Effects of machine planting density and depth on quality characteristics, yield formation and rhizosphere microorganisms of sweet potato

weet potato (Ipomoea batatas (L.) Lam.) is a typical labor-intensive crop, and the traditional manual planting mode is not only time-consuming and labor-intensive but also increases agricultural production costs. To explore the suitable planting density and transplanting depth for mechanized sweet potato production and provide solid scientific basis and technical support for its mechanized cultivation, a two-year field experiment was conducted in Rongchang District of Chongqing Municipality (29°40′N, 105°59′E) from 2022 to 2023. The experiment took Yushu 198, a major and widely promoted local sweet potato variety, as the test material and adopted a two-factor split-plot design. The main plots were set with three planting spacings: 25 cm (P5, 50,025 plants/hm²), 30 cm (P4, 41,688 plants/hm²) and 36 cm (P3, 34,739 plants/hm²), while the subplots were designed with two transplanting depths: 15 cm (D15) and 20 cm (D20). The study systematically investigated the responses of fresh tuber yield, starch content, soluble sugar content, tuber starch extraction rate, rhizosphere soil nutrients and soil microbes to different combinations of mechanized transplanting depths and densities. The results indicate that there are significant differences in the impact of mechanical planting density and depth on sweet potato yield, quality, and rhizosphere soil nutrients. Additionally, within a reasonable planting range, increasing mechanical planting density and depth is conducive to improving fresh sweet potato yield, starch content and tuber starch extraction rate, and also effectively increased the contents of alkaline nitrogen, available phosphorus, available potassium, microbial biomass carbon, microbial biomass nitrogen, microbial biomass phosphorus and the the number of bacteria, fungi, and actinomycetes. Among all treatment combinations, the P5D20 treatment (50,025 plants/hm², 20 cm) achieved the optimal fresh tuber yield and starch extraction rate, which were 37,619.0 kg/hm² and 30.4%, respectively. In addition, compared with the P5D15 treatment (50,025 plants/hm², 15 cm), the P5D20 treatment significantly increased the contents of alkaline nitrogen, available phosphorus, available potassium, microbial biomass carbon, microbial biomass nitrogen, microbial biomass phosphorus, as well as the quantities of bacteria, fungi and actinomycetes in the rhizosphere soil by 11.9%, 62.2%, 13.9%, 17.4%, 12.2%, 44.9%, 37.4%, 21.2% and 21.5%, respectively. Based on a comprehensive analysis of all measured indicators, the combination of a mechanized planting density of 50,025 plants/hm² and a transplanting depth of 20 cm is the optimal parameter for Yushu 198 to achieve high yield, superior quality and improved soil quality in mechanized production. This finding provides practical technical support for the local promotion of sweet potato mechanized cultivation and also offers a reference for similar regions planting homologous sweet potato varieties.