Effects of associating a N-fixer species to monotypic oak plantations on the quantity and quality of organic matter in minesoils

Two soils developed over 21 years on a homogeneous mine spoil bank — one under a pure plantation of Quercus robur L., the other under a mixed plantation Q. robur L. and N-fixer Alnus cordata Loisel. — were compared and contrasted in terms of quantity and quality of organic matter. The amount of both organic C and N are significantly higher in the mixed stand, with the differences confined to the mineral topsoil. Soil organic matter (SOM) was extracted by alkali and fractionated on a molecular weight basis. In terms of C, the extracted SOM amounted to about half of the total and the fraction  50,000 Da in the organo-mineral aggregates from the forest floor while the two fractions were equivalent in the mineral soil. In the mixed stand, the soil sampled under the oak canopy and that sampled under the alder canopy were processed separately. The whole extracted SOM and the soil residue were analysed for C/N ratio and spectroscopic properties (CP/MAS 13C NMR). The qualitative differences are less evident than the quantitative ones, and often hardly interpretable. Nonetheless, some of the differences are clear and, together with morphological features, such as the presence only in the mixed stand of patches of a darker A material, indicate an effective role of the N-fixer species on both pedogenic processes and C cycle in this human-made environment.