Microbes alter substrate from mineral-associated carbon to litterfall with nitrogen additions and warming

Nitrogen (N) additions often decrease soil respiration and increase soil organic carbon (C) stock. However, it is unclear how microbial substrates may shift with N additions and increasing temperature. Leveraging 12 years of N fertilization experiments and the associated shift in the dominant vegetation from C4 to C3, we explored the delta 13C-CO2 and temperature sensitivities of respired CO2 and extracellular enzyme activities in control and fertilized soils. N additions increased cellulose-decaying extracellular enzyme activity while respiration remained similar between the control and fertilized soils. Temperature sensitivity of cellulose-decaying extracellular enzyme activity decreased with the N additions. The delta 13C-CO2 data reveal that, as temperature increased, microbes in fertilized soils changed their dominant substrate from bulk soil organic C to plant litterfall. Our results suggest that long-term N fertilization imposed C limitation on microbes, leading to enhanced microbial efforts to acquire C. This study highlights how long-term N additions can promote the relative preservation of organic C in mineral soil while litterfall, the precursor to mineral-associated C, is increasingly decayed as temperatures increase.