Parent materials and the fertility levels of paddy soils are highly variable in subtropical China.  Bacterial diversity and community composition play pivotal roles in soil ecosystem processes and functions.  However, the effects of parent material and fertility on bacterial diversity and community composition in paddy soils are unclear.  The key soil factors driving the changes in bacterial diversity, community composition, and the specific bacterial species in soils that are derived from different parent materials and have differing fertility levels are unknown.  Soil samples were collected from paddy fields in two areas with different parent materials (quaternary red clay or tertiary sandstone) and two levels of fertility (high or low).  The variations in bacterial diversity indices and communities were evaluated by 454 pyrosequencing which targeted the V4–V5 region of the 16S rRNA gene.  The effects of parent material and fertility on bacterial diversity and community composition were clarified by a two-way ANOVA and a two-way PERMANOVA.  A principal coordinate analysis (PCoA), a redundancy analysis (RDA), and multivariate regression trees (MRT) were used to assess changes in the studied variables and identify the factors affecting bacterial community composition.  Co-occurrence network analysis was performed to find correlations between bacterial genera and specific soil properties, and a statistical analysis of metagenomic profiles (STAMP) was used to determine bacterial genus abundance differences between the soil samples.  The contributions made by parent material and soil fertility to changes in the bacterial diversity indices were comparable, but soil fertility accounted for a larger part of the shift in bacterial community composition than the parent material.  Soil properties, especially soil texture, were strongly associated with bacterial diversity.  The RDA showed that soil organic carbon (SOC) was the primary factor influencing bacterial community composition.  A key threshold for SOC (25.5 g kg^–1) separated low fertility soils from high fertility soils.  The network analysis implied that bacterial interactions tended towards cooperation and that copiotrophic bacteria became dominant when the soil environment improved.  The STAMP revealed that copiotrophic bacteria, such as Massilia and Rhodanobacter, were more abundant in the high fertility soils, while oligotrophic bacteria, such as Anaerolinea, were dominant in low fertility soils.  The results showed that soil texture played a role in bacterial diversity, but nutrients, especially SOC, shaped bacterial community composition in paddy soils with different parent materials and fertility levels.

Green manure can be used as a substitute for chemical fertilizer without reducing rice yield.  We studied the responses of soil fertility and rice yield to different combinations of Chinese milk vetch (CMV; Astragalus sinicus L.) and chemical fertilizer in a subtropical double-rice cropping system.  Our goal is to reduce chemical fertilizer use and decrease environmental contamination.  Compared with the recommended rate of chemical fertilizer (CF), both early- and late-rice yields in the two treatments supplied with 15 and 22.5 Mg CMV ha^–1 plus 60% CF (represented as 60A and 60B, respectively) showed no significant differences while the two treatments supplied with 30 and 37.5 Mg CMV ha^–1 plus 60% CF (represented as 60C and 60D, respectively) showed significantly higher values.  The sustainable yield index (SYI) values in the 60C and 60D treatments with double-rice croppong system were significantly higher than those in other treatments (P<0.05).  Early-rice yield showed a significant positive relationship with the Chinese milk vetch incorporation rate.  The coefficients increased annually from 2009 to 2013 and then decreased in 2014.  Soil organic matter increased over time by the end of the experiment in all of the treatment groups.  Soil organic matter in 60A, 60B and 60C showed no significant difference compared with that in CF, while soil organic matter in 60D was significantly higher than that in CF.  The slopes of soil organic matter and total nitrogen over six years were the highest in 60C and 60D.  The soil total nitrogen content in 60A, 60B, 60C and 60D was higher than that in CF, but the differences were not significant (P>0.05).  Therefore, a relatively high Chinese milk vetch incorporation rate (≥30 Mg ha^–1) was more effective in improving the productivity and sustainability of paddy soil.  The decreased coefficients of early-rice yield and the Chinese milk vetch incorporation rate in 2014 implied that the benefits of soil fertility and rice yield created by Chinese milk vetch input may decline after five years under a continuously high rate of Chinese milk vetch incorporation.