A Graphical-Statistical Analysis of Humic Acid Pyrolysis Compounds in Relation to Soil Hydrological Properties

The stable fraction of soil organic matter (SOM)—humic acids (HAs)—from semiarid soils in central Spain was characterized using pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). We examined the relationships between the relative abundances of pyrolysis-derived compounds and several soil hydrological properties: bulk density, porosity, aggregate stability, hydraulic conductivity (Kostiakov b and c parameters), and infiltration rate. Correlation coefficients for individual pyrolysis products were mapped in H/C–O/C van Krevelen space to generate density surfaces for each physical property. Methoxyphenols (lignin derivatives) and alkenes correlated positively with macroaggregation, porosity, and infiltration rate. In contrast, Kostiakov parameters b and c and cumulative infiltration were associated with non-methoxylated aromatic compounds and showed negative correlations with hydrophobic aliphatic structures. These patterns indicate that soil physical quality emerges from specific molecular signatures in HAs that reflect moderate hydrophobicity (alkyl domain) and lignin transformation toward unsubstituted aromatic structures. Total SOM content was significantly associated (p < .05) only with bulk density, porosity, aggregate stability, and cumulative infiltration, whereas other hydrological properties were better explained by HA molecular composition. Our results highlight the complementary value of SOM quantity and quality for predicting soil hydrological properties under Mediterranean conditions.