Straw retention, an alternative to artificial fertilization, commonly mitigates soil degradation and positively affects soil fertility. In this study, we investigated the succession of soil bacteria during two sugarcane straw retention treatments (control (CK) and sugarcane straw retention (SR)) and at four depths (0–10, 10–20, 20–30, and 30–40 cm) in fallow soil in a sugarcane cropping system. Using an Illumina MiSeq (16S rRNA) and soil enzyme activity, we explored the SR influence on soil bacterial communities and enzyme activities and its inclusive impact on soil fertility, with an emphasis on topsoil (0–10 cm) and subsoil (10–40 cm). Our results show that SR effectively improved soil fertility indicators (C, N, and P), including enzyme activities (C and N cycling), throughout the soil profile: these soil parameters greatly improved in the topsoil compared to the control. Sugarcane straw retention and soil depth (0–10 cm vs. 10–40 cm) were associated with little variation in bacterial species richness and alpha diversity throughout the soil profile. Subsoil and topsoil bacterial communities differed in composition. Compared to the CK treatment, SR enriched the topsoil with Proteobacteria, Verrucomicrobia, Actinobacteria, Chloroflexi, and Nitrospirae, while the subsoil was depleted in Nitrospirae and Acidobacteria. Similarly, SR enriched the subsoil with Proteobacteria, Verrucomicrobia, Actinobacteria, Chloroflexi, Gemmatimonadetes, and Bacteroidetes, while the topsoil was depleted in Acidobacteria, Gemmatimonadetes, and Planctomycetes compared to the CK. At the genus level, SR enriched the topsoil with Gp1, Gp2, Gp5, Gp7, Gemmatimonas, Kofleria, Sphingomonas, and Gaiella, which decompose lignocellulose and contribute to nutrient cycling. In summary, SR not only improved soil physicochemical properties and enzyme activities but also enriched bacterial taxa involved in lignocellulosic decomposition and nutrient cycling (C and N) throughout the soil profile. However, these effects were stronger in topsoil than in subsoil, suggesting that SR enhanced fertility more in topsoil than in subsoil in fallow land.

Exploring the effects of different soil improvement measures on soil physical properties, soil nutrients, soil microorganisms and sugarcane yield is of great significance to make the improvement strategy for mechanically compacted acidic soil. In the present study, four soil improvement treatments including applications of loosening soil essence (B₂), bacterial manure (B₃), organic fertilizer (B₄), and quicklime (B₅) were set up, with no soil amendment as control (B₁) to explore the physical properties, nutrients and microorganisms of soil and sugarcane yield components in two successive crop seasons. The application of organic fertilizer (B₄) significantly reduced soil bulk density, soil compactness, soil penetration resistance and shear strength resistance and significantly improved total porosity, aeration porosity and pore porosity of soil. The solid-phase volume ratio of soil was significantly lower than that of control, while the liquid-phase volume ratio of soil was significantly higher than that of control, and the organic matter content and total nitrogen of soil were also significantly improved in B₂ treatment. The soil compactness and total porosity of B₂ and B₃ treatments were reduced, improving the soil physical properties to some extent. The soil pH of B₅ treatment was significantly improved. The soil available phosphorus content of B₃ and B₅ treatments was significantly increased. The Shannon index, Chao1 index, and ACE index of soil bacteria and fungi treated with B₂, B₃, B₄, and B₅ were higher than those of the control. The four soil improvement measures improved the species diversity and richness of bacteria and fungi in tilled soil layer, reduced the relative abundance of Proteus, Acidobacteria, and basidiomycetes increased the relative abundance of Actinomycetes, Curvularia and ascomycetes, and changed the composition of other fungal communities. These four soil improvement measures improved the number of effective stems and single stem quality of sugarcane at maturity stage to varying degrees, resulting in increased yield of sugarcane. Among them, the effect of organic fertilizer was the best, and followed by that of bacterial manure. This study provides a scientific basis for screening suitable measures to improve the fertility of the mechanically compacted acidic soil, and increase sugarcane yield.

Calmodulin (CaM), as an important factor in the calcium signaling pathway, is widely involved in plant growth and development regulation and responses to external stimuli. In this study, the full-length sequence of the ScCaM gene (GenBank: GQ246454) was isolated from the leaves of a Saccharum spp. hybrid. Prokaryotic expression showed that ScCaM could be solubly expressed and purified in Escherichia coli BL21. Subcellular localization confirmed that ScCaM was localized in the plasma membrane and nucleus of cells. A quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis revealed that ScCaM can be induced by various stresses, including sodium chloride (NaCl), chromium trichloride (CrCl₃), salicylic acid (SA), and methyl jasmonate (MeJA). Ectopic expression in Arabidopsis thaliana demonstrated that ScCaM can affect the growth and development of transgenic plants. Moreover, the qRT-PCR analysis indicated that the overexpression of the allogenic ScCaM gene inhibits the expression of AtSTM, leading to the phenomenon of multiple-tillering in transgenic A. thaliana. The present study provided valuable information and facilitates further investigation into the function of ScCaM in the future.

Calmodulin (CaM) proteins play a key role in signal transduction under various stresses. In the present study, the effects of a sugarcane ScCaM gene (NCBI accession number: GQ246454) on drought and salt stress tolerance in transgenic Arabidopsis thaliana and Escherichia coli cells were evaluated. The results demonstrated a significant negative role of ScCaM in the drought and salt stress tolerance of transgenic lines of A. thaliana, as indicated by the phenotypes. In addition, the expression of AtP5CS and AtRD29A, two genes tightly related to stress resistance, was significantly lower in the overexpression lines than in the wild type. The growth of E. coli BL21 cells expressing ScCaM showed weaker tolerance under mannitol and NaCl stress. Taken together, this study revealed that the ScCaM gene plays a negative regulatory role in both mannitol and NaCl stresses, and it possibly exerts protective mechanisms common in both prokaryotes and eukaryotes under stress conditions.