Effects Of Low Molecular Organic Acids On Phosphorus Release In Red Soil And Growth And Physiological Characteristics Of Chinese Fir Seedlings

Chinese fir(Cunninghamia lanceolata(Lamb.)Hook)is one of the most important plantationtree species in South China.It has the advantages of fast growth,high yield and high economic benefit and is widely used in buildings,shipbuilding and Bridges etc.However,there are a series of ecological problems in the plantation of Chinese fir,such as the declining soil fertility and the decline of productivity which have greatly affected the sustainable development of Chinese fir plantation.Many scholars have done some researches and put forward various views due to the series of ecological problems including the litter decomposition and the self-toxic effect of the allelochemicals secreted by the root system,unbalance of soil nutrient,excessive nutrient consumption in woodland and so on.Litter is the main component of forest ecosystem.Especially,the decomposition process of litter is not only an important process of nutrient cycling but also an important guarantee for normal operation of the ecosystem nutrient cycle,which is a direct effect on the structure and function of ecosystem.However,the composition of litters is complex.The various substances that litter decomposition may have different function effect on the forest ecological environment.Previous studies have found that Chinese fir litter contains different kinds and amount of low molecular organic acids and high molecular organic acids,and the advantage of low molecular organic acids include oxalic acid,malonic acid,malic acid,citric acid,etc.Up to now,the effect of low molecular organic acids from Chinese fir litters on the aerial and underground part of Chinese fir seedlings is unclear.In this work,according to the results of our earlier study we select exogenous organic acids including oxalic acid,malonic acid,malic acid,citric acid and mixed acid(According to the content of low molecular organic acid in the branches,leaves and fruits of Chinese fir)to simulation of low molecular organic acids from the source of Chinese fir litter,and the southern Red Soil and Chinese fir seedlings as the research object to explore the effect of low molecular organic acids on soil phosphorus release and growth and physiological characteristics of Chinese fir seedlings under phosphorus deficiency stress.This work will provide a theoretical basis for the scientific evaluation the effect of litter source of low molecular organic acids on the forest ecosystem and provide scientific reference for promoting the sustainable development of Chinese fir plantation.The main results are as follows:(1)With the increase of low molecular organic acid concentration,the pH gradually decreased,while the ion activity gradually increased,and they both have significant concentration effect.At the same concentration,the pH of all kinds of low molecular organic acids demonstrates that malic acid>citric acid>malonic acid>oxalic acid.The ionic activity generally shows oxalic acid>malonic acid>citric acid>malic acid.(2)In one-time extraction,five types of low molecular organic acids under different concentration treatment can effectively promote the release of red soil phosphorus,iron and aluminum compared with control,and they were all significantly different(P<0.05).The amount of release showed a tendency to increase with the increase of concentration.The release effect of five types of low-molecular organic acids on phosphorus,iron and aluminum in red soil showed that oxalic acid?mixed acid>malonic acid?citric acid>malic acid.(3)Under the effect of low molecular organic acid,The phosphorus release of red soil increased with the increase of iron and aluminum release.The release pattern of iron and aluminum is similar to that of phosphorus.There was a significant positive correlation between the phosphorus release amount of red soil and the release of iron and aluminum(P<0.001)under low molecular organic acid treatment,and the correlation coefficient was 0.86 and 0.90.(4)In repeated sequential extraction,the phosphorus release of red soil in 2 mmol/L low molecular organic acids treatment showed a trenddency of increasing first and then decreasing with the increase of extraction number,which reached the peak in the third dip.Under the treatment of 8 mmol/L low molecular organic acids,the phosphorus release of red soil showed a tendency of decreasing with the increase of the number of extraction.After six extraction,the phosphorus accumulated release exceeded the effective phosphorus content of red soil itself indicating that low molecular organic acid had a dissolving effect on the slightly solubility phosphorus in red soil.(5)The relationship between the phosphorus release of red soil and the concentration of low molecular organic acid can be described by linear equation,and the fitting effect of Y=AX+B is significant(P<0.05).The relationship between times of repeated sequential extraction and the continuous release of red soil phosphorus can be described by nonlinear equations.The best fitting equation is parabolic equation(Y=b1X3+b2X2+b3X+b4)and the reciprocal equation(Y=b1/X+b2).(6)Compared to the control,low concentration(0.2,1,5 mmol/L)of low molecular organic acids can promote the growth of Chinese fir seedlings and increase the seedling height,ground diameter,total root length,root volume,and 5 mmol/L is the best treatment concentration.However,high concentration(50 mmol/L)becomes inhibitory.(7)Appropriate concentration of low molecular organic acids can improve Chinese fir seedling physiological characteristics,0.2,1,5 mmol/L low molecular organic acids can increase antioxidant enzymes(SOD,POD,CAT)activity,chlorophyll content(chlorophyll a and chlorophyll b)and reduce malondialdehyde(MDA)content,5 mmol/L is the optimum concentration;The low molecular organic acids of 50 mmol/L have different degrees of stress on Chinese fir seedlings.(8)The suitable concentration of low molecular organic acids can improve the biomass of fir seedlings.At low concentration(0.2,1,5 mmol/L),the biomass of the root,stem and leaf of Chinese fir seedlings increased with the increase of concentration while inhibitory effect on the biomass at high contentration.(9)Compared with the control,the low concentration(0.2,1,5 mmol/L)of low molecular organic acid can improve the content of nitrogen,phosphorus and potassium in the roots,stems and leaves of Chinese fir seedlings.In high concentration(50 mmol/L),there are different degrees of inhibition.(10)Appropriate concentration of low molecular weight organic acids can alleviate the low phosphorus stress of Chinese fir seedlings.In addition,the improvement of phosphorus level can promote the growth of Chinese fir seedlings in a certain extent in morphology,physiology,biomass and nutrient content.The changes of the indexes of the seedlings of Chinese fir seedlings under the treatment of low phosphorus,medium phosphorus and high phosphorus were basically the same.