procedure with the natural abundance of C-13 (delta C-13) to evaluate the effect of organic fertilizer applications, mineral fertilization, and tallow on changes in the organic C (C-org) associated with different particle-size fractions, The long-term agricultural field experiment was conducted since 1956 in Ultuna, Sweden, on a Eutric Cambisol, Organic C both in bulk soil samples and size fractions changed significantly since 1956, Fellow plots lost approximately one-third of their C-org from the topsoil layer (0-20 cm), whereas organic amendments based on an equivalent of 2000 kg C ha(-1) yr(-1) increased C-org up to twofold depending on the quality of the material applied (green manure < animal manure < sewage sludge < peat), Silt-sized particles increased in plots receiving sewage sludge or pest, Organic C in particle-size fractions responded significantly to treatments. Most C-org was found in the silt fraction, The relative contribution of the silt-sized particles to total C-org increased by 18% as C-org in bulk soil increased from 10.8 (fallow) to 32.0 (peat) g C-org kg(-1) soil; the contribution of clay-sized particles decreased by a similar proportion. Mass balance calculations showed that the proportion of C-org originating from organic amendments decreased with particle size and that sand fractions were the most sensitive to the treatments, The natural abundance of C-13 in bulk soil and size tractions increased significantly in the continuous fellow and was affected by organic amendments. The delta C-13 variations among size fractions were larger than among treatments and can be used as a fingerprint for differentiation. Our results suggest that silt-sized particles acted as medium-term sink for added C-org and that sand-sized fractions can be useful as sensitive indicators of changes in soil C status in response to land management.