Cellulose is the most abundant polysaccharide in plant biomass and an important precursor of soil organic matter formation. Fungi play a key role in carbon cycling dynamics because they tend to decompose recalcitrant materials. Here, we applied [12C]cellulose and [13C]cellulose to distinguish the effects of application of compost, nitrogen-phosphorus-potassium (NPK) fertilizer, and no fertilizer (control) for 27 years upon cellulose decomposition via RNA-based stable isotope probing (RNA-SIP). The loss ratio of added cellulose C in compost soil was 67.6 to 106.7% higher than in NPK and control soils during their 20-day incubation. Dothideomycetes (mainly members of the genus Cryptococcus) dominated cellulose utilization in compost soil, whereas the copiotrophic Sordariomycetes were more abundant in NPK and unfertilized soils. Compared with NPK and control soils, compost application increased the diversity of 13C-assimilating fungi. The 13C-labeled fungal communities in compost soil were more phylogenetically clustered and exhibited greater species relatedness than those in NPK and control soils, perhaps because of stringent filtering of narrow-spectrum organic resources and biological invasion originating from added compost. These changes led to an augmented decomposition capacity of fungal species for cellulose-rich substrates and reduced cellulose C sequestration efficiency. The RNA-SIP technique is more sensitive to responses of fungi to altered soil resource availability than DNA-SIP. Overall, long-term compost application modified fungal community composition and promoted fungal diversity and phylogenetic relatedness, accelerating the decomposition of substrate cellulose in soil. This work also highlights the RNA-SIP technique's value for comprehensively assessing the contributions of active fungi to the substrate decomposition process. IMPORTANCE Cellulose is a very rich component in plant biomass and an important precursor of soil organic matter formation. Fungal communities are known to be important drivers of organic carbon accumulation in arable soils. However, current understanding of responses of fungal species to cellulose amendment and the contributions of active fungi to substrate decomposition process is still very superficial. Here, we established a [13C]cellulose microcosm experiment with soils subjected to long-term application of compost, nitrogen-phosphorus-potassium (NPK) fertilizer, and no fertilizer (control). The novel 13C-RNA-SIP technique with subsequent high-throughput sequencing was used to investigate the linkages between active fungal taxa and cellulose decomposition. Our study demonstrated that Dothideomycetes dominated cellulose utilization in compost soil, whereas the copiotrophic Sordariomycetes were more enriched in both NPK and unfertilized soils. We also found that the compost amendment promoted fungal diversity and phylogenetic relatedness and strengthened the decomposition capacity of fungi for cellulose-rich substrates by enhancing synergistic interactions, thereby reducing cellulose C sequestration efficiency. Overall, our research has implications for our understanding of the role of active fungi in cellulose C transformation in soils undergoing different types of long-term nutrient management.
Keywords: RNA; [13C]cellulose; fungi; long-term fertilization; stable isotope probing.