Degradation Mechanism Of Continuous Cropping Poplar Plantations: Accumulation And Allelopathy Of Phenolic Acids

Soil degradation of continuous cropping plantation was a world wide problem. China was the fastest country in developing plantation and it had the largest area of plantation in the world, but serious soil degradation has been observed in its plantations of several tree species resulted from the continuous planting model and this problem has severely affected the sustainable management of the plantation. The mechanism of soil degradation and maintenance technique of forest productivity in planted forest has been hot spot issues in the research area of forestry for a long period. In recent years, with the application of chemical ecology, ecologic toxicology and environmental science in forestry research, especially with the deeper understanding of allelopathy, the allelopathic effect of plantation has become a new hot spot issue in the research of soil degradation mechanism in replanted forest, but there are still many problems need to be discussed deeply. In which, the accumulation mechanism and environmental behavior of allelochemicals in soil, the producing condition and stress introducing mechanism of allelochemicals, effects of allelochemicals on soil ecosystem etc. were all important problems waiting for deeper understanding.The rhizosphere effect of poplar plantation under different management models was studied by comparison and significant rhizosphere effect was found in replanted poplar forest. Comparison on the organic components of GC-MS fingerprint of rhizosphere soil and bulk soil of poplar plantation showed that, hydrocarbon of methane series in organic components of the rhizosphere and bulk soil was the most in content which followed with linear-chain acid ester and aromatic series. But the kind of organic components appeared remarkable difference between rhizosphere soil and bulk soil, and in which the most significant difference was found in aromatic series. There existed definite material basis for allelopathy of continuous cropping poplar plantation. By the way of HPLC techanics, the extracting solution of rhizosphere soils of continuous cropping poplar plantations was analyzed. The specific five components of phenolic acids were monitored, and its content in soil was found higher than the first rotation, which implied an accumulation tendency in the rhizosphere soil of continuous cropping poplar plantation. And this discovery has extrimely helpful to study allelopathy and soil degradation in continuous cropping poplar forest. Based on five phenolic acids found in the rhizosphere soil of continuous cropping poplar plantation, the environmental behaviors of those phenolic acids such as adsorption, desorption and maintenance etc., was studied through exogenous application. The result showed that, phenolic acids had high adsorptive capacity, in which vanillin, ferulic acid and cinnamic acid, the maximum adsorption Ymax was 3267.85μg·g^-1, 3281.58μg·g^-1, 3211.80μg·g^-1 respectively, had higher adsorptive capacity than p-hydroxybenzoic acid and benzoic acid, whose maximum adsorption Ymax was 3003.60μg·g^-1 and 1771.15μg·g^-1 respectively. In early period the adsorption of phenolic acids was larger, and with the increasing of phenolic acids on the surface of soil colloids, the adsorption gradually decreased. The adsorption of phenolic acids in the soil of poplar plantation belonged to physical adsorption, and the desorption could take place under influenc of certain external conditions. All above discovery could give an explanation to the accumulation mechanism of phenolic acids in the poplar rhizosphere soils. That was, p-hydroxybenzoic acid, vanillin, ferulic acid and cinnamic acid were more likely to be adsorbed in rhizosphere soil of poplar and their adsorption belonged to reversible adsorption, and those phenolic acids of free state in soil will increase rapidly when being leached and their allelopathic potential will meanwhile inhanced. Howere, benzoic acid was very hardly to be desorbed after asorption by soil colloids, so benzoic acid mostly existed in bound state in soil which inhabited its allelopathic potential. What's more, as vanillin, ferulic acid and cinnamic acid were more likely to be degraded than p-hydroxybenzoic acid and benzoic acid, their remain time in soil were shorter than the later two phenolic acids. In our study, the metenance half time T1/2 of phenolic acids in poplar plantation was significantly different. T1/2 of p-hydroxybenzoic acid was 4.24d, benzoic acid was 2.61d, vanillin was 1.66d, ferulic acid was 1.12d and cinnamic acid was 1.29d. Consequently, compared the phenolic acids content in the rhizosphere soil of poplar forest of different rotation by normal HPLC analysis, the content of p-hydroxybenzoic acid was found significantly higher than that of benzoic acid, vanillin, ferulic acid and cinnamic acid. Over time, the remain amount of phenolic acids in soil of plantation gradually reduced. Because of biological and abiological effects, phenolic acids in the environment was degraded rapidly and phenolic acids in soil could not remain for a certain long period. This discovery offered certain base for the deeper research of the mechanism of phenolic acids exudation in poplar roots.Took hydroponiccuttings of poplar(Populus deltoids) as material and established environment lack of N and P, the quantitative analysis of phenolic acids exudation dynamic of poplar roots under nutrient stress was carried out with the help of macroporous absorbent resin and circulating gathering device. The result showed that, under stress of N deficiency, the exudation of phenolic acid from poplar roots was more than normal conditions, which p-hydroxybenzoic acid was more 125%²63%, benzoic acid was more 230%³32%, vanillin more 201%³16% and cinnamic acid more 138%¹88%. The root exudation of all phenolic acids studied of poplar cuttings enhanced over treated time. Under stress of P deficiency, the exudation of phenolic acid from poplar roots was also more than normal conditions, which p-hydroxybenzoic acid was more 96%¹24%, benzoic acid was more 222%⁷34%, vanillin more 58%¹56% and cinnamic acid more 99³28%. The root exudation of benzoic acids and vanillin significantly inhanced while treated time went on but that of benzoic acid and cinnamic acid inhanced in early stage and reduced in late stage. This fact implied that the exudation rate of phenolic acids was closely related to the nutrient stress, and environmental stress could promote the exudation of phenolic acids in poplar roots. And this discovery offered scientific base to the explanation of the accumulation mechanism of phenolic acids in soil of continuous cropping poplar forest.Based on the each content(X) of five phenolic acids in the soil of the second rotation of poplar plantation, five levels i.e. CK, 0.5 X, 1.0 X, 1.5 X and 2.0 X were established, and took hydroponic poplar cuttings as material, the growth and development of poplar cuttings under phenolic acids were studied. The result showed that: root SOD and POD activity of poplar cuttings were gradually inhibited by exogenous phenolic acids, and this fact promoted the accumulation of ROS and the overoxidation of Membrane lipid which responsible for the increasing of MDA, and MDA could combined with protein, nucleic acid and amino acid and formed insoluble deposit which could interfere the normal metabolism of cell and cause damage to the structure of cell membrane, and root activity and the absorption and conversion capacity would as well be inhibited. The change of root physiologic characteristics would finally performed on the morphologic characteristics such as root tip number, root length, root area etc. The decline of root absorption function could affected the growth and development of leaves and stems to a certain degree. The extention of leaves was inhibited by phenolic acids. The shape of leaf tended to elongate in the lengthwise direction and grow in a way deviating the replete and circular shape, and the change in shape may cause decline in photosynthetic area of poplar leaves. The content of total chlorophyll and carotenoid was significantly decreased under phenolic acids and the proportion of them could meanwhile affected the photosynthesis of the plants. Because of the adverse change in leaf morphology and content of total chlorophyll and carotenoid, the photosynthetic rate and the comprehensive utilization capacity of plants to different intensity of light declined which affected the accumulation of assimilation substance. As the planted poplar forest usually applied the continuous cropping model, thecontent of soil nutrient declined significantly and caused nutrient deficiency in forest land, meanwhile, phenolic acids inhibited the absorption of plants to nutrients such as nitrate nitrogen in soil and then intensified the nutrient deficiency in plants and finally affected the growth and development of plants. All the findings above gave an explanation to the allelopathic effect of phenolic acids to the growth and development of poplar and revealed the autotoxic effect of phenolic acids exudated by poplar.By the way of ion exchange resin membrane, the dynamic process of nutrient availability, that is the transmission and supply rate of nutrient in the soil of poplar plantation after exogenous application of phenolic acids, was studied and the correlation of nutrient availability with soil enzyme and microbes abundance was also studied. The result showed that, the nutrient availability in soil was remarkbly affected by phenolic acids, in which N availability was significantly affected by high level phenolic acids. At 7 day after exogenous application, the extraction of NH4-N was only 67.62±2.31 ug·(20cm²Â·6h)^-1, while the CK was 144.21±10.60 ug·(20cm²Â·6h)^-1 at the same time. Meanwhile, the extraction of NO3-N was respectively 24.86±1.71 and 33.74±4.44 ug·(20cm²Â·6h)^-1 in treatment and CK. P availability in soil did not change significantly in low level phenolic acids but increased in certain degree under high level phenolic acids; K availability performed significantly decline in relatively high level phenolic acids; Mn availability increased while the level of phenolic acids rose; Fe and Cu availability were significantly inhibited by high level phenolic acids. After the introducing of phenolic acids, as culture time went on, the availability of NH4-N, PO4 and Mn were gradually inhanced while that of NO3-N, K, Fe, Zn and Cu were inhibited. The result of simple and typical correlation analysis of soil nutrient availability with soil enzyme activity and microbe number showed that: there existed close relation between the conversion and supply of NO3-N, K, Fe, Mn etc. and the activity of urease, polyphenol oxidase and phosphatase; close relation between the conversion and supply of NH4-N, NO3-N, K, Mn etc. and fungi, actinomycete, ammonifiers and nitrosomas were also found. The conclusion above gave an explanation to the change law of soil nutrient availability and its relation to soil enzyme and mocrobes under phenolic acids.Based on the research mentioned above, the allelopathic mechanism model of soil degradation in continuous cropping forest was put forward.