Effect of drought stress on symbiotic nitrogen fixation, soil nitrogen availability and soil microbial diversity in forage legumes

Background and aims Forage legumes form mutualistic interactions with specialized soil rhizobia bacteria that inhabit root nodules and fix atmospheric nitrogen. However, legumes are sensitive to drought stress, which can interrupt nodulation and symbiotic nitrogen fixation (SNF). We hypothesize that drought-impaired SNF may influence soil nitrogen availability and soil microbial diversity. Methods Here, we evaluated the effects of drought on nodulation, plant growth, physiological parameters, SNF, soil nitrogen availability, soil extracellular enzyme activity, and soil microbiome of alfalfa ( Medicago sativa ) and red clover ( Trifolium pratense ). The drought treatments were imposed at the flowering stage by maintaining soil moisture contents at 20% field capacity (FC) (severe drought), 40% FC (moderate drought), and 80% FC (well-watered) for three weeks. Results Drought significantly reduced nodulation, root and shoot growth, and SNF in alfalfa and red clover. Soil available nitrogen was significantly increased following severe drought conditions. The enzyme assays showed reduced activity of N-acetyl-glucosaminidase and β-D cellobiosidase enzymes under drought stress in alfalfa and red clover, respectively. Microbiome data showed shifts in the relative abundance of some key bacterial taxa under drought stress. Conclusion Overall results indicate that drought has deleterious effects on SNF and plant growth, affecting carbon and nitrogen cycling enzymes, soil nitrogen availability, and soil microbial diversity.