Abstract: The soil bacterial community composition, abundance and diversity were studied in alfalfa stands between 1-6 years of age (i.e. established 2012-2017), in the Ningxia Yellow River irrigation area. The determination of the soil bacterial community characteristics was achieved by DNA extraction followed by PCR amplification, and sequencing using the HiSeq platform. The 16S rRNA amplifier sequencing technique was used in the study. The correlation between soil nutrients and bacterial population diversity was analyzed. The results indicate that there are about 40 phyla, 78 classes, 151 orders, 275 families and 416 genera of bacteria in the soils of the various alfalfa stands of different ages. At the phylum level, the dominant genera of bacteria detected were Proteobacteria, Acidobacteria and Bacteroidetes, accounting for 68% of the total bacterial population. The ranking of bacterial abundance and diversity with stand age was: 1 yr>5 yr>3 yr>4 yr>2 yr>6 yr. Soil pH, organic carbon, total nitrogen and alkali-hydrolysable nitrogen all decreased initially, and then increased with stand age, reaching their maxima at year 5. The result is basically consistent with findings of other studies of factors affecting bacterial community composition. Analysis using Spearman's correlation showed that organic carbon, total nitrogen and alkaline nitrogen were significantly (P<0.05) correlated with bacterial community composition. In general, the dominant taxa of soil bacterial communities present in the Yellow River irrigation area did not change with stand age, but population numbers and bacterial community diversity did vary from year to year, though not with any consistent trend over time. Soil organic carbon, soil alkali-hydrolysable nitrogen and soil total nitrogen were the main factors associated with changes in bacterial taxa present across years.

Key words: alfalfa, bacterial community, bacterial abundance, diversity, soil nutrients