| Organic acids were ubiquitous in forest ecosystems.There was active-COOH in organic acids'structure,so they played a central role in forests and forest soils.In the paper, based on Larix olgensis, Fraxinus mandshurica, Pinus koraiensis,Betula platyphylla and dark brown forest soil, kinds and content of organic acids in 4 kinds of litter leachates were researched by leaching L and F layer litters; by simulating organic acids concentrations in forest litter leachates and application experiment,effects and mechanism of exogenous organic acids/salts on P availability of dark brown forest soils,effects of exogenous organic acids/salts on physiological characteristics,growth,biomass accumulation and biomass allocation of Fraxinus mandshurica seedlings were researched.The results were as follows:(1)A capillary gas chromatography procedure was established to determine organic acids in forest leachates,and the method was satisfactory by feasibility test.Some low-molecular-weight organic acids could be leached from forest litters,including oxalic,malonic,fumaric, succinic,maleic,malic,tartaric and citric and so on.Contents of citric were greater than any other organic acids,followed by oxalic,succinic and fumaric,contents of malic,malonic and tartaric were lower,and contents of maleic were fewest in all samples.As to the same organic acid,the contents in leachates of Larix olgensis and Fraxinus mandshurica were greater than of Pinus koraiensis and Betula platyphylla.(2)Effects of exogenous organic acids/salts on P availability of dark brown forest soils following extractions,there were optimal extraction time(12h) to oxalic, citric and malic.As to P determination in extraction,"soil coloration method" and "water coloration method" both could be used.By studying the effects of different concentration of organic acids on coloration of phosphorus standard solutions,and organic acids added to A1 and B horizon soils, it founded that,"water coloration method" was better,so it was chosen to determine P in extraction and leachate.When concentration of organic acids/salts were lower than 5mmol/L,P could be determined directly,concentration were 10 and 20mmol/L,P should be diluted to 2 and 4 times respectively,and coloration time was all 15min.(3)Oxalic acid/salts substantially stimulated soil P release of A1 horizon following one-time extraction,showing a linear relationship.But its effect on B horizon soil was relatively negligible.Organic salts had a much higher soil-P-release effect than corresponding organic acids the same in concentration,therefore,the mechanism of organic acids/salts triggering P release were assumed to be dominated by reactions of organic anion.P release from A1 and B horizons by organic acids/salts followed the series:citric>malic>oxalic,citrate>malate>oxalate, and it was related to structures,dissociation constants of organic acids and stabilities of organic ligand-metal.(4)Soil P release from A1 horizon under different concentrations of organic acids/salts could not be finished following one-time extraction,and P could be released following latter nine-time extractions,so the effects of organic acids/salts on P release was somewhat cumulative.P release following one-time extraction was not much,but P release following sequential extractions would be very conspicuous.Leaching experiment also proved that,citrate stimulated soil P release of A1 horizon,and P cumulative release were higher with longer leaching times.(5)Exogenous organic acids/salts could improve photosynthesis level of Fraxinus mandshurica seedlings,including photosynthesis curve and dynamic,of which 5.0mmol/L concentration had better capacity than other concentrations.Effects of organic acids/salts on photosynthesis followed the series:citric>malic>oxalic,citrate>malate>oxalate,oxalate>oxalic, citrate>citric and malate>malic.(6)Organic acids/salts had important impacts on some physiological characteristics of Fraxinus Mandshurica seedling,including activity of ALA biosynthesis rate,activity of PBG synthase,chlorophyl content,soluble protein concentration,soluble sugar content,Rubisco initial activity,total activity and root activity.Organic acids/salts of 0.2 to 50.0mmol/L concentration were all profitable to the characteristics above,and 5.0mmol/L concentration had best effect.To effects on chlorophyl content,chlorophyl a and chlorophyl b contents varied more than carotenoid content.Rubisco activity ratio did not show obvious rule.Effects of organic acids/ salts on physiological characteristics above followed the series:citric>malic>oxalic,citrate> malate>oxalate,oxalate>oxalic,citrate>citric and malate>malic.(7)Organic acids/salts were helpful to increase growth of Fraxinus mandshurica seedlings, leaf area,height and collar diameter under organic acids/salts treatments were all higher than distilled water.Growth of 5.0mmol/L concentration organic acids/salts were higher than other concentrations.Effects of organic acids/salts on growth of Fraxinus mandshurica seedlings followed the series:citric>malic>oxalic,citrate>malate>oxalate,oxalate>oxalic,citrate>citric and malate>malic.(8)Organic acids/salts increased photosynthesis level,chlorophyl contents of Fraxinus mandshurica seedlings and nutrient availability in soils,so they also were helpful to increase biomass accumulation.Organic acids/salts of 0.2 to 50.0mmol/L concentrations all increased root,stem,leaf,aboveground,underground and total biomass,and 5.0mmol/L concentration had best effect.Increase scope of different organs showed different rules:increase percents of root and leaf biomass were bigger than that of stem.Effects of organic acids/salts on biomass accumulation of Fraxinus mandshurica seedlings followed the series:citric>malic>oxalic, citrate>malate>oxalate,oxalate>oxalic,citrate>citric and malate>malic,and it was obvious with all concentration treatments. (9)Trends of biomass allocation of Fraxinus mandshurica seedlings under different concentrations of organic acids/salts were same as distilled water treatment.The biomass mostly were distributed to root,i.e.underground,so root biomass were higher than aboveground biomass,and much higher than leaf or stem biomass,which was related to tree species.To the effects of organic acids/salts on biomass allocation of Fraxinus mandshurica seedlings,root and underground biomass percents increased,stem biomass percent decreased,leaf biomass percent showed no obvious rule,aboveground biomass percent decreased,ratio of root to stem increased, and 5.0mmol/L concentration had best effect.Effects of organic acids/salts on biomass allocation of different organs followed the series:citric>malic>oxalic,citrate>malate>oxalate, oxalate>oxalic,citrate>citric and malate>malic.
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