Alfalfa is a perennial herbaceous forage legume that is significantly and adversely affected by monocropping. Crop rotation is the most effective measure to overcome continuous cropping obstacles. However, the mechanisms of how bacterial communities are affected and the potential links between these effects and cropping systems remain poorly understood. Based on a long-term field experiments with continuous alfalfa and forage crops with alfalfa rotation in the black soil region of the western Songnen Plain in Northeast China, the alterations in soil bacterial community structure using high-throughput sequencing of the 16S rRNA gene and soil chemical properties and enzyme activities were analyzed. The alfalfa–forage oats–silage maize–alfalfa and alfalfa–silage maize–forage oats–alfalfa system significantly increase the levels of total phosphorus and available phosphorus, and promote the activities of acid phosphatase, β-glucosidase, leucine aminopeptidase, and N-acetyl-β-glucosaminidase in comparison to continuous alfalfa. While alfalfa crop rotation did not affect the α-diversity of soil bacteria, it significantly altered the bacterial community composition and structure. Some key taxa are significantly enriched in the crop rotation system soils, including Bacillus, Sphingobium, Paenibacillus, Hydrogenispora, Rubrobacter, Haliangium, and Rubellimicrobium. Additionally, crop rotation with alfalfa increased the stability and complexity of the soil bacterial co-occurrence network. Based on our findings, we recommend promoting the alfalfa–forage oats–silage maize–alfalfa and alfalfa–silage maize–forage oats–alfalfa rotation systems as ideal practices for overcoming the challenges associated with continuous cropping of alfalfa. These systems not only enhance soil nutrient content and enzyme activities but also foster a beneficial microbial community, ultimately improving soil functionality and crop performance.

Background: Selection for fall dormancy (FD) in alfalfa (Medicago sativa) can impact other traits, including dry matter yield (DMY), winter survival rate (WSR) and nutrient compositions. The current study was aimed to screen alfalfa varieties with greater yield, good quality and strong winter survival ability and thus are more suitable for planting in Heilongjiang, China and may be suitable for similar upper-latitude semiarid environments in the world. Methods: Eight alfalfa varieties (Zhaodong, Gongnong No. 2, Caribou, Gold Empress, Adrenalin, Sanditi, Sardi and WL525) were planted in regional test based on randomized block design and mowed during the early flowering stage from 2019 to 2021. The DMY, WSR and nutrient compositions were measured by weighing method, field investigation and chemical test, respectively. Result: There was a significant negative correlation (P less than 0.01) between the FD rating and WSR, with a correlation coefficient of 0.956. The three-year average DMY of Gongnong No. 2 was the greatest (12,745.65 kg ha-1) and there was a significant negative correlation (0.766, P less than 0.01) between the FD rating and DMY. The crude protein content of Gongnong No. 2 was greater (19.29%) than the other varieties. The Zhaodong had the least neutral detergent fiber (47.76%) and acid detergent fiber (30.79%).