4.2. Effects of straw return on microorganism diversity
Straw application alters the biotic and abiotic environmental conditions of the soil and affects its local ecology, which in turn affects the structural diversity of soil bacterial and fungal communities (Philippot et al., 2023). In the present study, soil fungal populations were significantly affected by straw return, as revealed by the results of the ACE and Shannon indices, whereas the alpha diversity of soil bacterial communities did not respond significantly to different straw return rates. Bacteria usually have shorter life cycles than those of fungi and decompose unstable nutrient pools, leading to the dominance of bacteria in the early stage of straw decomposition, whereas fungi have more stable population dynamics with straw return in the later stages of decomposition; thus, the alpha diversity of fungi was more considerably affected by straw return than that of bacteria (Yang et al., 2019; Wang et al., 2021c).
The returned straw is a substrate for microbial growth and improves environmental conditions, improving microbial activity (Yang et al., 2019; Shan et al., 2021). In the present study, the bacteria that were most responsive to the straw return were Proteobacteria, Acidobacteriota, and Bacteroidota (Fig. 2a, b). Kim et al. (2021) exhibited that the Proteobacteria and Acidobacteriota levels in the soil were correlated with soil pH, terminal electron acceptors, and toxic metals, with Proteobacteria enriched at high soil organic matter content and Acidobacteriota having contrasting results. The increase in straw return quantities provided organic material and improved the environmental conditions of the soil. Consequently, both factors exhibited significant differences across different straw return rates in the present study. Bacteroidota are carbohydrate degraders in the soil, breaking down complex compounds and playing a role in the breakdown of hemicellulose in the later stages of straw decomposition. The increase in straw polymers, along with the increase in microbial accessibility, increased Bacteroidota abundance in the late stage of straw decomposition (Huang et al., 2023). Fungal communities responded differently to low (S1/2) and high (S1 and S2) amounts of straw return. Ascomycota, Basidiomycota, and Mortierellomycota were predominant in the fungal community at the phylum level, and Mortierellomycota andChaetomium exhibited significant variations between the treatments (Figs 2c and d). However, the relative abundance of each of these fungal groups varied in S1/2, inconsistent with the results in the other treatments. Ascomycota are saprophytic fungi that decompose plant and animal debris and organic matter. Chaetomium have an inhibitory effect on phytopathogenic fungi, and Mortierellomycota, which are saprophytic fungi, enhance phosphorus and iron in the soil. These fungi are eutrophic fungi (Abuduaini et al., 2021; Ozimek et al., 2020). Contrastingly, Basidiomycota, are important for lignin and substrate decomposition, are ectomycorrhizal fungi that consume more resources than they produce, and are oligotrophic (Li et al., 2021). In the present study, the Basidiomycota relative abundance increased (decrease) in S0–S1/2 and decreased (increase) in S1–S2, suggesting that low and high straw inputs may have different stimulatory effects on fungal communities. The high abundance of oligotrophic bacteria in the soil at the early stage of plant residue decomposition and that of eutrophic bacteria at the later stage of decomposition, coupled with the longer life cycle of fungi than that of bacteria, may have increased oligotrophic fungi with increasing amounts of straw returned to the field (Wang et al., 2022b), consistent with findings from this study. Additionally, the input of organic materials changes the structure of soil microorganisms and activates many soil-borne microorganisms, thereby increasing microbial diversity (Semenov et al., 2021). Microorganisms with high relative abundances did not show significant differences, probably due to their differences in periods of action in the decomposition process and competitiveness in utilising straw polymers (Huang et al., 2023).
Our results showed that straw application enhanced the ability of microorganisms to utilise carbon sources (AWCD) and enhanced their functional diversity (Shannon, Simpson, and Pielou indices) (Figs S2 and S3). This was because plant residues and root exudates under straw return provide substrates for microbial carbon metabolism, affecting the habitat of microorganisms and enhancing the carbon metabolism process (Zhang et al., 2022; Zhu et al., 2022). As a lignocellulosic biosaccharification feedstock, straw is composed of cellulose (28–45%), hemicellulose (12–32%), and lignin (5–24%) (Srivastava et al., 2023). Wheat straw has many carbohydrates and is rich in protein, minerals, silica, and ash (Tufail et al., 2021). Straw application increases the amounts of carbohydrates, amino acids, and carboxylic acids in the soil, considering composition and products of decomposition; thus, enhancing the biomass attachment sites for microorganisms on these substances. Therefore, the observed differences in microbial carbon metabolism between the different amounts of straw returned to the soil were correlated with increased carbohydrates, amino acids, and carboxylic acid. The results of PCA and cluster analyses, indicating the utilisation of carbon sources in the different treatments, exhibited that S1/2 was distinguished from S0 and S1, whereas S2 was distinguished from the rest of the treatments (Figs 3 and S4). This may be because of the low homogeneity of the microbial community in S1/2 and the difference in the types of microorganisms that decompose straw between the low and high amounts of straw returned to the field (Zhang et al., 2017). Compared to bacterial communities, fungal communities are more efficient at decomposing cellulose, hemicellulose, and lignin (Zhang et al., 2017). The fungal community analyses indicate that S1/2 is distinct from S0, S1, and S2, suggesting a correlation between the soil fungal community structure and the capacity for carbon metabolism.