Fungistatic Activity of Multiorigin Humic Acids in Relation to Their Chemical Structure

Humic acid (HA) has an inhibitory effect on phytopathogenic fungi, but the structure–activity relationship remains unclear. HAs were extracted from 14 different materials, and their fungistatic activities and elemental C, N, S, and O contents were measured. Cross-polarization magic-angle spinning 13C nuclear magnetic resonance (CPMAS 13C NMR) was used to measure the organic carbon composition. The results showed that all HAs suppressed phytopathogenic fungi growth, with Yunnan lignite HAs showing the highest inhibition (85.3%) against Physalospora piricola. The soil and compost HA aromaticity (ARO) was <50%, except for black soil HAs, while the ARO of all coal HAs was >60%. The ARO of meadow and moss peat HAs was <50%, while the ARO of woody peat HAs was 50.61%. Mantel test and redundancy analysis (RDA) were applied to evaluate the structure–activity relationship. The Mantel test revealed that the N, S, O, N/O, carbonyl C, aromatic C–O, and anomeric C contents were significantly correlated with fungistatic activity. The RDA analysis showed that the S content was positively correlated with fungistatic activity, while the O content was negatively correlated. The carbonyl C content had a positive correlation with fungistatic activity, while the anomeric C and aromatic C–O content had a negative correlation. A high S content and an active composition (carbonyl C) in HAs would lead to a high degree of fungistatic activity. Phytotoxicity test indicated all HAs were beneficial to plant growth. This work identified the basic properties of HAs from various raw materials that control their fungistatic activities.