Pig Slurry Fertilization Changes the Pyrolytic Signature of Humic Substances in Calcareous Soil

The aim of this study was to determine the effect of progressive pig slurry (PS) rates, applied over a 12-year period, on the molecular composition of soil organic matter in a calcareous soil. Annual organic matter rates of PS ranged from 1.0 to 4.8 Mg ha−1. Humic acids (HAs) were extracted from field plots treated with PS, including a control (no PS applied). These HAs were analysed using pyrolysis–gas chromatography–mass spectrometry. The proportions of the 122 major compounds released from the soil HAs indicated that PS stimulated humification processes, with the degree of enhancement depending on the application rate. The applied PS contained a high proportion of aliphatic compounds, but only steroids and triterpenes accumulated in the HA soil fraction, and this was only observed at low PS rates. These results suggest that the application of PS leads to a dose-dependent increase in alkyl compounds, mainly alkanes and olefins. Aromatic compounds also showed a dose-dependent increase, but not in terms of the demethoxylated compounds typical of mature humic substances found in the original soil. Instead, the increase in aromatics was observed in the form of methoxyphenols, suggesting a recent incorporation of lignin derivatives from crop residues into the HA.