Effects of Water and Nitrogen on Growth, Rhizosphere Environment, and Microbial Community of <i>Sophora alopecuroides</i>: Their Interrelationship

The effective management of water and nitrogen is crucial in the artificial cultivation of medicinal plants. <i>Sophora alopecuroides</i>, a perennial herbaceous plant in the Fabaceae family, is extensively used in medicine, with alkaloids as its primary bioactive constituents. Nevertheless, there remains a significant knowledge gap regarding how rhizospheric microbial communities respond to varying water and nitrogen conditions and their intricate relationships with soil environments and the growth of <i>S. alopecuroides</i>. In this study, two-year-old <i>S. alopecuroides</i> were used in a two-factor, three-level water-nitrogen interaction experiment. The irrigation levels included W1 (30–35% of maximum water holding capacity), W2 (50–55%), and W3 (70–75%), while nitrogen levels comprised N1 (32 mg/kg), N2 (64 mg/kg), and N3 (128 mg/kg). The study assessed plant growth indicators, total alkaloid content, and rhizospheric soil physicochemical parameters of <i>S. alopecuroides</i>. High-throughput sequencing (16S rRNA and ITS) was employed to analyze variations in rhizospheric microbial community composition and structure. The results showed that Proteobacteria and Ascomycota are the predominant bacterial and fungal phyla in the rhizosphere microbial community of <i>S. alopecuroides</i>. The highest biomass and alkaloid accumulation of <i>S. alopecuroides</i> were observed under the N1W3 treatment (50% nitrogen application and 70–75% of maximum water holding capacity). Specifically, six bacterial genus-level biomarkers (<i>TRA3_20</i>, <i>MND1</i>, <i>env_OPS_17</i>, <i>SBR1031</i>, <i>Haliangium</i>, <i>S0134_terrestrial_group</i>) and six fungal genus-level biomarkers (<i>Pseudeurotium</i>, <i>Rhizophagus</i>, <i>Patinella</i>, <i>Pseudeurotium</i>, <i>Patinella</i>, <i>Rhizophagus</i>) were identified under the N1W3 treatment condition. In the partial least squares path modeling (PLS-PM), water and nitrogen treatments demonstrated markedly positive direct effects on soil physicochemical parameters (<i>p</i> < 0.01), while showing significant negative direct impacts on alkaloid accumulation and plant growth indicators (<i>p</i> < 0.05). Soil physicochemical parameters, in turn, significantly negatively affected the rhizosphere fungal community (<i>p</i> < 0.05). Additionally, the rhizosphere fungal community exhibited highly significant negative direct effects on both the plant growth indicators and total alkaloid content of <i>S. alopecuroides</i> (<i>p</i> < 0.01). This study provides new insights into the interactions among rhizosphere soil environment, rhizosphere microbiota, plant growth, and alkaloid accumulation under water and nitrogen regulation, offering a scientific basis for the water and nitrogen management in the cultivation of <i>S. alopecuroides</i>.