Impact of bare fallow management on soil carbon storage and aggregates across a rock fragment gradient

Background: Our understanding of C storage in soils lacks insights investigating organic matter (OM) depletion, often studied in bare fallow systems. The content of coarse rock fragments is often excluded, whereas it may affect C storage. Aims: We aim to contribute to a better understanding of the impact of bare fallow on C storage mechanisms in the soil as influenced by its coarse rock fragment contents. We investigated whether bare fallow induced a depletion of C in OM fractions and analyzed to which extent this affected soil aggregate size distribution and the C loading of the clay-sized fraction. Methods: A comparison of 14 years bare fallow management with adjacent cropped soils located in Selhausen (Germany) provided a gradient of coarse rock fragments of 34%–71%, from which sites with three different fine earth (FE) contents were compared. Across the FE gradient, we isolated particulate OM and mineral-associated OM fractions, obtained microaggregate and macroaggregate size fractions, and quantified the C loading. Results: Bare fallow management induced an OM depletion at lower contents of FE. There, the management influence was more concentrated onto less FE volume. The contribution of both particulate and mineral-associated OM fractions to the C in the low-FE soils decreased. The C loading increased under bare fallow, compared to cropped soil. In the low-FE soil, we also found less macroaggregates, whereas the C content decreased in some microaggregate size fractions. Conclusions: A high content of coarse rock fragments can enhance OM depletion decreasing mineral-associated and particulate C under bare fallow.