The Variation Of Soil Nutrientsin Eucalyptus Plantations Of Different Generations And Ages

The extensive cultivation of Eucalyptus in southern China has caused a series of ecological problems,andcaused by pure Eucalyptus forest.which has attracted wide attention of scholars from all walks of life and has become a hot issue in current research.Weidu forest farm is a state-owned forest farm in Guangxi.The area of eucalyptus planting is about 80%of the total area of forest land,and the phenomenon of continuous planting in the field is serious,which may also be the main cause of site quality decline.In this study,the effects of different generations(one generation,two generations)and one to five years old eucalyptus forest on the growth,carbon sequestration and oxygen release of eucalyptus plantation were studied.Soil nutrient elements(total nitrogen,total phosphorus,total potassium,nitrate nitrogen,ammonium nitrogen,available phosphorus and available potassium PH value,organic matter content),microbial biomass carbon nitrogen,microbial quantity(bacteria,actinomycetes),Fungi)and five enzyme activities(sucrase,urease,acid phosphatase,catalase,polyphenol oxidase),and the correation between them,In order to improve the site quality of eucalyptus plantation and to manage eucalyptus plantation scientifically and reasonably,the site quality of continuous planting and Eucalyptus plantations of different ages were analyzed comprehensively in order to provide the basis for improving the site quality of eucalyptus plantation in the later stage.The results of this study are as follows:(1)The high breast diameter of two generation Eucalypus with continuous planting has a small increase compared with that of the first generation,and the growth rate in the first 3 years is larger,and the growth rate decreases gradually after fourth years.The biomass and carbon sequestration oxygen content are all two generation forest higher than the one generation forest,the change rule is basically consistent with the change of the diameter of the chest.(2)The soil nutrient content decreased gradually with the increase of successive cropping times,andPH,the content of organicmatter,TN,TK,TP,NO3-N,NH4+-N,AP,AK weer lower in the second generation forest than in the first generation fo,rest,but the decrease of phosphorus clement and nitrogen was not consistent in the second generation,however,the content of organic matter was lower in the second generation forest than in the first generation forest,the content of organic matter was lower than that in the second generation forest.The content of organic matter and PH decreased greatly,but the content of potassium remained the same in two generations of Eucalyptus plantations.With the increase of forest age,the content of soil PH,organic matter,nitrogen,potassium and other nutrients decreased firstly and then increased with the age of the forest.The content of organic matter,TK,nitrate nitrogen and available potassium gradually decreased from 1 to 3 years old and increased year by year.Total nitrogen content,ammonium nitrogen content and available phosphorus content decreased in 1-4 years,but phosphorus content decreased year by year,but phosphorus content did not rise,but the decrease range decreased year by year.The vertical variation of soil PH value and nutrient content was consistent.The depth of soil PH value and nutrient content decreased layer by layer,and the decrease of 20-40cm layer was much larger than that of 10-20cm layer.(3)The change of microbial biomass carbon and:itrogen was similar to that of nutrient content.Microbial biomass carbon and nitrogen had a great influence on the effect.The second generation of microbial biomass carbon in 0-10cm layer was higher than that in the first generation,but the biomass carbon content under 10cm was lower.Microbial biomass nitrogen increased with the generation increase,and the whole change range was between 17.29%-98.99%.Microbial biomass carbon and nitrogen decreased with the vertical deepening of soil,and the soil depth decreased gradually.The effect of forest age on it was basically consistent with the change rule of nutrient,which was first decreased and then increased.(4)Bacteria accounted for a large proportion of the three groups of soil microbes,and there was a very significant positive correlation between bacteria and nutrient content.Fungi,actinomycetes and most of the nutrient content showed a very significant positive correlation,but the correlation was weak.The number of bacteria,actinomycetes and fungi decreased by 3.38%-30.49%with the increase of generations,respectively.The number of bacteria in soil decreased by 0.00%-26.58%and 6.25%-26.31%,from the first year to the fourth year,and the number of actinomycetes and fungi decreased year by year,but the number of microbes increased in the fifth year.There was significant difference between the number of microorganism and that of first year.With the vertical variation of soil and the change of nutrient content index,the microorganism quantity was 0-10 cm layer>10-20cm layer>20-40cm layer,and with the deepening of soil depth,the decreaser.(5)The response of the five enzymes to the environment was not consistent.Continuous cultivation decreased the activities of sucrase,acid phosphatase and catalase,but increased the activities of urease and polyphenol oxidase.Because of the difference of soil nutrient conversion catalyzed by different enzymes,enzyme activity varies with forest age,urease and sucrase have similar changing rules,and they are strongly related to soil nitrogen,potassium,PHand organic matter content.The change trend of catalase and polyphenol oxidase was basically the same,which was affected by different nutrient content of soil.The vertical variation of soil enzyme was consistent with other indexes and decreased layer by layer.To sum up,soil site quality decreased after continuous planting,mainly by reducing the number of microbes,thereby weakening soil biochemical reaction and other activities to reduce soil nutrient content,inhibit soil enzyme activity and so on.With the increase of forest age,especially after the fifth year,soil site nutrients began to return to improve the soil site quality.Although the soil site quality was decreased by continuous planting,the forest growth and nitrogen fixation and oxygen release were increased,which contributed greatly to the carbon balance of the ecosystem.