Spectroscopic properties of bulk and dichromate oxidation resistant soil organic matter from an anthroposequence in a Mediterranean environment

The shallow soils of Pianosa-a small calcareous island in the Mediterranean Sea, facing Tuscany, central Italy-developed under the same pedogenic factors with the exception of vegetation and, in the last two centuries, human impact. Originally, they were covered by "macchia," a tangled mix of bushy species, or, in a few more fertile sites, by Quercus ilex L. Currently there are six land uses on the island: (1) pristine macchia, (2) groves of Olea europaea L. totally reinvaded by macchia, (3) degraded residual thickets of Q. ilex, (4) cropland, (5) recently abandoned pastures, and (6) stands of Pinus halepensis Mill. The related soils were analysed by solid-state cross polarisation/magic angle spinning C-13-nuclear magnetic resonance (NMR) to compare the spectroscopic properties of their organic pool. The prevalence of O-alkyl C over other C forms is evident in all cases, while there is large variability with respect to the alkyl C to aromatic C ratio between both the different soils, and the horizons of a same soil. On the whole, the results show that on Pianosa Island, human impact has profoundly altered not only the amount but also the quality of soil organic matter (SOM). The SOM residual of the dichromate oxidation in the Walkley-Black method-a widely used method to measure soil organic C-was also analysed by NMR. It comprised almost exclusively alkyl C and aromatic C, as a result of the removal of carbonyl and O-alkyl C. Most of the aromatic C seems to belong to charcoal.