Integrated Flame Weeding and Biochar Reutilization Enhance Soybean Yield by Promoting Soil Microbial Abundance

Weed interference and crop residue are major constraints on sustainable soybean production, particularly in intensive tropical agriculture. Flame weeding provides a non-chemical approach for weed management, but the agronomic value of carbonized residues generated during this process remains largely unexplored. In this study, flame-weeding temperatures ranging from 300 to 800 °C were first screened based on weed carbonization efficiency and air-quality condition, after which 500 and 600 °C were selected for field evaluation. A two-season field experiment was conducted using a 3 × 2 treatment structure combining three pre-sowing flame-weeding temperatures (0, 500, and 600 °C) with or without biochar from flame-weeding (BFW) foliar spraying. Each treatment included four replicates. Flame weeding was applied before soybean sowing, whereas BFW foliar spraying was performed at the V4, R2, R4, and R6 growth stages. Flame weeding altered the soil environment, increasing electrical conductivity and reducing soil nematode abundance. Combined treatments produced the highest yields in soybean. In particular, T600-BFW increased the yield per plant by 73.32% and 32.43% relative to the CK in the two seasons, respectively. Flame weeding exerted a significant lethal effect on soil nematodes. Soil nematode density declined from 149.75 to 2.92 ind·20 g−1 of soil at week 10 following T600 treatment. Compared to CK, T600-BFW increased the root nodule number by 31.71%. The quantities of bacteria, fungi, and actinomycetes in T500 and T600 increased significantly after flame weeding. Redundancy analysis identified soil temperature, electrical conductivity, and pH as major variables associated with microbial community variation. The PLS-PM model showed that soil microorganisms exerted a significant positive effect on soybean yield. These findings indicate that flame weeding and BFW reutilization enhance soybean yield through complementary soil–microbe–root and leaf physiological improvement. This integrated strategy provides a potential pathway for simultaneously managing weeds, reutilizing field-derived carbonized biomass, and improving soybean production.