Nitrogen input level modulates straw-derived organic carbon physical fractions accumulation by stimulating specific fungal groups during decomposition

Microbial-driven straw-derived soil organic carbon (SOC) sequestration plays pivotal role in promoting soil productivity and achieving C neutrality, while nitrogen (N) and phosphorus (P) input levels are the impetus of this process. However, our understanding of the microbial mechanism underpinning the straw-derived SOC fractions accumulation within aggregates under contrasting N and P application rates is rudimentary. Here, a 90-day incubation experiment was conducted to observe the dynamics of straw-derived SOC fractions accumulation within aggregates by adding C 3 straw (natural 13 C-labeled substrate) to a Mollisol (in which C 3 plants had never been grown). Treatments included straw combinations of half N and regular P (N 1/2 +P + SR), regular N and half P (N + P 1/2 +SR) and regular N and P (N + P + SR). Regular N input accelerated the succession of the microbial community from bacteria-dominated to fungi-dominated and increased the contents of straw-derived particulate OC (POC) at 10 days and mineral-associated OC (MOC) at 30 days within aggregates. Differential abundance analysis and slopes of linear regression models showed that the fungal community, especially the core enriched taxon of Chaetomiaceae, was strongly positively correlated with straw-derived MOC after N input. Random forest models strengthened the view that N input rate and Chaetomiaceae richness may directly mediate straw-derived POC and MOC accumulation process, respectively, during incubation. To verify our conjecture, another 60-day incubation experiment was carried out to elucidate the effects of Chaetomium globosum inoculation and N application on SOC turnover after straw addition. Chaetomium globosum inoculation with regular N input promoted straw decomposition and SOC sequestration than other treatments, but no difference was observed in soil respiratory quotient compared with control. The slopes of linear regression model show the more significant and positive correlations between SOC content and Chaetomiaceae richness in the present study than data from previous references, impling the important role of Chaetomiaceae in SOC sequestration in Mollisols. Our work provides new ecophysiological angles for understanding the effect of Chaetomiaceae richness on the accrual of SOC in response to varying N levels in agroecosystems on the Northeast China Plain. • We reveal the pathway of straw to SOC fractions by determining δ 13 C. • N input levels regulate straw-derived SOC accumulation process, rather than P. • Succession of guides from bacteria to fungi optimized straw-derived SOC accumulation. • Chaetomiaceae’s function in SOC accumulation was validated via culture inoculation. • Chaetomium globosum inoculation promoted SOC sequestration under regular N input.