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).