Effects And Mitigation Factors Of Soil Aggravates Root Rot Of Panax Notoginseng

Panax notoginseng is one of the precious Chinese medicinal materials in China,but the root rot of P.notoginseng is serious in high input–based intensive cropping systems.Cultivation of P.notoginseng under the forest is an effective strategy to reduce the occurrence of diseases and insect pests and the pollution of pesticides as well as improve the quality.However,it was found that unhealthy growth of P.notoginseng root and outbreaks of root rot disease under the forest in some areas.Firstly,this study studied the causes of unhealthy growth of P.notoginseng root in the soil under Cunninghamia lanceolata forest through the determination of soil chemical properties of unhealthy growth of P.notoginseng and experimental verification;Secondly,we undertook a meta-analysis using published literature and field investigation data to further identify the key chemical factors leading to the occurrence of root rot disease,and the relationship between these key factors and the occurrence of P.notoginseng root rot was confirmed by soil amelioration experiment;Finally,the mitigation effect and mechanism of soil leaching and quicklime improve acidified soil under forest were studied through pot experiment and plot experiment.The main conclusions are as follows:1.Allelopathic inhibition of phenols on P.notoginseng roots in soil under C.lanceolate forest.The comparative study found that the pH value of the soil in the unhealthy growth area of P.notoginseng was significantly decreased,but the electronic conductivity（EC）value and the content of phenols(（p-hydroxybenzoic acid（HA）,vanillin（V）,syringic acid（SA）,ferulic acid（FA）and vanillic acid（VA））was significantly increased.The root growth of lettuce in unhealthy soil was significantly inhibited.Pearson correlation analysis showed that the number of fibrous roots,fresh and dry biomass of P.notoginseng were significantly positively correlated with soil pH but negatively correlated with EC values（p<0.05）.The contents of p-hydroxybenzoic acid and vanillic acid in soil were significantly negatively correlated with fresh and dry biomass of P.notoginseng（p<0.05）.Further analysis showed that the accumulation of phenols could reduce soil pH and increase EC value.Compared with CK,when the concentrations of syringic acid,ferulic acid,vanillic acid and the mixture were between 5 and 30 mg L^-1,the root activity decreased significantly.When the concentrations of vanillin,syringic acid and ferulic acid reached 15 mg L^-1,the relative conductivity of root solution increased significantly.Therefore,the accumulation of phenolic allelochemicals under the forest of C.lanceolata would reduce the soil pH and increase the electrical conductivity,affecting the healthy growth of P.notoginseng roots.2.The decrease of soil pH value and the accumulation of phenols were the key factors leading to the occurrence of P.notoginseng root rot disease:Through literature collection and field investigation data,meta-analysis found that the decrease of soil pH value was the key soil chemical factor to aggravate the occurrence of P.notoginseng root rot disease（ln RR=-0.34,p<0.001）.In depth analysis found that the increase of available nitrogen（AN）and phenolic substances was the main driving factor for the decrease of soil pH value（r<-0.45,p<0.01;r<-0.21,p<0.01）.The results of hydroponic culture experiment showed that the wilting rate and water loss of P.notoginseng increased with the increase of phenol concentration;In the soil environment,the synergistic effect of phenols and pathogens（Ilyonectria destructans）could aggravate the occurrence of P.notoginseng root rot disease,and reduce root dry weight.These results of field experiment showed that exogenous quicklime,calcium magnesium phosphate fertilizer and organic fertilizer with suitable dosage in acid soil could effectively increase pH and alleviate root rot disease.Overuse of nitrogen fertilizer could furtherly decrease the pH value and aggravate the root rot disease.These results further prove that the decrease of soil pH value and the accumulation of phenols were the main reasons for the aggravate occurrence of P.notoginseng root rot disease.3.Mechanism of reducing root rot of P.notoginseng by leaching.Soil with high level of phenols was leached with five intensities and their effect on the root rot of P.notoginseng was assessed.Then,the relationship of root rot with changes in soil chemical properties and phenol content as well as rhizospheric microorganisms was analyzed.With increasing leaching intensity,both the levels of phenols HA,V,SA,FA and acid VA in soils and the incidence of root rot decreased significantly.Further analysis demonstrated that decreased phenol leaching could alleviate root rot disease by modifying the soil microbial community.Network analysis showed that root rot incidence was positively correlated with the HA-enriched pathogenic fungus Ilyonectria but negatively correlated with HA-,FA-or VA-suppressed rhizospheric bacteria,including Sphingomonas,Lysobacter,Massilia and Burkholderia.Further culture experiments confirmed that individual HA,FA and VA as well as a mixture of the five phenols with increasing concentrations could stimulate the growth and sporulation of I.destructans but suppress the growth and biofilm formation of bacteria（Burkholderia sp.and Lysobacter sp.）.Moreover,Burkholderia isolates showed antagonistic activity against the growth of I.destructans.Therefore,leaching could relieve the stimulation of pathogenic fungi by phenols and reduce the pressure on phenolic-suppressed bacteria,thereby reducing the incidence of P.notoginseng root rot disease.4.Modification of quicklime on acid soil under forest and their effect on the growth of Panax notoginseng.Different amounts of quicklime(0,500,1000,1500 and 2000 kg hm^-2)were amended into acid soil under forest then evaluated their effect on the soil chemical properties,phenols,rhizosphere microorganisms and growth of P.notoginseng.The results shown that an appropriate amount of quicklime(500-1000 kg hm^-2)could significantly increase the soil pH value,decrease the content of phenols（HA,V,SA,FA and VA）,promote the growth of P.notoginseng,and reduce the incidence of root rot disease.Further in-depth study showed that an appropriate amount of quicklime(500-1000 kg hm^-2)could significantly reduce the ratio of fungi to bacteria,increase the diversity of bacteria,and increase the relative abundance of the phylum Ascomycota and Proteobacteria as well as the genus Massilia and Sphingomonas.However,excessive quicklime(1500-2000 kg hm^-2)could reduce the content of available nitrogen and organic matter,and inhibit the growth of P.notoginseng.Therefore,an appropriate amount of quicklime amendment(500-1000 kg hm^-2)could improve the chemical properties and microbial community of acid soil under forest,thereby promoting the growth of P.notoginseng and reducing the incidence of root rot disease.In conclusion,soil acidification and accumulation of phenols in coniferous forests were important factors to inhibit root growth and increase the incidence of P.notoginseng root rot disease.In the natural state,the decomposition and root exudates produced by coniferous forest metabolism caused soil acidification and the accumulation of phenols.The synergistic effect of phenols and pathogens aggravates the serious occurrence of P.notoginseng root rot disease.Both leaching and quicklime could effectively alleviate the severity of root rot of P.notoginseng by reducing the content of soil phenols and changing soil microbial community.This study revealed the negative effects of soil acidification on P.notoginseng,and explored effective mitigation measures to improve soil acidification,so as to provide theoretical and technical support for the planting of P.notoginseng under the forest.