| Boron is an essential micronutrient element for the growth and development of eucalyptus.Lack of boron can lead to treetop degradation,red leaf dieback disease,growth retarded or even death of eucalyptus,and seriously affect the biomass and yield of eucalyptus plantation.However,what changes of soil boron occur during the cultivation of eucalyptus?What factors affect the change of boron?What are the influencing mechanisms?None of these questions is clear.Thus,to explore the soil boron nutrition level changes of eucalyptus plantation resulting from long-term management,the change regularity and influencing factors of soil boron status in eucalyptus plantation of different stand ages were studied.In this study,E.urophylla×E.grandis plantations of 5 years(5yr),7 years(7yr),8 years(8yr)and 11 years(llyr)and non-forest site to be used for eucalyptus plantations(0yr)were selected as research objects.Soil samples of 0～20 cm,20～40 cm and 40～60 cm were collected from each plot,and the contents of different forms of boron,soil physical properties,soil chemical properties,soil microbial biomass and soil enzyme activities were determined.Using principal component analysis,redundancy analysis,canonical correlation analysis and other statistical methods,this study clarified the changes and influencing factors of soil boron pools in eucalyptus plantations of different stand ages,in order to provide reference for boron nutrition management of eucalyptus plantations and theoretical basis for sustainable development of eucalyptus plantations.The main conclusions are as follows:(1)Eucalyptus planting changed the status characteristics of water-soluble boron and specifically absorbed boron in soil,which may be related to the absorption of boron by eucalyptus trees and the change of soil properties caused by eucalyptus management.In 0～20cm and 20～40cm soil layers,the content of water-soluble boron was lower than that in non-forest site at 5yr and llyr eucalyptus plantations,but higher than that in non-forest site at 7th and 8th years eucalyptus plantations.In the 11th year of eucalyptus planting,the content of specifically absorbed boron was the lowest,and the supplementation of water-soluble boron and specifically absorbed boron should be paid attention to at the same time.(2)Long-term planting of eucalyptus decreased the levels of nonspecifically adsorbed boron,boron occluded in manganese(Mn)oxides and boron occluded in noncrystalline iron(Fe)and aluminum(Al)oxides oxides in the soil layers of 20～40 cm and 40～60 cm,and changed the distribution of nonspecifically adsorbed boron in the soil profile,which may be related to the transformation of boron morphology caused by the change of soil properties.In the soil layers of 20～40 cm and 40～60 cm,the average contents of boron occluded in noncrystalline Fe and A1 oxides were the highest in the soil layers of non-forest site,and the nonspecifically adsorbed boron and boron occluded in Mn oxides were the highest in the soil layers of 20-40 cm and 40-60 cm.At non-forest site,the content of nonspecifically adsorbed boron in soil layer of 0-20 cm was significantly higher than that in soil layers of 20-40 cm and 40-60 cm,but there was no significant difference in the content of nonspecifically adsorbed boron in different soil layers of eucalyptus plantation of the same age.(3)Since residual boron is difficult to be released and utilized,eucalyptus planting has a very limited influence on the total boron,residual boron.Soil residual boron accounted for 80.83%～95.49%of total soil boron in the sample plots.There was a significant positive correlation between total soil boron and soil residual boron,but there was no significant correlation between total soil boron,soil residual boron and other forms of boron.(4)With the increase of management years,the improvement of soil properties of eucalyptus plantation is the key to maintain soil boron nutrient level.Eucalyptus planting can change soil physical properties,and soil physical properties are closely related to nonspecifically adsorbed boron,boron occluded in Mn oxides and boron occluded in noncrystalline Fe and Al oxides.The storage of soil carbon,nitrogen and phosphorus in eucalyptus plantations can improve the availability of soil boron.The contents of soil organic matter,total nitrogen and total phosphorus in eucalyptus plantations are positively correlated with specifically absorbed boron,boron occluded in Mn oxides and amorphous iron and boron occluded in noncrystalline Fe and Al oxides.The improvement of soil microbial biomass and enzyme activity has a positive effect on the improvement of boron pool structure,releasing the difficult part of soil boron pool,and increasing the content of boron that is easy to be absorbed and utilized by plants.There were positive correlations between soil microbial biomass,enzyme activities and boron forms except residual boron and total boron.Soil residual boron and total boron were negatively correlated with microbial biomass carbon,acid phosphatase activity,invertase activity,catalase activity and urease activity.(5)Different forms of boron in eucalyptus plantation soil were classified and studied,which simplified the diagnosis and management techniques of boron nutrition in eucalyptus plantation soil.According to the result of principal component analysis,the soil boron forms were divided into three categories:nonspecifically absorbed boron,boron occluded in manganese oxides and boron occluded in noncrystalline iron and aluminum oxides oxides,soil water-soluble boron and specifically absorbed boron,and soil total boron and residual boron.The results of principal component analysis showed that the highest Norm values were soil water-soluble boron,soil nonspecifically absorbed boron,and soil total boron,indicating that they were the most representative forms of each type of boron.The contents of soil water-soluble boron and soil nonspecifically absorbed boron could be used to represent the plant availability level of soil boron pool. |
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