Carbon Emissions from a Whole-Soil Warming Experiment are not Attenuated in the Long-Term
Margaret Torn1* (MSTorn@lbl.gov), Elaine Pegoraro1, Sigrid Dengel1, Cristina Castanha1, Caitlin Hicks-Pries2, Jennifer Soong3, Rachel Porras1
1Climate and Ecosystem Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA; 2Department of Biological Sciences, Dartmouth College, Hanover, NH; 3Corteva Agriscience, Indianapolis, IN
By the end of the 21st century, global mean surface temperatures are projected to increase by 2.6–4.8°C, relative to 1986 to 2005, under a Representative Concentration Pathway of 8.5. Soil temperatures are projected to increase at a similar rate to air temperatures, both at the near-surface (~1 cm) and at depth (100 cm; Soong et al. 2020). This means that soil carbon in deeper soil layers, which on average cycle on centennial to millennial timescales, can become increasingly vulnerable to microbial decomposition. Since the subsoil (>20–30 cm) stores a large proportion of soil carbon in the top meter, this depth response can have significant consequences for terrestrial carbon emissions to the atmosphere.
In 2013, researchers established a whole-soil warming experiment at Blodgett Experimental Forest, a temperate forest comprising a thinned, 80-year-old stand of mixed conifers. The top meter of the soil profile was warmed by 4°C using heating rods installed 2.4 m into the soil around 3 m diameter plots, with surface heating cables buried at 5 cm depth at radii of 0.5 and 1 m. In the first five years of warming, carbon dioxide (CO2) respiration increased 30% from decomposition throughout the entire soil profile (Hicks-Pries et al. 2017; Soong et al. 2021). In the subsoil, warming shifted soil organic matter toward more decomposed material and decreased microbial abundance (Ofiti et al. 2021; Zosso et al. 2021). The effects of warming can be large in the initial phase of disturbance and attenuate over time due to acclimation or a decrease in substrate availability for the microbial response. Thus, long-term soil warming experiments are crucial to inform Earth system models and avoid over- or underestimating soil carbon loss based on the initial warming response.
After almost a decade of warming, researchers have found that the mean annual CO2 flux ratio (heated/control) has not significantly changed; from 2014 to 2021 it averaged between 1.2–1.4, meaning respiration in heated plots was 20–40% higher than in control plots. The CO2 response to warming has not subsided overtime, despite some changes in carbon substrate stocks, and support long-term projections of soil carbon loss with warming.
Hicks-Pries, C. E., et al. 2017. “The Whole-Soil Carbon Flux in Response to Warming,” Science 355(6332), 1420–1423. DOI:10.1126/science.aal1319.
Ofiti, N. O. E., et al. 2021. “Warming Promotes Loss of Subsoil Carbon Through Accelerated Degradation of Plant Derived Organic Matter,” Soil Biology and Biochemistry 156, 108185. DOI:10.1016/j.soilbio.2021.108185.
Soong, J. L., et al. 2021. “Five Years of Whole-Soil Warming Led to Loss of Subsoil Carbon Stocks and Increased CO2 Efflux,” Science Advances 7(21). DOI:10.1126/sciadv.abd1343.
Soong, J. L., et al. 2020. “CMIP5 Models Predict Rapid and Deep Soil Warming Over the 21st Century,” Journal of Geophysical Research: Biogeosciences 125(2), e2019JG005266. DOI:10.1029/2019jg005266.
Zosso, C. U., et al. 2021. “Whole-Soil Warming Decreases Abundance and Modifies the Community Structure of Microorganisms in the Subsoil but not in Surface Soil,” Soil 7(2), 477–494. DOI:10.5194/soil-7-477-2021.