Chemical-structural changes of organic matter in a semi-arid soil after organic amendment

A 9-month incubation experiment using composted and non-composted amendments derived from vine pruning waste and sewage sludge was carried out to study the effects of the nature and stability of organic amendments on the structural composition of organic matter (OM) in a semi-arid soil. The changes of soil OM, both in the whole soil and in the extractable carbon with pyrophosphate, were evaluated by pyrolysis-gas chromatography and chemical analyses. By the end of the experiment, the soils amended with pruning waste exhibited less organic carbon loss than those receiving sewage sludge. The non-composted residues increased the aliphatic-pyrolytic products of the OM, both in the whole soil and also in the pyrophosphate extract, with the products derived from peptides and proteins being significantly higher. After 9 months, in the soils amended with pruning waste the relative abundance of phenolic-pyrolytic products derived from phenolic compounds, lignin and proteins in the whole soil tended to increase more than those in the soils amended with sewage sludge. However, the extractable OM with pyrophosphate in the soils amended with composted residues tended to have higher contents of these phenolic-pyrolytic products than that in non-composted ones. Thus, despite the stability of pruning waste, the composting of this material promoted the incorporation of phenolic compounds to the soil OM. The pyrolytic indices (furfural/pyrrole and aliphatic/aromatic ratios) showed the diminution of aliphatic compounds and the increase of aromatic compounds, indicating the stabilization of the OM in the amended soils after 9 months. In conclusion, the changes of soil OM depend on the nature and stability of the organic amendments, with composted vine pruning waste favouring humification.