High-Intensity Hurricanes Reduce Soil Carbon Mean Transit Times in a Humid Tropical Forest

The Science

Tropical forests account for over half of the global terrestrial carbon sink, but climate change, including increasing intensity of extreme events, threatens to alter the carbon balance of these ecosystems. A team of researchers quantified changes in soil carbon storage and transit time across a forested watershed over 30 years—a period that included four high-intensity hurricanes. The pulses of carbon inputs associated with defoliation of the forest canopy during these hurricanes and the reduction of litter inputs during the post-hurricane recovery period altered the distribution and accelerated decomposition and cycling rates of soil carbon.

The Impact

High-intensity hurricanes defoliate forest canopies, resulting in a large pulse of plant debris to soils and creating gaps that alter soil microclimate and forest structure as the forest recovers from disturbance. This study demonstrates through measurements and modeling that high-intensity hurricanes result in a younger total soil carbon pool with faster mean transit times because hurricane disturbances increase the replacement of older soil carbon with new carbon from plant debris. This finding suggests increasing frequency of intense hurricanes will speed up carbon cycling rates in tropical forests, making soil carbon more sensitive to future tropical forest stressors.

Summary

The team sampled soils from the same locations across Bisley Watershed in Luquillo Experimental Forest in Puerto Rico in 1988 and 2018. Researchers quantified and compared over time carbon storage, distribution with depth and across pools that differ in residence time and degree of protection from soil microbes, and soil carbon pool radiocarbon and stable isotope signatures. Carbon increased slightly from 1988 to 2018 in the physically protected organic matter pool, but changes in the particulate and mineral associated pools as well as total soil carbon were not detected. Changes in radiocarbon values of soil carbon pools over time suggest that mean carbon transit times decreased from 1988 to 2018.

A reduced complexity soil carbon and radiocarbon model simulated the plant input pulses associated with hurricanes followed by reduced inputs over a 5-year recovery period post-hurricane. The model was fit to observed data to identify the best structure and initialization parameters. The model showed hurricane disturbances resulted in faster incorporation of carbon from plant debris into the physically protected organic matter pool coupled with higher rates of older soil carbon loss, relative to no-hurricane control conditions. These results suggest that hurricanes’ increasing intensities are amplifying soil carbon cycling, which could make hurricane-impacted ecosystems more vulnerable to future events.

Principal Investigator

Karis McFarlane
Lawrence Livermore National Laboratory
[email protected]

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This work was supported by the Office of Science through Grant/Award Number SCW1572; National Institute of Food and Agriculture through Grant/Award Number CA-B-ECO-7673-MS; National Science Foundation Division of Earth Sciences through Grant/Award Number EAR-1331841 and EAR-2012878; Lawrence Graduate Research Scholarship at Lawrence Livermore National Laboratory; and National Science Foundation Division of Environmental Biology through Grant/Award Number DEB-1546686 and DEB-1831952.

References