Abstract
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.
Highlights
China ranks as the largest apple producer and consumer in the world with a planting area and an annual output accounting for 2.22 million hectares and 41.39 million tons, respectively.[1,2] Most apple orchards in China were established in the 1980s and 1990s, and due to the aging of fruit trees, the acceleration of the optimization of tree species, and popularization of dwarfing rootstocks, the renewal of orchards is becoming increasingly frequent
Gel permeation chromatography (GPC) analysis indicated that the relative ratio of PFA with the molecular weight of less than 1000 Da is 19.4%, the fraction of 1000−5000 Da is 59.5%, and the remaining fraction is 21.1% (Figure 1a)
The Fourier transform infrared (FTIR) spectrum of PFA (Figure 1b) revealed a broad peak at around 3400 cm−1 that is usually produced by the stretching vibration of the O−H bond
Summary
China ranks as the largest apple producer and consumer in the world with a planting area and an annual output accounting for 2.22 million hectares and 41.39 million tons, respectively.[1,2] Most apple orchards in China were established in the 1980s and 1990s, and due to the aging of fruit trees, the acceleration of the optimization of tree species, and popularization of dwarfing rootstocks, the renewal of orchards is becoming increasingly frequent. Recent studies noticed that the organic matter in paper mill effluent of the straw-based ammonium sulfite pulping process exhibited similar characteristic functional groups as well as biostimulatory activity to that in FAs, naming it as a “fulvic acid-like substance from paper mill effluent” (PFA).[22,23] In general, FAs/PFA can increase the nutrient utilization efficiency of plants through its biostimulatory effects[24,25] and enhance the photosynthesis of plants by regulating the opening/closing of leaf stomata,[26] thereby regulating the metabolism of plants to enhance plant resistance under adverse conditions.[27] For the soil, active groups in PFA such as carboxyl and phenolic hydroxyl groups are important carbon energy sources, which are conducive to improving the physical properties of the soil.[28] It has been shown that PFA can accelerate the formation of macro/microaggregates by increasing the cohesive force between clay particles and fine particles when PFA is associated with the large surface area of soil.[29] research studies on PFA to date have mainly focused on alleviating abiotic stresses such as salt,[22,23] drought,[30] or nutrients,[31] but its effects on ARD that are dominated by biological factors remain unclear. This research will help provide practical implications, open research avenues in the efficient application of PFA for controlling ARD, and provide an application prospect for paper mill effluent
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