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. While most long-term nitrogen (N) fertilization studies show reduced decomposition of soil C, 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.
Results/Conclusions
Across experiments, N fertilization suppressed total soil CO2 respiration. Further, long-term N fertilization increased microbial CUE as respiration of 13C glucose was lower and 13C incorporation into DNA was greater in N fertilized soils. One plausible mechanism for this result appears to be reductions in the investment of energetically expensive ligninolytic enzymes by the soil microbial community. For example, across studies, phenol oxidase activity was lower in fertilized soils and was correlated with total CO2 respiration. The magnitude of this response was largest at the longest running N fertilization experiments. These results indicate that shifts in CUE and turnover may be an important mechanism that explains reduced soil decomposition and enhanced soil C storage across N fertilization studies.