Yundong Wei

Continuous cassava cropping can lead to soil degradation and disturbances in soil microbial functions, threatening the sustainability of cassava production systems. To address these challenges, this study examined the effects of biochar on soil physicochemical properties and microbial functional profiles in a continuous cassava cropping system. Two biochar application rates (0 and 3 Mg ha ⁻¹ ) were applied, and metagenomic sequencing was conducted to evaluate microbial community composition and functional genes related to carbon and nitrogen cycling in rhizosphere and bulk soils. The results demonstrated that biochar was associated with higher soil pH, soil organic matter, and available nutrient content, with a stronger effect in rhizosphere soil. Biochar application altered the genetic potential of microbial communities, particularly in the rhizosphere soil. In terms of functional categories, biochar was related to higher gene abundances in “homologous recombination” and “DNA replication” categories (Kyoto Encyclopedia of Genes and Genomes (KEGG) database), as well as the “replication, recombination, and repair” category (evolutionary genealogy of genes: Non-supervised Orthologous Groups (eggNOG) database) in rhizosphere soil. Biochar also affected the abundance of carbon cycling genes, particularly in the rhizosphere. The abundance of the aerobic respiration-related gene coxA was increased, while the abundance of the anaerobic fermentation gene L-lactate dehydrogenase ( LDH ) was decreased. Additionally, in rhizosphere soil, biochar significantly increased the abundance of norB (denitrification), GDH2 (nitrogen mineralization), and nifD (nitrogen fixation), while decreasing the abundance of genes involved in nitrogen assimilation ( gltB ), assimilatory nitrate reduction ( nirA ), nitrogen mineralization ( cynS ), and nitrogen uptake ( nrtA, nasF, cynA, nrtC, and nasD ). Together, these results suggest that biochar application may enhance nutrient availability and reshape microbial functional potential primarily in the cassava rhizosphere, providing field evidence for biochar use in continuous cassava cropping.

Continuous monocropping of cassava (Manihot esculenta Crantz) often leads to soil degradation and yield decline, commonly referred to as continuous cropping obstacles (CCOs), which are closely linked to changes in soil physicochemical properties and microbial communities. Biochar has been widely used as a soil amendment to improve soil quality and microbial activity and is considered a potential strategy for alleviating CCOs. Understanding the effects of biochar on soil bacteria and metabolites under field conditions is essential, as it provides insights into its practical effectiveness in reducing CCOs and improving soil health in cassava cultivation systems. In this study, a field experiment was conducted in a continuous cassava system to investigate the effects of a single biochar application rate on soil bacterial diversity, community composition, and metabolite profiles in both rhizosphere and bulk soils. High-throughput 16S rRNA gene sequencing and UHPLC–MS/MS-based non-targeted metabolomics were employed to analyze soil bacterial and metabolic patterns. Biochar was associated with increased α-diversity in rhizosphere soil and distinct shifts in β-diversity. Biochar increased the relative abundance of Chloroflexi and Actinobacteriota in the bulk soil, while Cyanobacteria and Nitrospirota were more abundant in the rhizosphere. Network analysis revealed the compartment-specific differences after biochar application, with higher network complexity in the rhizosphere and lower complexity in the bulk soil relative to the control. Metabolomic profiling identified 402 metabolites in positive ion mode and 357 in negative ion mode. In the rhizosphere, biochar-treated soil exhibited higher relative abundances of alkaloids (e.g., trigonelline, berberine, vincristine) and flavonoids (e.g., catechin, naringin, rutin, and taxifolin), which are commonly linked to plant stress responses. In the bulk soil, biochar application resulted in lower levels of several anthropogenic organic compounds (e.g., monobutyl phthalate, terephthalic acid, and p–toluenesulfonic acid). These findings provide preliminary field evidence that biochar application can lead to compartment-specific changes in soil bacterial communities and metabolite profiles. Such changes are closely related to soil quality and nutrient cycling, pointing to a possible role of biochar in mitigating soil degradation under continuous cassava cultivation.

Due to limited land resources and traditional farming practices, continuous cassava cropping is common in China. This practice leads to soil degradation, including reduced fertility, imbalanced microbial communities, and lower crop yields. In this study, we investigated the impacts of continuous cassava cropping (CC) and cassava–maize rotation (RC) systems on soil physicochemical properties, microbial community composition, and functional gene abundance related to carbon and nitrogen cycling. The RC system consists of a five-year rotation cycle: cassava is planted in the first year, followed by two consecutive years of maize, and then, cassava is planted again in the last two years. The soil type is classified as Haplic Acrisols with a clay loam texture in this research. Soil samples from both cropping systems were analyzed for physicochemical properties and enzyme activities, and the results showed significant decreases in soil pH, available nitrogen, available phosphorus, and available potassium in CC. Using metagenomic sequencing, 1,280,928 and 1,224,958 unigenes were identified under RC and CC, respectively, with differences in microbial taxonomic and functional profiles. Bacteria accounted for 89.257% of the soil community in RC, whereas the proportion was 88.72% in CC. The proportions of eukaryota and viruses in RC were 0.031% and 0.006%, respectively; in contrast, their proportions were 0.04% and 0.02% in CC, respectively. Cassava–maize rotation promoted the metabolic activities of soil microbes, leading to a significant enhancement in functional genes related to nitrogen and carbon cycling, such as nasA, nasD, nrtC, coxA, porA, and frdA. This shows that microbial activity and nutrient cycling improved in the crop rotation system. Thus, these findings highlight the importance of crop rotation for maintaining soil health, enhancing microbial functions, and improving sustainable cassava production. This study provides valuable insights into the management of cassava agroecosystems and the mitigation of the adverse effects of continuous cropping.