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 13 C (δ 13 C) to evaluate the effect of organic fertilizer applications, mineral fertilization, and fallow 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. Fallow 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 peat. 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 13 C in bulk soil and size fractions increased significantly in the continuous fallow and was affected by organic amendments. The δ 13 C 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.