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.