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 13C (delta13C) 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 13C in bulk soil and size fractions increased significantly in the continuous fallow and was affected by organic amendments. The delta13C 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.