Abstract
The present study combined a physical fractionation procedure with the natural abundance of 13C (δ13C) to evaluate the effect of organic fertilizer applications, mineral fertilization, and fallow on changes in the organic C (Corg) 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 Corg from the topsoil layer (0–20 cm), whereas organic amendments based on an equivalent of 2000 kg C ha−1 yr−1 increased Corg 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 Corg was found in the silt fraction. The relative contribution of the silt‐sized particles to total Corg increased by 18% as Corg in bulk soil increased from 10.8 (fallow) to 32.0 (peat) g Corg kg−1 soil; the contribution of clay‐sized particles decreased by a similar proportion. Mass balance calculations showed that the proportion of Corg originating from organic amendments decreased with particle size and that sand fractions were the most sensitive to the treatments. The natural abundance of 13C in bulk soil and size fractions increased significantly in the continuous fallow and was affected by organic amendments. The δ13C 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 Corg and that sand‐sized fractions can be useful as sensitive indicators of changes in soil C status in response to land management.
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