Network analysis reveals bacterial and fungal keystone taxa involved in straw and soil organic matter mineralization

Understanding the effects of straw return and N fertilization on soil organic matter (SOM) transformations will help maintain crop production and soil function, which can ultimately contribute to mitigating climate change. In this study, we conducted a 100-day soil incubation experiment with the addition of 13C-labeled maize straw and/or N fertilization. Soils that were used in the study included soil without fertilizers (Control), with mineral fertilizer alone (NPK), and with mineral fertilizer and straw (NPK+Straw). Compared with the control, the NPK- and NPK+Straw-treated soils showed higher straw decomposition by 59% and 55%, and SOM mineralization by 27% and 37%, respectively, although the priming effect was decreased by 59% and 39%, respectively. The priming effect (PE) was higher with increased N content and lower with decreased N additions because of an improved C/N ratio for microorganisms. Straw additions compared to without straw increased the bacterial and fungal abundance by 1.4- and 4.9-fold, respectively. N fertilization lowered C/N ratios resulting in decreased fungal diversity. Although the bacterial abundance decreased, the diversity increased with the duration of incubation as the bacteria preferred to utilize the labile organic compounds that were abundant in the initial stages of incubation. In addition to the depletion of labile organic compounds, the fungal abundance increased. Bacteria (Firmicutes, Actinobacteria, and Proteobacteria phyla) and fungi (Ascomycota, Basidiomycota, and Mucoromycota phyla) dominated straw and SOM decomposition. Firmicutes were mostly involved in straw and SOM mineralization during the first day after straw addition. The edge number and ratio for pairwise correlations between environmental factors and fungal taxa (22.1–24.6%) were greater than those with bacterial taxa (1.0–2.9%) in the microbial correlation network. Overall, straw combined with a low level of added N benefited soil C sequestration by decreasing the PE. Compared to bacteria, the functional role of fungi in SOM mineralization is more prominent and should be considered during agricultural management.