Biochar combined with magnesium fertilizer improves cabbage (Brassica oleraceae L.) yield by modulating physicochemical characteristics and the bacterial community in acidic soil

Biochar and magnesium (Mg) fertilizers have gained increasing attention in agriculture for their potential benefits. However, their combined effects on vegetable growth in southern acidic soils have not been fully elucidated. To address this issue, a study was conducted with six treatments all of which included a baseline amount of compound fertilizer (CK), an increased amount of compound fertilizer (F), Mg fertilizer (M), biochar (BC), biochar combined with Mg (BCM) and an increased amount of compound F with Mg (FM), to investigate their synergistic effects on soil physicochemical properties, enzyme activity, microorganisms and cabbage yield. The results of the study showed that compared with CK, the M, BC and BCM treatments all increased soil organic matter (SOM), exchangeable magnesium (Ex-Mg), acid phosphatase (ACP) and yield, while the F treatment led to a 5% decrease in SOM. The BCM treatment significantly increased SOM, Ex-Mg, ACP by 14%, 40%, and 25%, respectively. The most abundant phyla in BCM processing were Proteobacteria (21%), Chloroflexi (19%) and Actinomycetes (17%); the most abundant genera were Bacillus (8%). Further, the effect of BCM on crop growth promotion was better than that of FM, and the economic analysis showed that compared with FM, the benefit of BCM increased by 14%. Through structural equation modelling analysis, the synergistic increase in yield was because of the increase in soil TP, Ex-Mg, NO3--N and beta-GC caused by BCM, improving soil nutrients and microbial communities, thereby promoting cabbage growth. Therefore, the combined application of biochar and Mg in acidic soil could significantly improve soil fertility and crop productivity, while also reducing the need for costly chemical fertilizers, thereby offering a promising and cost-effective approach for sustainable agriculture.