Fungi Outcompete Bacteria for Straw and Soil Organic Matter Mineralization

Background: Understanding the effects of straw return and nitrogen (N) fertilization on soil organic matter (SOM) transformations will help to mitigate climate change and maintain crop production and soil function. A 100-day soil incubation experiment was conducted using a two-factorial design with three fertilization levels and four 13 C-labeled maize straw and N addition treatments. The competition and contributions of the bacterial and fungal communities were assessed with relation to straw mineralization. Results: Mineral fertilizer alone and with straw increased straw decomposition by 59% and 55% and SOM mineralization by 27% and 37%, respectively, compared with the unfertilized soil, due to raised β-N-acetylglucosaminidase and cellobiohyrolase activities. Conversely, priming effect was decreased by 59% and 39%, respectively. Priming effect increased with higher N additions and decreased with lower N additions because an improved C:N ratio for microorganisms. Straw additions increased bacterial and fungal abundance by 1.4 and 4.9 times. Fungal diversity decreased with N fertilization because lower C:N ratios increased the bacterial competition. Bacterial abundance decreased but diversity increased with the duration of incubation as bacteria preferred to utilize labile organic compounds abundant in the initial stages. Along with labile organic compounds depletion, fungal abundance was increased. Firmicutes, Actinobacteria, and Proteobacteria bacterial as well as Ascomycota, Basidiomycota, and Mucoromycota fungi dominated straw and SOM decomposition. Firmicutes were mostly involved in straw and SOM mineralization on day one because of their capacity for labile compound decomposition. Integrated co-occurrence networks revealed that fungal taxa had a stronger correlation with straw decomposition than bacterial groups. Straw and N addition increased the number of negative edges among bacterial taxa but these decreased within fungal groups when compared to trials without straw and N. The ratio for pairwise correlations between abundant fungal taxa, straw, and SOM mineralization (29.9%) was greater than with bacteria (1.2%). Conclusions: Straw with low N additions increased soil C sequestration by decreasing priming effect. Straw alone and with N addition decreased competition for C and N among fungal groups, but increased competition within bacterial taxa. Fungi outcompete bacteria for straw and soil organic matter mineralization in long-term fertilized soils.