| Eucalyptus is a major fast-growing and productive tree in China.It has been widely planted due to its wide adaptability,strong resistance to adversity,and rapid growth,and has generated enormous economic benefits.However,due to various unreasonable management measures,ecological problems such as soil degradation,decline in forest productivity and ecological service functions have repeatedly emerged.Although research has shown that combining Eucalyptus and N2-fixing species can significantly improve soil ecological status,the stability and nutrient of soil aggregates remain unknown.A pure plantation of Eucalyptus urophylla（PP）and a mixed plantation containing E.urophylla and Acacia mangium（N2-fixing tree species）（MP）were selected as the research objects.The changes in soil aggregate nutrients and soil aggregate stability,as well as their interactions with organic matter components and soil microbial community structure and function are the focus of this research.It is expected to strengthen the scientific foundation for improving soil structure,soil fertility,and long-term Eucalyptus plantation management.The main results are as follows:（1）After 17 years of introducing N2-fixing tree species into Eucalyptus plantations,the litter and fine root quality(C/Nlitter and C/Nroot)were increased significantly（P<0.05）.The soil acidification degree（p H）was decreased significantly（P<0.05）,while the contents of organic carbon（SOC）,total nitrogen（TN）and nitrate nitrogen（NO3--N）increased significantly（P<0.05）in0-10 cm and 10-20 cm soil layers.However,the soil porosity（SP）was only increased significantly（P<0.05）in the 0-10cm soil layer,and the soil bulk density（BD）was only decreased significantly（P<0.05）in the 10-20cm soil layer.（2）After 17 years of introducing N2-fixing tree species into Eucalyptus plantations,the content of organic matter components（humic acid（HA）,fulvic acid（FA）,iron-bound humin（HMi）,clay-bound humin（HMc）and insoluble humin（HMr））of soil aggregates were increased significantly（P<0.05）in most particle sizes and the content in 0～10 cm was higher than that in 10～20 cm soil layer.The changes in the proportions of the organic matter components showed a significant increase（P<0.05）in the proportion of HA（HA/SOC）and the degree of humification（PQ）,a significant decrease（P<0.05）in the proportion of HMr（HA/HMr）.（3）After 17 years of introducing N2-fixing tree species into Eucalyptus plantations,the microbial biomass（MBC,MBN and MBP）of soil aggregates were increased significantly（P<0.05）in most particle sizes and the contents of0-10 cm soil layer was greater than that of 10-20 cm soil layer.The enzyme activities（BG,NAG,LAP and ACP）of soil aggregates were increased significantly（P<0.05）in most particle sizes and the enzyme activities of 0-10cm soil layer were higher than that of 10-20 cm soil layer.The species richness（ACEB,Chao1B）and diversity（ShannonF）of bacterial community were increased significantly（P<0.05）in the 0-10 cm and 10-20 cm soil layers,but only the species diversity（ShannonF,SimpsonF）of fungal community was increased significantly（P<0.05）in the 0-10 cm soil layer.NMDS analysis showed significant changes in soil bacterial and fungal community structure（βdiversity）.The network topology parameters such as the number of nodes and edges in the bacterial and fungal co-occurrence network of MP were larger than those of PP,and the number and proportion of positively correlated edges were higher.（4）After 17 years of introducing N2-fixing tree species into Eucalyptus plantations,mean weight diameter（MWDD）and geometric mean diameter（GMDD）of mechanically stable aggregates in 0～10 cm soil layer were increased significantly（P<0.05）;mean weight diameter（MWDW）,geometric mean diameter（GMDW）,water stable aggregates（WSA）,and aggregate stability index（ASI）of water stable aggregates in 0～10 cm and 10～20 cm soil layers were increased significantly（P<0.05）,while fractal Dimensionality（DW）and percentage of aggregate destruction（PAD）were decreased significantly（P<0.05）.The mechanical stability and water stability of soil aggregates in 0～10cm were higher than those in 10～20 cm soil layer.The relative dissipation index（RSI）and relative mechanical crushing index（RMI）of MP were significantly lower than those of PP（P<0.05）.Correlation analysis and PLS-PM showed that soil properties（SOC,TN and NO3--N）,αdiversity index of bacterial（ACEB,Chao1B and ShannonB）,microbial biomass（MBC and MBN）,enzyme activity（BG,NAG,LAP and ACP）and organic matter components（HA,FA,HMi,HMc,and HMr）could promote the formation and stability of aggregates.The factors explained 42.20%of the variation of soil aggregate mechanical stability and56.80%of the variation of soil aggregate water stability.（5）After 17 years of introducing N2-fixing tree species into Eucalyptus plantations,the contents of SOC,TN and NO3--N of soil aggregates were increased significantly（P<0.05）and the contents in 0～10 cm soil layer was higher than those in 10～20 cm soil layer.The contents of total phosphorus（TP）,ammonium nitrogen（NH4+-N）and available phosphorus（AP）decreased to a certain extent in all particle sizes.There was almost no significant difference in the soil carbon-nitrogen ratio(C/Nsoil)of soil aggregates,while carbon-phosphorus ratio(C/Psoil)and nitrogen-phosphorus ratio(N/Psoil)were increased significantly in all particle sizes（P<0.05）.The nutrient contribution rate of>2mm particle size was the highest,and the nutrient contribution rate of different particle size aggregates decreased with the decrease of aggregate size.Only the nutrient contribution rate of PP and MP aggregates reached a significant difference in a small part of particle size（P<0.05）.Correlation analysis showed that litter and fine root quality,organic matter composition in soil aggregate,microbial biomass and enzyme activity in soil aggregate,αdiversity index of soil microbial and soil aggregate stability were positively correlated or significantly positively correlated（P<0.05）with SOC,TN and NO3--N.Correlation network analysis showed that the effect of microbial community on SOC,TN and NO3--N was higher than that of TP,NH4+-N and AP.Redundancy analysis showed that the influencing factors explained 82.72%of the nutrient variation of soil aggregates,and the main influencing factors were HA,FA,RSI,Clitter and MBC.In summary,the introduction of N2-fixing tree species has a significant improvement effect on the stability and nutrients of soil aggregates in Eucalyptus plantations.This study can provide a theoretical guidance for soil and water conservation,soil nutrient management and sustainable management of Eucalyptus plantations in the South Asian tropics. |
