Influence of Alfalfa (Medicago sativa L.) at Various Growing Years on the Physico-chemical Properties and Microbiology of Irrigated Desert Soils

Background: The arid area of northwest China is the main producing area of alfalfa (Medicago sativa L.) characterized by low rainfall, low vegetation coverage and vulnerable ecosystem. However, there are still very few reports about the influence of alfalfa at various growing years on the physico-chemical properties and microbial community structure in typical northwestern irrigated desert soil. This study has important theoretical and practical guiding significance for providing reference and scientific evidence for local alfalfa industry development and protection of soil ecosystem to maintain efficient alfalfa production. Methods: The rhizosphere soils of "Sanditi" alfalfa and Longdong" alfalfa were collected to assess the influence of alfalfa varieties, growing years and growth stages on the physico-chemical properties, microbial biomass and microbial community structure. There were four treatments in this experiment: 1. "Sanditi" alfalfa planted in 2015 (S1); 2. "Sanditi" alfalfa planted in 2016 (S2); 3. "Sanditi" alfalfa planted in 2017 (S3); 4. "Longdong" alfalfa planted in 2017 (LD). Result: Soil of all treatments had consistent pH at 8.11-8.45. The average soil organic matter content was 11.02 g . kg(-1), 11.43 g . kg(-1), 11.26 g. kg(-1) and 11.20 g. kg(-1) respectively for LD, S1, S2 and S3 (P>0.05). The total nitrogen content was 0.451 g. kg(-1), 0.468 g. 0.437 g. kg(-1) and 0.428 g. kg(-1), respectively for growing seasons of LD, S1, S2 and S3 (P>0.05). The results suggested that alfalfa cropping did not significantly influence the soil organic matter content in 3 years. S1 significantly changed the ratio of fungal to bacterial typical fatty acids, which was significantly higher than S3 and LD, but insignificantly higher than S2. Continuous cropping of alfalfa in arid area affected the diversity of the fungi community and reduced soil bacterial community richness and diversity.