Influence of change in land use on the refractory organic macromolecular fraction of a sandy spodosol (Landes de Gascogne, France)

The refractory (insoluble and resistant to drastic laboratory hydrolyses) organic macromolecular material (ROM), isolated from a sandy spodosol from Cestas (Landes de Gascogne, France), was examined via a combination of isotopic (δ13C measurements), spectroscopic (FTIR and solid-state 13C NMR) and pyrolytic (conventional pyrolysis and thermally assisted hydrolysis and methylation) methods. This soil was collected from a plot cleared 22 years ago and since then continuously used for intensive maize cropping. The ROM of this cultivated soil was also compared to the ROM from the reference forest soil. It thus appeared that (i) the carbon of the ROM accounts for a substantial part (20%) of the total organic carbon of the cropped soil, (ii) the ROM shows a heterogeneous composition with contributions of altered lignin, polysaccharides, suberans, bacterial and higher plant fatty acids incorporated in the macromolecular structure, melanoidin-type components and suberin, (iii) extensive degradation of the ROM inherited from the forest soil (ca. two thirds) occurred upon cropping, a conspicuous uncoupling is thus observed between the resistance of this material to drastic laboratory hydrolyses (i.e. refractoriness) and its resistance to degradation under natural conditions, (iv) a low input of maize components took place in the ROM of the cropped soil so that, in spite of such a large degradation of the refractory carbon inherited from the forest soil, the latter is still predominant (ca. 85%) in this ROM, (v) differential degradation took place, upon cropping, in the ROM inherited from the forest soil: some components were eliminated (condensed alkyl lipids of higher plants, cellulosic units) or heavily degraded (other polysaccharides), a part of the lignin was retained but underwent side-chain oxidation, while increase in relative abundance was observed for suberin, melanoidins and suberans.