Soil Fertility Quality Evaluation And Aggregate Organic Carbon Distribution Characteristics Of Eucalyptus Pure Forest And Its Mixed Forest

Eucalyptus robusta is an important fast-growing tree species in China,which is widely distributed in southern China.However,with the expansion of Eucalyptus planting area,coupled with short-term continuous planting,high-intensity and pure forest management,a series of ecological and environmental problems have occurred in Eucalyptus plantations.Among them,the problem of soil fertility decline and soil organic carbon content reduction is more serious.The mixed treatment is considered to be a better solution to alleviate the soil quality problem of plantation.In this study,Pinus massoniana,Erythrophleum fordii and Castanopsis fissa were introduced as mixed tree species to improve soil conditions.In this study,Eucalyptus pure forest and its mixed forest were taken as the research object.By analyzing the soil fertility quality and aggregate organic carbon distribution characteristics of Eucalyptus continuous planting and three Eucalyptus mixed forests,the improvement ability of Eucalyptus mixed forest on soil fertility quality and soil organic carbon of Eucalyptus pure forest was explored.From many aspects to study the law of continuous planting of eucalyptus and the improvement of mixed soil,it provides more possibilities for eucalyptus mixed,and also provides a reference for the sustainable development of eucalyptus economy.The main conclusions are as follows :(1)Except for bulk density and aeration,the physical properties of Eucalyptus pure forest and its mixed forest have significant differences in maximum water holding capacity,capillary water holding capacity,minimum water holding capacity,non-capillary porosity,capillary porosity and total porosity among different soil layers.The soil bulk density of Eucalyptus pure forest decreased first and then increased with the increase of continuous planting generations.The soil water holding capacity and porosity decreased with the increase of continuous planting generations.Continuous planting of eucalyptus increased soil bulk density,decreased soil water holding capacity,decreased soil porosity,and decreased soil aeration and storage capacity.The soil physical properties of the three eucalyptus mixed forests were improved to varying degrees compared with the third-generation eucalyptus pure forest,indicating that the eucalyptus mixed forest could improve the soil performance decline caused by the continuous planting of eucalyptus,and the soil condition of the eucalyptus ×Pinus massoniana ×Erythrophleum fordii mixed forest was the best.(2)There were significant differences in p H,organic matter and available potassium in soil chemical indexes of Eucalyptus pure forest and mixed forest in different soil layers.There were significant differences in total nitrogen,total phosphorus,total potassium and alkali-hydrolyzable nitrogen only in surface soil,and surface soil was significantly higher than deep soil.The chemical indexes in the soil of Eucalyptus pure forest were gradually reduced by continuous planting,and some nutrient elements had a brief increase in the second generation,but the overall trend was still downward.The soil p H value,soil organic matter content,nitrogen,phosphorus and potassium content in the three mixed forests were higher than those in the third generation eucalyptus pure forest.It shows that Eucalyptus mixed forest can curb the loss of nutrient elements caused by Eucalyptus,and the nutrient content of Eucalyptus × Pinus massoniana ×Erythrophleum fordii mixed forest is higher than that of the other two mixed forests.(3)The soil enzyme activities of Eucalyptus pure forest and mixed forest were significantly different in the soil surface,and the enzyme activity decreased with the increase of soil depth.The soil enzyme activity in Eucalyptus pure forest decreased with the increase of continuous planting generations.The mixed forest with the highest soil catalase,sucrase and urease activities was Eucalyptus × Pinus massoniana ×Erythrophleum fordii,and the mixed forest with the highest acid phosphatase activity was Eucalyptus × Pinus massoniana..(4)Principal component analysis and correlation analysis were used to comprehensively evaluate the soil fertility quality of Eucalyptus pure forest and its mixed forest.There were significant differences in soil fertility quality index between Eucalyptus pure forest and its mixed forest.From large to small,the order was Eucalyptus × Pinus massoniana ×Erythrophleum fordii > Eucalyptus × Pinus massoniana > Eucalyptus 2 > Eucalyptus × Pinus massoniana × Erythrophleum fordii > Eucalyptus 1 > Eucalyptus 3.The evaluation results showed that the short-term multi-generation continuous loading of Eucalyptus pure forest would cause the decrease of soil fertility quality.Eucalyptus mixed forest could effectively improve the adverse effects caused by Eucalyptus continuous loading and prevent further decline of soil fertility.The selection of mixed tree species and reasonable tending measures should be paid attention to in forest management.(5)The distribution of soil aggregate content in Eucalyptus pure forest and its mixed forest decreased with the decrease of particle size,and the content of different particle size aggregates decreased with the deepening of soil layer.The content of > 2mm aggregates was the highest,and the content of < 0.053 mm aggregates was the lowest.The mean weight diameter(MWD)and geometric mean diameter(GMD)increased with the deepening of soil layer,and the fractal dimension(Dm)decreased with the deepening of soil layer.The higher the stability of soil aggregates,the lower the organic carbon content of soil aggregates with the deepening of soil layer,reaching the maximum in < 0.053 mm aggregates,and the contribution rate of > 2mm aggregates to soil organic carbon was the highest.Continuous planting of eucalyptus will reduce the content of large aggregates,resulting in an increase in the content of small aggregates.The structural stability of soil aggregates and the content of organic carbon also decrease with the increase of continuous planting generations.Eucalyptus mixed forest can increase the content of large aggregates in soil and the stability of aggregate structure.