Abstract: This research aimed to explore the effects of different alfalfa crop rotations on soil nutrient, microorganism abundance, and allelochemical (autotoxin) levels. The alfalfa cultivar used was ‘Gannong No. 3’ and rotations compared were alfalfa-wheat (AW), alfalfa-wheat-wheat (AWW), alfalfa-corn (AC), alfalfa-corn-corn (ACC), and continuous alfalfa (CK). The results showed that: 1) Compared with CK, after the AC rotation, soil organic matter, ammonia, alkali decomposed ammonia, quick-test phosphorus were all reduced; after the AC rotation, the same soil parameters were more depleted than in AW (P<0.05). 2) In the various alfalfa-corn and alfalfa-wheat rotations, the numbers of soil bacteria and actinomycetes were increased significantly (P<0.05), with the treatment rankings being AWW>AW>ACC>AC. However, the number of fungi decreased, compared with CK, with percentage decreases being 48.96%, 65.19%, 66.72%, and 74.91% (P<0.05) for AC, ACC, AW, and AWW, respectively. 3) In the various alfalfa-corn and alfalfa-wheat rotations, soil levels of major allelochemicals, including chlorogenic acid, ferulic acid, caffeic acid, p-hydroxybenzoic acid, and coumarin, were all reduced. Average reductions after one year were, respectively, 17.90% and 27.50% (P<0.05) compared with CK, for AC and AW rotations. In AWW the allelochemical reduction compared to CK was 24.20%. It is concluded that well managed rotation of alfalfa, corn, and wheat could significantly improve soil microbial ecology, reduce the soil autotoxin content, and that soil nutrient deficiency was not the primary limiting factor to continuous cropping of alfalfa. In the semi-arid area in which the study was conducted, alfalfa-wheat rotation appeared the most suitable of the tested rotations and 2 years in wheat had a more pronounced soil detoxification effect. Alfalfa-corn rotations should be avoided, as they excessively deplete farmland fertility. Additionally, the population counts of soil microorganisms (particularly actinomycetes) and levels of soil allelochemicals assayed (as listed above) were generally significantly related.

Key words: alfalfa-corn rotation, alfalfa-wheat rotation, soil nutrients, soil microorganism, soil autotoxin