Abstract
Soil bacterial community structure and carbon-fixation functional genes are easily influenced by straw application, especially in the microenvironment of the aggregate. Here, soil aggregate samples were collected from a 35-year rice-wheat rotation experiment field with the following treatments: no straw or chemical fertilizer (Ctrl); chemical fertilizer (NPK); and straw application + chemical fertilizer (NPKS). Compared with the control and NPK treatments, NPKS treatment enhanced the contents of soil organic carbon (SOC) and microbial biomass carbon (MBC) significantly in 0.25-1 mm aggregate soil. Similarly, higher ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) activity (172.92 nmol CO(2) g(-1) soil h(-1)) and abundance of cbbL (6.26 × 10(8) copies g(-1)) in the 0.25-1 mm aggregate fraction were observed under the NPKS treatment. Bacterial community diversity in various soil aggregate fractions decreased under the NPK treatment but increased under the NPKS treatment, compared with the control. The relative abundances of Proteobacteria and Actinobacteria in the 0.25-1 mm aggregate under the NPKS treatment were greater than those in the other treatments, indicating sufficient nutrient supply in this aggregate fraction, beneficial to eutrophic bacterial growth. Redundancy analysis showed that bacterial community composition strongly affected SOC and MBC distribution across soil aggregate fractions. Bacterial community contributed to RuBisCO activity by affecting the cbbL gene (path coefficient = 0.67, P < 0.001), which positively affected SOC content. In conclusion, the dominant soil aggregate fraction (0.25-1 mm) is mainly involved in carbon fixation in paddy, and straw application promoted carbon storage by affecting bacterial community structure in soil aggregates. IMPORTANCE: Paddy soils have been under frequent disturbance through field management activities (fertilization and straw application) for a long time, and carbon pool change is active and frequent. Soil aggregate plays an important role in carbon capture and storage. Variations in aggregate could alter microbial habitats and life strategies, thus triggering the renewal of soil organic carbon (SOC) encapsulated in aggregates. Straw application has both economic and ecological benefits, and it has been widely promoted in paddy fields with obvious effects. Here, a 35-year long-term positioning experiment was carried out to explore the mechanisms through which microbial communities increase SOC content as influenced by soil aggregate size. The findings enhance our understanding of carbon storage and aggregate-associated microbial mechanisms in paddy soil, in addition to facilitating the enhancement of paddy productivity and promoting the rational utilization of straw resources.