Nitrogen fertilization increases carbon use efficiency of soil microbial communities across 10 long-term N fertilization studies

Soils store more carbon (C) than both the atmosphere and vegetation combined. Thus, any changes in the rate at which soil microbes release CO₂ into the atmosphere through respiration has critical implications for atmospheric CO₂ concentrations and ultimately Earth's climate. Historically, N deposition has altered the decomposition of soil C in temperate forests. While most studies show reduced decomposition of soil C under long-term N fertilization, the underlying mechanisms that drives this reduction remain unclear. We hypothesized that elevated soil N as the result of N fertilization may reduce the need for microbes to mine N from soil organic matter (SOM) resulting in a community shift that favors microbial scavengers over miners. As such, there is a greater return on extracellular enzyme investment, higher microbial carbon use efficiency (CUE) and enhanced production of microbial necromass. Given that microbial necromass is the precursor of stable SOM, these shifts in CUE and turnover may also contribute to greater soil C protection. To test this hypothesis, we measured microbial community CUE in fertilized and control plots across ten long-term N fertilization experiments spanning Eastern temperate forests. We incubated soils in the lab with ¹³C labelled glucose and measured the fraction of this substrate released during respiration vs. what was incorporated into microbial DNA. We found that long term N fertilization increased microbial CUE as respiration of ¹³C glucose was lower in N fertilized soils along with greater incorporation of ¹³C into DNA. Moreover, we found that N deposition enhanced microbial turnover. Collectively, our results suggest that shifts in CUE and turnover may be an important mechanism that explains reduced soil respiration across N fertilization studies as well as enhanced soil C stocks, owing to microbial necromass being the primary precursor of stable SOM.