We isolated 65 rhizobial strains from faba bean (Vicia faba L.) from Panxi, China, studied their plant growth promoting ability with nitrogen free hydroponics, genetic diversity with clustered analysis of combined ARDRA and IGS-RFLP, and phylogeny by sequence analyses of 16S rRNA gene, three housekeeping genes and symbiosis related genes. Eleven strains improved the plant shoot dry mass significantly comparing to that of not inoculated plants. According to the clustered analysis of combined ARDRA and IGS-RFLP the isolates were genetically diverse. Forty-one of 65 isolates represented Rhizobium anhuiense, and the others belonged to R. fabae, R. vallis, R. sophorae, Agrobacterium radiobacter and four species related to Rhizobium and Agrobacterium. The isolates carried four and five genotypes of nifH and nodC, respectively, in six different nifH-nodC combinations. When looking at the species-nifH-nodC combinations it is noteworthy that all but two of the six R. anhuiense isolates were different. Our results suggested that faba bean rhizobia in Panxi are diverse at species, plant growth promoting ability and symbiosis related gene levels.

Exogenous carbon addition is widely regarded as an effective soil management strategy for rapidly increasing soil organic carbon, improving soil structure and function. However, a systematic comparison of the effects of diverse organic amendments on key soil attributes and processes is needed to inform their targeted application. We evaluated the impacts of seven organic amendments (biochar, organic fertilizer, corn straw, soybean straw, rapeseed straw, green manure, and carbon material) on a purple soil (Luvic Xerosols) in a pot experiment. The results showed that organic fertilizer and carbon material performed best in enhancing soil nutrient availability and promoting soil organic carbon content. Straw amendments promoted the formation of macro-aggregates. Green manure and straws enhanced carbon transformation-related β-glucosidase and cellobiohydrolase activities. Random Forest and structural equation modeling indicated that the organic amendments enhanced maize carbon sequestration capacity and biomass by improving aggregate stability and regulating the fungal community and by increasing nutrients and enhancing active carbon fractions. Green manure and organic fertilizer demonstrated the most significant agronomic effects. These findings provide guidelines for targeted organic amendment selection in purple soil regions.

Thirty-one nodulating rhizobium strains were collected from root nodules of spring and winter type faba bean cultivars grown in micro ecoarea, i.e. the same field in Chengdu plain, China. The symbiotic efficiency and phylogeny of these strains were studied. Effectively nitrogen fixing strains were isolated from both winter type and spring type cultivars. Based on phylogenetic analysis of 16S rRNA gene and concatenated sequence of atpD, glnII and recA genes, the isolates were assigned as Rhizobium anhuiense and a potential new Rhizobium species. The isolates were diverse on symbiosis related gene level, carrying five, four and three variants of nifH, nodC and nodD, respectively. Strains carrying similar gene combinations were trapped by both winter and spring cultivars, disagreeing with the specificity of symbiotic genotypes to reported earlier faba bean ecotypes.

Phosphate fertilizer is essential for crop production but poses environmental risks in agriculture. The agronomic and environmental thresholds for phosphorus application in Southwest China’s purple soils (Luvic Xerosols) remain poorly defined. We combined soil phosphorus fractionation, root phenotyping, and microbial community analysis (16S rRNA and ITS amplicon sequencing) to explore soil–microbe–plant interactions in a 12-year field experiment with five P application rates (0, 37.5, 75, 112.5 and 150 kg P₂O₅ ha⁻¹ yr⁻¹). Results showed that beyond 75 kg ha⁻¹, the medium-soluble phosphorus pool increased significantly while stable phosphorus decreased. Fungal diversity was more sensitive to high phosphorus than bacterial diversity. Maize yield plateaued at 75–150 kg ha⁻¹, mainly due to increased grain weight and optimized root architecture. An environmental risk threshold was identified at 83.54 kg ha⁻¹ and an optimal yield threshold at 84.65 kg ha⁻¹, enabling high yield with low environmental risk via microbially mediated phosphorus activation. Therefore, this research reveals that the phosphorus input threshold can provide a basis for reducing phosphorus application, regulating phosphorus components, and maintaining microbial diversity and network complexity in purple soil dryland farming systems, thereby ensuring maize yield.