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 (WoliƄska 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.