Effect of nitrogen application level on abundance and community structure of paddy soil bacteria under ricerice-Chinese milk vetch(Astragalus sinicus L.)cropping system

In order to study the effect of nitrogen rate on soil microbial ecology and provide a basis for the promotion of soil nutrient management in paddy fields, we investigated the abundance and community structure of paddy soil bacteria in a rice-rice-vetch cropping system. Soil samples were taken from an 8-year continuous field trial, with three treatments(no nitrogen application, 0 kg N·hm-2, CN0; middle nitrogen application level, 100 kg N·hm-2, CN100; high nitrogen application level, 200 kg N·hm-2, CN200) under milk vetch returning condition, and fallow was set as control(CK). Soil bacterial abundance and community structure were estimated by using fluorescence quantitative PCR and Illumina MiSeq high-throughput sequencing platform, respectively. The results showed that the abundance of soil bacteria ranged from 1.25×108 to 8.47×109 copies per gram dry soil in all soil samples. Sequencing results indicated that community diversity indexes(Shannon and Simpson) and community richness indexes(Ace and Chao) had significant variation among different treatments at 0.05 probability level. Proteobacteria, Chloroflexi and Nitrospirae were three mainly soil bacterial groups under Phylum classification level, accounting for total proportion of OTU 42.22%~54.54%, 8.56%~20.73% and 10.98%~15.08%, respectively. The relative abundance of Proteobacteria of soils from CN100 was lower than those from the other three treatments, while Chloroflexi relative abundance was 2.26, 1.58 and 1.17 times higher than CN200, CN0 and CK, respectively. Correlation analysis showed that bacterial 16S rRNA gene copy numbers was positively correlated with soil ammonium nitrogen. Factors analysis of soil bacterial community structure indicated that there was a close correlation between soil bacteria and soil chemical indicators. A forementioned results demonstrated that soil microbial quantity and community structure could be significantly altered by nitrogen application combined with vetch incorporation in double cropping soils.