| Guizhou Baili Rhododendron National Forest Park has the largest alpine rhododendron reserve in China and even in the world,known as the "ribbon on the earth",with unique and rich rhododendron resources.However,at present,there are older trees,a thick litter layer,high canopy closure,and few new seedlings,which undoubtedly form obstacles to the self-renewal of rhododendron.At the same time,phenolic acids released by litter decomposition not only exist widely in nature but also have a close relationship with soil fertility reduction and plant growth renewal.However,there are few reports on the changes in phenolic acids with allelopathic effects during the decomposition of litters under Rhododendrons.In this paper,four kinds of Rhododendron delavayi Rhododendron irroratum,Rhododendron agastum and Deciduous Broadleaved Mixed forest in the Baili Rhododendron forest are divided into research objects.Through a field litter decomposition experiment of18 months,six kinds of phenolic acidifying substances and eight kinds of common organic acids of low molecular weight were measured in the humus and deep soil under the litter bags and litters,and their changes with time were analyzed.The correlation between the physical and chemical properties in the deep soil and the measured organic acids was analyzed to provide basic data for the regeneration of rhododendron forests.The main research results are as follows:(1)In the first three months of decomposition,litters release a large amount of various phenolic acids.After that,the content of litter increases in spring and summer and decreases in autumn and winter,and the changes in seasons and stands are important factors affecting the changes.The total amount of phenolic acid released from litters of different stands was R.irroratum Franch forest>R.delavayi Franch forest>R.agastun Balf.f.et W.W.Smith forest>Deciduous Broadleaved Mixed forest.(2)The contents of phenolic acids in litters of four rhododendrons were higher than those in soil and humus,and the contents of protocatechuic acid,chlorogenic acid and ferulic acid were higher in different layers.There was no significant correlation between the phenolic acids of the litter layer and soil layer,except for the significant positive correlation between the gallic acid of the litters of Rhododendron irroratum and the protocatechuic acid of soil(p< 0.05).(3)The composition and content distribution of low molecular weight organic acids in litters of four rhododendrons were different,among which oxalic acid and tartaric acid were the main organic acids,followed by citric acid.Oxalic acid,tartaric acid and citric acid were the most abundant in the humic soil and soil layer.(4)The relationship between phenolic acids and soil physical and chemical properties varied in different stands,and the p H,water content,available phosphorus,available potassium and phenolic acids in the Rhododendron irroratum forest mostly had significant negative effects(p<0.05).There were positive correlations between water content,available phosphorus and phenolic acid in Rhododendron delavayi forest,and significant positive correlation between organic matter and phenolic acid in Rhododendron agastum forest(p<0.05).The organic matter of deciduous Deciduous Broadleaved Mixed forest was negatively correlated with phenolic acid(p<0.05).There were significant positive effects of available iron,available copper and phenolic acid in the four stands(p<0.05).Therefore,there is a close relationship between phenolic acids and soil physical and chemical properties of the four stands,and the interaction effect is significant.The specific degree of influence is related to the type of phenolic acids,the type of stand,and soil physical and chemical properties.(5)The relationship between low molecular weight organic acids in the four stands and the physicochemical properties of different soils was different.For example,malic acid in Rhododendron delavayi forest had a significant negative effect on p H,water content and available copper(p<0.05).There were significant negative effects of formic acid on p H and organic matter in Rhododendron irroratum forest(p<0.05).There were significant positive effects of citric acid on p H,available copper and ammonium nitrogen in Rhododendron agastum forest(p<0.05).There was a significant positive effect between citric acid and ammonium nitrogen,and a negative effect between citric acid and water content in Deciduous Broadleaved Mixed forest(p<0.05).Therefore,the specific correlation and the degree of influence are closely related to the types of low molecular weight organic acids,stand types and soil physicochemical properties.In summary,the contents of organic acids(six potential allelochemicals phenolic acids and eight common low molecular weight organic acids)during litter decomposition were affected by stand types and seasons.The changes of soil organic acid content were closely related to the contents of a variety of soil nutrients,which provided data reference for further understanding nutrient restitution and the alleloeffects of phenolic acids in rhododendron litter. |
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