| By changing the soil microbial biomass,community composition and soil enzyme activities,nitrogen addition can affect soil fertility.In order to investigate the effects of nitrogen addition on soil fertility of Fraxinus mandshurica plantations,a 17-year-old Fraxinus mandshurica plantations was selected,and four nitrogen addition treatments were set up(CK:no fertilization,N1:5 g·m-2,N2:12.5 g·m-2,N3:20 g·m-2),soil microbial C,N and P contents,soil microbial community structure and four hydrolase activities related to soil C,N and P cycles were determined after fertilization.The main results are as follows:（1）Compared with CK,nitrogen addition increased the number of soil bacterial and fungal taxa at all levels,but there was only a significant difference in the class and order taxa of fungal classification.The dominant phyla of bacteria were proteobacteria,acidobacteria and actinobacteria,while the dominant phyla of fungi were ascomycetes,mortierella and basidiomycetes.Under different nitrogen application treatments,the Chao1 index and Simpson index of bacterial community were N1>CK>N2>N3,while the Chao1 index,Shannon index and Simpson index of fungal community were decreased.（2）The contents of MBC,MBN and MBP in 0-10cm soil layer of Fraxinus mandshurica plantations under different treatments were higher than those in 10-20cm soil layer.In 0-10cm soil layer,except In June,nitrogen application reduced soil MBC content,and soil MBP content showed a trend of first decreasing,then increasing and then decreasing.Except August,N1 and N3 treatments reduced soil MBN content.In the 10-20cm soil layer,N3 treatment reduced soil MBC and MBP contents.With the increase of nitrogen application concentration,soil MBN content reached the highest in N2 treatment.（3）The activities ofβG,βX,NAG and AP in 0-10cm soil layer of Fraxinus mandshurica plantations under different treatments were higher than those in 10-20cm soil layer.In 0-10cm soil layer,the activities ofβG,βX,NAG and AP decreased under N3 treatment except In May.In the soil layer of 10-20cm,nitrogen application increased soilβG activity except in August.Except September,soilβX,NAG and AP activities increased under N2 treatment.N3treatment decreased soilβX and NAG activities,and increased soil AP activities.（4）The C,N and P contents in 0-10cm soil layer of Fraxinus mandshurica plantations were higher than those in 10-20cm soil layer.In 0-10cm soil layer,with the increase of nitrogen application concentration,soil C content under N1 and N2 treatment was higher than CK treatment,but it was opposite under N3 treatment.Soil N content increased first and then decreased,which were higher than CK treatment.Soil P content decreased firstly,then increased and then decreased,and reached the highest value under N2 treatment.In the 10-20cm soil layer,with the increase of nitrogen application concentration,soil N content decreased firstly,then increased and then decreased,and reached the highest under N2treatment.Soil C,N and P contents were highest in July.The results showed that nitrogen addition could change the soil microbial community structure of Fraxinus mandshurica plantations.During the growing season,soil microbial biomass,soil hydrolase activity and soil nutrients all had significant dynamic changes,but the synergistic relationship among these indicators needed further analysis. |
