The use of green manure plants for soil restoration is a viable agricultural practice that can mitigate soil degradation and biodiversity loss caused by the long-term application of inorganic fertilizers. However, the effects of green manure on soil microbial communities, rice yield, and quality in the karst mountains are largely unknown. The effects of no chemical fertilizer, chemical fertilizer, chemical fertilizer + different Astragalus sinicus L. (Chinese milk vetch, CMV) treatments on the microbial community, soil enzyme activities, soil nutrient content, and crop yield were investigated through field experiments. A moderate application of chemical fertilizer with green manure can increase chlorophyll content, increase effective rice spikes, positive impact on rice yield, and increase crude protein, etc. Additional application of the moderate amount of CMV can increase alkali-hydrolyzable nitrogen and available phosphorus (a significant increase of 54.87–72.65%), improve microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) (CFMV2 significantly increased by 22.16%), improve soil urease and phosphatase activities, and the urease activity increased by 43.43–69.24% with CMV application compared to CK. Moreover, all bacterial communities were dominated by three major phyla (Proteobacteria, Chloroflexi, and Acidobacteria), where the application of chemical fertilizer with CMV increased the ratio of abundance of Proteobacteria and Acidobacteria in rice soils, and the effect of chemical fertilizer application on the dominant bacteria was regulated to some extent by additional green manure application, which may have a beneficial effect on rice yield. Therefore, we conclude that the rational use of chemical fertilizers with CMV (22,500 kg ha−1) in karst landscapes is one of the effective measures to achieve efficient and sustainable use of rice fields.

Xanthomonas oryzae pv. oryzicola (Xoo) is a type of bacteria that causes bacterial leaf blight disease in rice plants. This disease is substantially harmful, and the current prevention and control measures are facing challenges. This study has investigated the effectiveness of the control activity that the endophytic fungus NS7 fermented from Dendrobium candidum possessed against Xoo. Twenty-eight novel mesoionic compounds were designed and synthesized based on the natural compound D. These compounds displayed moderate to excellent anti-Xoo activity in vitro. Notably, compound 24 exhibited prominent anti-Xoo activity in vitro with an EC 50 value of 40.3 mg/L, which was better than that of the positive control thiodiazole copper (TC)(71.2 mg/L) and the lead compound D (108.1 mg/L). In vivo pot experiments on Xoo showed that compound 24 exhibited protective and curative activities of 39.4 and 30.4%, respectively, which were better than those of TC (35.7 and 28.8%, respectively). Further, a preliminary mechanism study indicated that compound 24 could enhance the activity of defense enzymes to improve the ability for anti-Xoo. Meanwhile, compound 24 could also regulate the carbon fixation in photosynthetic organisms, which might be related to the enhanced immune function of rice. This study offers a new strategy for discovering antibacterial agents based on natural products.

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