Comprehensive Analysis of the Influence of Fulvic Acid from Paper Mill Effluent on Soil Properties, Soil Microbiome, and Growth of Malus hupehensis Rehd. Seedlings under Replant Conditions

[Image: see text] In this study, the potential regulatory effects of fulvic acid extracted from paper mill effluent (PFA) in apple replant disease (ARD) were investigated through a comprehensive experimental evaluation of the effects of PFA on soil properties, growth inhibition of apple replant pathogens, and growth of replanted Malus hupehensis Rehd. seedlings. PFA with a relatively lower molecular weight was mainly composed of carbohydrates, lignin derivatives, and polysaccharides and was rich in functional groups such as carboxyl and phenolic hydroxyl groups. Treatment with PFA dosages ranging from 2 to 3 g/pot significantly increased available phosphorus (P) in soil by 47.5 to 57.5% when compared with the control without PFA, indicating that PFA had a positive effect in activating P. In addition, PFA stimulated the growth of replanted seedlings by promoting root elongation, enhancing leaf photosynthesis, and increasing the activity of root antioxidant enzymes including superoxide dismutase, peroxidase, and catalase. However, no convincing evidence was found that application of different dosages of PFA had remarkable effects on soil pH, inorganic nitrogen, available potassium, organic matter, and the numbers of bacteria and fungi. Notably, PFA had no effect on the copy number of the main pathogenic fungi causing ARD, including Fusarium oxysporum, Fusarium solani, Fusarium proliferatum, and Fusarium moniliforme. Overall, PFA can alleviate ARD to a certain extent mainly through its effects on improving the resilience of replanted young seedlings rather than by affecting soil microorganisms or providing nutrients.