Long‐term mineral fertilization in paddy soil alters the chemical structures and decreases the fungistatic activities of humic acids

Fungistatic activity is a newly identified ecological function for soil humic acids. However, the relation between fungistatic activities of soil humic acids and their chemical structures is still unclear. In addition, how fertilizer application affects fungistatic activity is still unknown. We sampled paddy soils with different long‐term fertilizer schemes, namely: unfertilized control (CK), mineral fertilizer applied (NPK) and mineral fertilizer applied plus straw return (NPKRS). Soil humic acids were extracted and their fungistatic activities against 14 phytopathogenic fungi were investigated to evaluate their ecological functions. The humic acids from NPK and NPKRS treatments showed much smaller fungistatic effects than those from CK treatments. The chemical structures of humic acids were investigated by 13C nuclear magnetic resonance (13C NMR) spectroscopy. Long‐term fertilization significantly changed the chemical structure of soil humic acid, and the variation in chemical structure affected the fungistatic activities. The relative content of alkoyl C and alkyl C, the alkoyl C/alkyl C ratio and aliphaticity all had positive correlations with the fungistatic effects of humic acids. That indicated humic acids with more aliphatic and decomposable structures showed more fungistatic activity. From the results, we believe that long‐term application of mineral fertilizer in paddy fields makes the soil humic acids less aliphatic and more recalcitrant, and concomitantly weakens their ability to inhibit phytopathogenic fungi. Highlights We studied the inhibitory effects of paddy soil humic acids on 14 phytopathogenic fungi. Fertilization‐induced changes in humic acids affect inhibition of phytopathogenic fungi. Long‐term mineral fertilization significantly changed chemical structures of humic acids. Humic acid with more aliphatic and decomposable structures had more fungistatic activity.