Abstract: In order to determine the effects of different crop rotations on alfalfa yield and soil quality in the Jiang-huai area, an experiment was set up with the following three crop rotation treatments: (i) a Control (CK) with four years of continuous monoculture alfalfa, then replanted for one year of measurement; (ii) a 3-year sequence of alfalfa-maize-alfalfa (T₁); and (iii) a 3-year sequence of alfalfa-sorghum-alfalfa (T₂). Traditional methods were used to determine the alfalfa yield and physical and chemical properties of the soil in different soil depths. Enzyme activities in rhizosphere soil were determined using proprietary analytical kits, and the diversity of the bacterial community in the rhizosphere soil was determined by Illumina Miseq analysis. Incorporation of maize and sorghum into the rotation significantly improved alfalfa yield in the final measurement year. Specifically, alfalfa yield in T₁ and T₂ was, respectively, 1.27 and 1.13 times higher than that in the continuous alfalfa (CK) treatment. Maize rotation (T₁) significantly (P<0.05) increased soil total nitrogen, while inclusion of sorghum in the rotation (T₂) significantly (P<0.05) increased soil pH and organic matter. Both maize and sorghum rotations significantly (P<0.05) increased the soil available potassium content. Sorghum rotation (T₂) significantly (P<0.05) increased soil pH and soil organic matter content. T₁ and T₂ treatments reduced soil bulk density; the sorghum rotation more so than the maize rotation. Compared with alfalfa continuous cropping, catalase and urease activities in soils of the maize and sorghum rotations were significantly (P<0.05) higher than those in CK-treatment soil. The Illumina Miseq analysis indicated that the soil bacterial community diversity was significantly higher in T₁ and T₂ soils than in CK, and the OTU count was, respectively, 1.25 and 1.39 times higher than that in the continuous alfalfa CK treatment. In addition, the Shannon, Chao, and Sobs diversity indices in T₂ soil were significantly higher than in CK (P<0.05). The relative abundance of Proteobacteria and Firmicutes in CK soil was significantly higher than that in T₁ and T₂ soils, while the relative abundance of Chloroflexi and Planctomycetes in CK soil was significantly lower than in the T₁ and T₂ soils. It was concluded that the maize and sorghum crop rotations improved soil fertility and soil enzyme activities, stabilized bacterial community structure and thus increased the alfalfa yield.

Key words: alfalfa, rotation, soil bacteria community composition, soil enzyme activities, Illumina Miseq