Storage of organic carbon and Black carbon in density fractions of calcareous soils under different land uses

The association between soil particles and their spatial arrangement plays a key role in soil organic matter (OM) dynamics. Density fractionation combined with ultrasonic dispersion enables separation and study of soil OM fractions, considered on the basis of the mechanisms of physical protection: non-physically protected OM (FF), OM occluded into aggregates (OF), and OM stabilized in organo-mineral complexes (DF).In the present study, whole soils and density fractions of calcareous soils under three different management systems — native Quercus ilex forest, a Pinus halepensis plantation and cropped land — were analyzed for organic C (OC), total N, and Black carbon (BC) content. Black carbon is often considered as a very recalcitrant pool in the soil. However, as well as BC content of soils has seldom been quantified, long-term studies on BC stability are scarce and conclusions about BC stability are not widespread.About 67% of the total C in the topsoil was lost as a result of converting the natural Q. ilex forest to cropped land, 100 years ago. After crop abandon, the stock of OC in the topsoil greatly increased upon reforestation of the studied plot with P. halepensis. An average recovery of 71% of the previously lost OC had been recovered, after 40 years of pine plantation. The changes in OC stocks affected mainly the free fraction (FF). Black carbon represented between 1.2 and 2.3% of the TOC of soil with the highest concentrations in OF. The maintenance of BC proportion through land uses suggests an equilibrium between inputs and outputs, and leads to the suspicion that BC could be less stable and less resistant to biodegradation than is often taken for granted.