Image courtesy of Oak Ridge National Laboratory; designed by Thomas Splettstoesser
A large contributor to atmospheric carbon dioxide concentration is degradation of soil organic matter by various biological and chemical processes.
A key question in Earth system science is whether warming will lead to increased soil organic matter decay and an accelerated release of soil carbon as carbon dioxide (CO2). If so, the increase in atmospheric CO2 would, in turn, contribute to more warming. This cycle of events thus would represent a self-reinforcing feedback, with warming begetting warming.
A decades-long study suggests that soil microbial communities will adapt to long-term warming in a way that will deplete recalcitrant (i.e., stable, long-stored) carbon stocks in soil, leading to a self-reinforcing feedback to the climate system.
In 1991, a replicated, in situ soil-warming experiment was established at the Harvard Forest in central Massachusetts to address this question. Rates of CO2 production have been measured monthly for microbial and root respiration from April through November. Initially, warmed plots had higher respiration than controls, but after about a decade, the warming-accelerated CO2 production decreased and returned to background levels. However, during the last 7 years of the study (years 16–22), soil respiration again increased in the heated plots relative to the control plots, a long-term response to soil warming never before documented. Based on measurements made over the first 15 years that showed the depletion of the soil’s labile carbon pool, investigators hypothesized that much of the carbon respired over the last 7 years has come from the recalcitrant soil carbon pool. Using 13C compound-specific soil incubation studies, they found that long-term soil warming increases the microbial carbon-use efficiency (CUE) associated with the degradation of complex (recalcitrant) carbon compounds such as phenol, but that the CUE of simple carbon compounds such as glucose was not temperature sensitive. Additional preliminary data show a shift in microbial community structure in the heated plots that indicates an increase in taxa or pathways adapted to recalcitrant carbon decomposition.
Serita D. Frey
Department of Natural Resources and the Environment, University of New Hampshire
Durham, NH 03824
This work, including maintenance of the long-term soil warming experiments, was supported by a National Science Foundation (NSF) Faculty Early Career Development Award, the NSF Long-Term Ecological Research (LTER) Program, a Department of Energy National Institute for Climatic Change Research (NICCR) grant, and a Harvard Forest Bullard Fellowship.
Frey, S.D., Lee, J., Melillo, J.M. and Six, J. “Soil carbon cycling: The temperature response of microbial efficiency and its feedback to climate.” Nature Climate Change 3, 395–398 (2013). [DOI: 10.1038/nclimate1796]