As microbes and plant roots metabolize organic matter in the soil, they release carbon dioxide into the atmosphere, a process called soil respiration. In the Northeast, where forests store more than half of all their carbon in the soil, changes in the rate of soil respiration have profound implications for forests’ continuing role as sinks – not sources – of greenhouse gas emissions.
A report in the August 2024 issue of Nature Geoscience suggests that continued warming may increase soil respiration rates more than scientists have previously projected. The report draws on the findings from a 13-year experiment led by ecologist Peter Reich, director of the Institute for Global Change Biology at University of Michigan, and conducted in collaboration with colleagues at the institute, as well as scientists at University of Minnesota, University of Illinois, and the Smithsonian Environmental Research Center. As compared to a 26-year soil-only warming experiment at Harvard Forest, this experiment more closely replicated actual climate warming effects in forests by simultaneously heating forest soil and aboveground plants and other surface materials.
From 2009 to 2021, from April through November each year, the researchers monitored impacts on soil respiration in subplots within two young southern boreal forests in northern Minnesota. Using infrared lamps and underground heating cables, they measured the effects of two warming scenarios: a 1.7-degree Celsius (3-degree Fahrenheit) increase in ambient air and soil temperatures, and 3.3 Celsius (5.9 Fahrenheit) increase. Within each group of subplots subjected to each warming scenario, they also tested two different levels of canopy cover (80 percent full sunlight versus 8 percent sunlight) and exposures to rainfall (100 percent of rainfall compared to 60 percent of rainfall).
Data from this experiment suggest a significant increase in soil respiration under both warming scenarios. Respiration varied significantly over time, however. As noted in the report, “When soil water was low…warming effects on [soil respiration] were minor or even negative, but became positive when ambient soil moisture was higher.” The researchers attributed this difference at least in part to metabolic activity’s dependence on nitrogen. (In dry conditions, nitrogen moves more slowly through the soil.)
The report suggests that in moist soil conditions, continued warming may prompt greater soil respiration – and therefore a higher rate of carbon emissions – while summer dry spells may have a mitigating effect. “If we fail to account for such drivers, mechanisms and patterns, we may inaccurately predict global forest [carbon] cycling under future climate change,” the team noted.