| Eucalyptus,a vital timber species in southern China,has faced challenges due to monoculture practices and successive generations of continuous cultivation.These practices have led to soil degradation,decreased productivity,and an unstable ecological environment,severely impeding the sustainable development of Eucalyptus plantations in China.Planting mixed forests is a primary solution to these issues,effectively enhancing soil nutrient conditions,enriching biodiversity,and improving productivity.However,the current research focuses on the whole soil level,and the distribution characteristics of carbon（C）,nitrogen（N）,and phosphorus（P）in the soil microenvironment of pure and mixed Eucalyptus stands remain elusive.Therefore,from the perspective of aggregates,the stability of soil aggregates,soil carbon,nitrogen and phosphorus content and stoichiometric characteristics of different types of Eucalyptus plantations were discussed in order to provide a theoretical basis for the maintenance and improvement of soil nutrients in Eucalyptus plantations and the sustainable management of soil ecosystems.In this study,the undisturbed soil samples of 0%20cm and 20%40 cm soil depths were collected from the mixed forest of Eucalyptus and Mytilaria laosensis（I）,the mixed forest of Eucalyptus and Erythrophleum fordii（II）and the pure Eucalyptus forest（III）,respectively.Soil aggregates with four different sizes were obtained through the dry sieve method,including the>2 mm,1%2 mm,0.25%1 mm,and<0.25 mm aggregates,and then determined the indexes of soil carbon,nitrogen and phosphorus of each aggregate size and analyze their correlation.The main research conclusions are as follows:（1）The proportion of soil aggregates increased with aggregate size increasing in Eucalyptus plantations with different stand types,and the>2 mm aggregate proportion was dominated in the three stands.The order of average weight diameter and geometric average diameter of soil aggregates is Eucalyptus-Mytilaria laosensis mixed forest>Eucalyptus-Erythrophleum fordii mixed forest>Eucalyptus pure forest,and they increase with the deepening of soil layer.（2）The content and storage of the organic carbon,labile organic carbon,recalcitrant organic carbon,medium-labile organic carbon and low-labile organic carbon in the whole soil and aggregates of different Eucalyptus forest types were higher in the mixed forest than in the pure forest,and decreased with the deepening of the soil layer.The content of organic carbon and its components in different Eucalyptus stands were concentrated in aggregates with a particle size of 1-0.25 mm in both soil layers,and the content of particles with a particle size of>2 mm was the lowest.However,the stock and contribution rate of carbon and its fractions in soil aggregates were the highest in>2 mm aggregates.（3）The content of total nitrogen,acid-hydrolyzable nitrogen,non hydrolyzable nitrogen,ammonium nitrogen,amino acid nitrogen,and amino sugar nitrogen in the whole soil and aggregates of different particle sizes of different Eucalyptus forest types is mainly higher in the mixed Eucalyptus forest than in the pure Eucalyptus forest,with forest I significantly higher than forest II.There was no significant difference in the content of NO3--N and NH4+-N among the three forest stands in the 0%20cm soil layer.The content of total nitrogen and its components in different Eucalyptus stands was mainly distributed in aggregates with a particle size of 0.25%1 mm in both soil layers,and the content of particles with a particle size of>2 mm was the lowest.The stock and contribution rate of nitrogen and its fractions in soil aggregates were the highest in>2 mm aggregates.（4）The total phosphorus content of the soil in the Eucalyptus-Mytilaria laosensis mixed forest was significantly higher than that in the pure Eucalyptus forest.The contents of soil resin-Pi（inorganic phosphorus）,Na HCO3-Po（organophosphorus）,Na HCO3-Pi,Na OH-Po,Na OH-Pi,dil.HCl-P,conc.HCl-Po,conc.HCl-Pi and residue phosphorus were generally distributed in the order of Eucalyptus-Mytilaria laosensis mixed forest>Eucalyptus-Erythrophleum fordii mixed forest>Eucalyptus pure forest.The content of total phosphorus and its components in different Eucalyptus stands is generally distributed in 0.25%1mm aggregates in both soil layers,while the content of Na OH-Po and Na OH-Pi were mainly distributed in>2 mm aggregates.The stock and contribution rate of phosphorus and its fractions in soil aggregates were the highest in>2 mm aggregates.（5）The soil C:N:P of mixed Eucalyptus forest was higher than that of pure Eucalyptus forest.In the 0%20cm soil layer,there was no significant difference in the total soil C:N and N:P among the three stands,and C:N and C:P in>2 mm aggregates were smaller than other particle sizes,with no significant difference in N:P.In the soil layer of 20%40 cm,the C:N,C:P,and N:P of the three stands with particle sizes>2 mm were all smaller than the other particle sizes.（6）Redundancy analysis and correlation analysis showed that the proportion of<0.25 mm aggregate was the main factor affecting the content of soil C,N,P and its components.The proportion of<0.25 mm aggregate was significantly negatively correlated with soil C,N,P and their fractions contents.In conclusion,compared to pure Eucalyptus forests,mixed forests exhibit a more stable soil structure and higher C,N,and P reserves.Establishing Eucalyptus mixed forests to improve soil fertility can provide a theoretical basis for promoting healthy soil environmental quality and facilitating sustainable soil ecosystem management.Among the studied forests,the Eucalyptus-Mytilaria laosensis mixed forest displayed generally superior soil structure,C,N,and P content,and storage compared to the Eucalyptus-Erythrophleum fordii mixed forest.Furthermore,>2 mm aggregates serve as the primary carriers of soil C,N,and P storage.Increasing the proportion of>2 mm soil aggregates may contribute to enhanced soil C,N,and P storage.Lastly,it is recommended to apply phosphorus fertilizer appropriately in Eucalyptus plantations to prevent phosphorus limitations on sustainable soil utilization. |
