The return of straw and manure to agricultural fields can impact soil organic carbon (SOC) and biological properties. However, there is a lack of research on how to use swine manure, maize straw, and various conditioners together affects soil bacterial and fungal populations. This study aimed to investigate six treatments, namely, only maize straw (S00), maize straw combined with swine manure (S0Z), maize straw combined with biochar and swine manure (SCZ), maize straw combined with boron slag and swine manure (SBZ), maize straw combined with biological agent and swine manure (SJZ), and maize straw combined with bio-organic fertilizer and swine manure (SFZ). The results showed that after the two-year return, all treatments increased the SOC content in 2023, which was 12.55–26.89% higher than S00. And the SCZ treatment significantly increased the soil organic carbon (SOC), dissolved organic carbon (DOC), easily oxidizable carbon (EOC), particulate organic carbon (POC), and microbial biomass carbon (MBC) content by 26.89%, 25.44%, 56.88%, 16.08%, and 43.54%, compared to S00. A redundancy analysis (RDA) showed that the continuous application of manure, maize straw, and conditioners has a positive impact on the diversity and abundance of soil microbial communities, enhancing the accumulation of soil carbon. Furthermore, our research revealed that soil fungi exhibited higher sensitivity in soil carbon composition following the addition of manure, straw, and conditioners to agricultural fields than bacteria. In conclusion, the addition of different conditioners to the fields is beneficial to biodiversity conservation from the perspective of achieving soil carbon storage and soil protection. Our findings suggested that the combination of maize straw, biochar, and swine manure was been proven to be the most effective treatment for increasing labile organic carbon fractions and enhancing the microbial community.

Returning straw and biochar to the fields can change the structure of humic acid in soil and affect the content of soil organic matter. However, returning straw and biochar to the fields in successive years also increases the insect pests of crops and causes nitrogen competition, thus increasing the soil burden. Therefore, to provide a theoretical basis for the study of the time effect of returning fields, three treatments were set in this paper to study the effects of straw and biochar returning every other year on the soil organic matter and humic acid (HA) structure. Elements, Fourier transform infrared spectroscopy (FTIR), thermogravimetric spectroscopy (TG), three-dimensional fluorescence spectroscopy, and solid state ¹³C-NMR spectroscopy were used to comprehensively investigate the structural characteristics of humic acid in soil. The results showed that after straw and biomass charcoal are returned to the field every other year, compared with 2018, after planting in 2020, the average content of soil organic carbon will increase by 29.49% and 36.14%, respectively. HA showed a phenomenon of increased aliphatic property and decreased aromaticity; aromatic carbon of HA decreased, while alkyl carbon and alkoxy carbon increased; the molecular structure of HA developed toward simplification. In the short term, the effect of biomass charcoal returning was more significant. This study can be used as a reference for the research on the time effect of returning straw and biomass carbon to the fields, and can also provide a basis for the research on the composition of humus after returning organic materials to the fields.