3. Results

3.1. Soil carbon and nitrogen fractions

Table 1 presents the effects of different straw return rates on soil carbon fractions. S2 increased SOC content (P < 0.05) by 39.8% and 31.4% compared with those of S0 and S1/2, respectively. Soil DOC content was significantly (P < 0.05) higher (25.3%–37.9%) in S2 than those in the other treatments. Soil MBC content increased by 26.3% (S1/2, P < 0.05), 16.7% (S1, P < 0.05), and 55.3% (S2, P < 0.05) with increased straw return amount. The soil MBC in S2 was 129.3% higher than that in S0. S2 significantly increased the LFOC content by 155.9% and 67.9% and significantly decreased the HFOC content by 26.7% and 24.1% compared with those of S0 and S1/2, respectively) (P < 0.05).
All soil nitrogen fractions increased with increased straw return (Table 2). S2 significantly increased the soil TN content by 15.1% (P < 0.05) compared with that of S0. The DON content in S1 and S2 was significantly higher than that in S0 by 29.3% and 33.1% higher, respectively (P < 0.05). The S2 treatment resulted in an average increase of 55.4% in the MBN content compared with those of the S0, S1, and S1/2 treatments. (P < 0.05). MBN sensitivity in S2 was considerably higher than that in the other treatments. LFON increased by 72.6%, 20.3%, and 28.5% in S2, S1/2, and S1, respectively (P < 0.05), compared with that in S0. The HFON content increased with increasing amount of straw returned, with an average increase of 3.7%, but no significant difference was observed between the treatments (P < 0.05).

3.2. Alpha and beta diversity of soil microbial communities

The richness and diversity of soil microbial communities were assessed using the ACE and Shannon indices of soil microbial alpha diversity, respectively (Fig. 1). The results revealed that increasing the amount of straw returned affects the alpha diversity of soil fungal microbial communities (Fig. 1A). PCoA revealed that different straw return rates affected the soil bacterial and fungal community composition. No significant differences in beta diversity were detected between bacterial (p = 0.365, R-squared = 0.2796) and fungal communities (p = 0.063, R-squared = 0.3296) (Fig. S1).

3.3. Structural diversity of soil microbial communities

Figure 2 reveals that 10 dominant bacterial phyla and genera were detected in the different treatments. Proteobacteria accounted for 21.6%–28.8% of the total sequences at the bacterial phylum level, with maximum relative abundance values. The Proteobacteria relative abundance was significantly increased by 12.6% (P < 0.05) in S2 compared with that in S0. Acidobacteriota relative abundance was significantly reduced by 40.3% (P < 0.05) in S2 compared with that in S0. Additionally, the Bacteroidota relative abundance significantly differed between S1/2 and S2, and increasing the amount of straw returned significantly increased Bacteroidota relative abundance by 27.0% (P < 0.05). No significant difference between the treatments was observed at the bacterial genus level (P > 0.05). (Fig. 2b).
At the fungal phylum level, S2 significantly increased the Mortierellomycota relative abundance by 54.8% and 56.5% (P < 0.05), compared with those of S0 and S1/2, respectively (Fig. 2c). Additionally, the relative abundance of unclassified_k__Fungi differed significantly between S1/2 and S1 (P < 0.05). Ascomycota, Basidiomycota, Mortierellomycota, and unclassified_k__Fungi were highly enriched in all treatments, accounting for 98.1% to 99.2% of the total fungal relative abundance. S2 significantly reduced the relative abundance ofChaetomium by 48.9% (P < 0.05) compared with that of S1/2 (Fig. 2d). Furthermore, compared with those of S0 and S1/2, S1 and S2 significantly increased the relative abundance ofunclassified_o_Polyporales (P < 0.05).