| The traditional pure forest model of Cunninghamia lanceolata has caused many ecological problems such as biodiversity reduction,soil fertility decline and productivity decline.Coniferous and broad-leaved mixed forests can enhance the stability of stands,improve soil fertility and microbial activity,etc.However,there are few studies on the soil microbial characteristics of mixed forests in the microdomain environment.Soil aggregates are carriers of microorganisms at the microscopic scale and are closely related to soil microbial communities.Therefore,soils of pure Cunninghamia lanceolata forest(PCL),mixed forest of Cunninghamia lanceolata and Michelia macclurei(MCM)and mixed forest of Cunninghamia lanceolata and Mytilaria laosensis(MCL)were used as the research objects.The microbial community structural and functional characteristics of soil aggregates with different particle sizes(> 2 mm,0.25-2mm and <0.25 mm)in 0-10 cm and 10-20 cm soil layers were analyzed by phospholipid fatty acid method and Biolog microplate method.Exploring the relationship between soil aggregate microbial community characteristics and soil fertility factors,and revealing the key factors affecting soil aggregate microbial community characteristics,can provide theoretical references for improving soil quality of Cunninghamia lanceolata plantations and promoting the healthy and sustainable development of Cunninghamia lanceolata plantations.The main findings are as follows:(1)In the 0-10 cm soil layer,the total amount of microbial PLFAs in soil aggregates of mixed forest was higher than that of pure forest;but only MCM was significantly higher than PCL in 10-20 cm soil layer.The content distribution rules of bacteria,fungi,actinomycetes and arbuscular mycorrhizal fungi were basically similar to the total amount.The total amount of soil microbial PLFAs and the contents of various groups of microorganisms in the0-10 cm soil layer were not significantly different among the three particle size aggregates.The total amount of soil microorganisms and the contents of bacteria,actinomycetes and arbuscular mycorrhizal fungi in the 10-20 cm soil layer were the least in >2 mm aggregates.In general,the total amount of microorganisms and the contents of various groups of microorganisms decreased with the deepening of the soil layer.(2)Soil microbial communities of the Cunninghamia lanceolata and Michelia macclurei mixed forest were more abundant.The performance was as follows: in different soil layers,the microbial structure richness index of >2mm aggregates and 0.25-2mm aggregates was the highest in MCM.In the 0-10 cm soil layer,richness increased with the decrease of aggregate particle size,and the change trend of Pielou’s evenness index was the opposite.In the 10-20 cm soil layer,the four structural diversity indices showed no significant differences among different aggregates(3)The overall utilization capacity of microbial communities for 31 carbon sources in the soil aggregates of the two mixed forests was higher than that of the pure forest.The overall utilization ability of microbial communities to 31 carbon sources decreased with the increase of soil depth,and the overall utilization ability of microbial communities to carbon source was poor in >2 mm aggregates.There were also differences in the utilization of various carbon sources by microbial communities in soil aggregates.The overall performance was that the utilization of various carbon sources by microbial communities in soil aggregates in mixed forests was higher than that in pure forests.(4)The utilization of carbon sources by the microbial communities in the soil of the Cunninghamia lanceolata and Michelia macclurei mixed forest was relatively concentrated.The performance was as follows: in the 0-10 cm soil layer,the functional Simpson and Pielou’s evenness indices of >2mm aggregates and 0.25-2mm aggregates were the lowest in MCM;in the 10-20 cm soil layer,the Pielou’s evenness index of >2 mm aggregates was lowest in MCM.In the0-10 cm soil layer,the four functional diversity indices were not significantly different in aggregates of different sizes;in the 10-20 cm soil layer,the Simpson index was the highest in <0.25 mm aggregates.(5)The total amount of microbial PLFAs in soil aggregates and the content of microbial PLFAs in various groups were positively correlated with OC,TN,AN and AP,and negatively correlated with AK and p H.In >2 mm aggregates,the composition of microbial community was mainly affected by OC,and the ability of microorganisms to metabolize various carbon sources is mainly affected by p H.In soil aggregates with different particle sizes in different soil layers,the effects of OC,TN,AN,AP,AK and p H on the microbial community diversity index were different.The soil aggregate microbial communities had stronger ability to utilize three types of carbon sources: carbohydrates,polymers and amino acids.In conclusion,soil aggregate related microbial biomass(especially the 0-10 cm soil layer)of the pure Cunninghamia lanceolata forest was lower than that of the mixing Cunninghamia lanceolata with Michelia macclurei or Mytilaria laosensis.The ability of microorganisms to utilize carbon sources in soil aggregates of two mixed forests was also higher than that of pure forest.It could be seen that the two mixed forests improved the microbial community structure of soil aggregates to a certain extent,and were beneficial to the growth and metabolism of microorganisms.The results can provide more theoretical references for the sustainable management of pure Cunninghamia lanceolata plantations. |
