4.3. Soil microbes influence changes in soil carbon and nitrogen
fractions
Soil microorganisms alter the soil carbon and nitrogen stocks through
decomposition and formation of soil organic matter (Fan et al., 2021).
The results of the present study exhibited that straw return to the
field significantly affected the soil carbon pool by affecting the
abundance of bacteria such as Proteobacteria, Acidobacteriota, and
Bacteroidota and consequently affecting the soil carbon pool content
(Fig. 6). Wu et al. (2021) reported that straw application significantly
increased the soil unstable carbon and nitrogen fractions and that
bacterial communities responded differently to carbon and nitrogen
mineralisation, with Brevundidanas (a member of the subzonal
family of Proteobacteria ) stimulating carbon mineralisation,
suggesting that changes in the abundance of Proteobacteria affected the
soil active carbon pool content, consistent with our results that
revealed that Proteobacteria was strongly correlated with soil active
carbon pools (DOC and MBC) (Fig. 5). Acidic bacteria are some of the
most important microbial populations that dominate during soil
denitrification, and they decrease soil NO and N2O,
thereby affecting soil N fractions (Pessi et al., 2022).Wang et al.
(2023b) suggested that elevated levels of soil nitrogen supply increase
the relative abundance of Proteobacteria and Bacteroidota, which are the
key bacteria influencing soil nitrogen cycling and carbon effectiveness.
In the present study, we observed positive correlations (to varying
degrees) between the Proteobacteria and Bacteroidota and the soil carbon
and nitrogen fractions. Mortierellomycota exhibited a high degree of
interpretability considering changes in soil carbon and nitrogen pool
content (Fig. 4). Mortierellomycota are mostly present in the soil as
living organic material and are efficient in decomposing carbon sources
such as chitin polymers and cellulose (WoliĆska et al., 2022; Ozimek et
al., 2020), indicating their importance in the soil nutrient pools. Zhao
et al. (2020b) reported that compared to fungi, bacteria play a more
important role in carbon and nitrogen cycling, where genes involved in
carbon degradation are from bacteria in eutrophic phyla. In the present
study, as oligotrophic fungi grew, bacteria had a stronger effect on the
carbon pool in the PLS-PM results compared with that of fungi (P < 0.05) (Fig. 6); however, a greater functional potential may
occur at a later stage of decomposition because of the strong
degradation of fungi and their longer growth cycle. Additionally, the
effects of microorganisms differ under different environmental
conditions because of the differences in the mechanisms of carbon and
nitrogen cycles. Fuchslueger et al. (2019) revealed that the
microorganisms involved in the carbon cycle are more sensitive to
temperature changes. Contrastingly, those involved in the nitrogen cycle
are more affected by moisture. Microorganisms involved in the carbon and
nitrogen cycles are affected by the fungal: bacteria ratio, soil
environment and properties, and other factors (Ma et al., 2020).
In the present study, among the different types of carbon sources
utilised by soil microorganisms, amino acid-based carbon sources
explained the most variation in soil carbon and nitrogen fractions (Fig.
4). Amino acids circulate very rapidly in the soil cycle, and the strong
dynamics of carbon and nitrogen in the soil are correlated with the high
turnover rate of amino acids, which can be rapidly mineralised and
assimilated by microorganisms compared with other types of substances;
thus, amino acids are important nutrient drivers in the soil and had a
strong correlation with the soil active carbon and nitrogen pools in our
study (Jones et al., 2009) (Fig. 5). Ning et al. (2022) found that
optimal supplementation of amino acid-based carbon sources favoured
straw decomposition in the soil. For example, glycyl-L-glutamic acid
played an important role in straw decomposition, especially in the
pre-decomposition stage, which could provide favourable environmental
conditions for soil microbial communities to proliferate and increase
their populations and balance the soil carbon and nitrogen ratio.
L-asparagine facilitates nutrient uptake by microorganisms (Han et al.,
2022), and it is effective in the late stage of straw decomposition.
Therefore, we can speculate that these amino acid carbon sources can
increase the active carbon and nitrogen pools of the soil by promoting
nutrient uptake during different periods of straw decomposition. In the
present study, we observed different degrees of significant positive
correlations between these carbon sources and active carbon and nitrogen
fractions (Fig. 5). The above analyses revealed that straw return
significantly increased the soil carbon and nitrogen pools and
stimulated the effect and driving mechanism of soil microorganisms.
Therefore, the influence of straw input on soil nutrients should not be
ignored.