Ecohydrological Controls on Root and Microbial Respiration in the East River Watershed of Colorado

Authors

Austin Simonpietri (ats327@nau.edu), Mariah Carbone*, Andrew Richardson

Institutions

Center for Ecosystem Science and Society, Northern Arizona University, Flagstaff, AZ

URLs

• https://carbone-lab.nau.edu/index.php/snodgrass-mountain-transect/

Abstract

Belowground in the soil, microbes breakdown organic matter, releasing carbon dioxide (CO₂). Plant roots produce CO₂ also, via their metabolism. This research aims to understand how moisture inputs, such as snow and rain, influence the amount of CO₂ produced belowground in the East River watershed, near Crested Butte, CO. In June 2021, researchers instrumented four sites along Snodgrass Mountain in the two main forest types of the region, aspen and spruce/fir conifer. To date, researchers have continuous measurements of the flux of CO₂ from the soil to the atmosphere, and its primary environmental and biological drivers (soil moisture, air and soil temperature, and snowpack and plant phenology). In 2022, researchers applied a radiocarbon source partitioning approach to determine the contributions from roots and microbes to the total soil CO₂ flux during the summer months. The work is motivated by the overarching hypothesis that quantifying belowground plant and microbial processes separately, and how they are influenced by snow and rain inputs, is necessary for understanding and predicting how the belowground East River watershed ecosystems will respond to changes in the environment.

Total water inputs differed between measurement years due to winter snowfall (428 mm water in 2020 to 2021 vs. 560 mm H₂O in 2021 to 2022). Monsoon rain amounts were similar between summers, but differed how they were distributed, with few, large events in 2021, and smaller more evenly distributed rains in 2022. These differences in precipitation translated into wetter June to July soils (by 0.05 m³ m^-3 VWC) and cooler June to September soils (by -0.31 °C) in 2022. Larger soil CO₂ fluxes (June to September) were observed across all sites in 2022, with an average increase of 133 g C m^-2. Between both years, the largest fluxes were observed at the middle elevation aspen (377.9 ± 132 g C m^-2) and conifer (421.13 ± 74 g C m^-2) stands, likely due to greater soil moisture and ground water availability. The soil CO₂ flux seasonal patterns were strongly driven by soil temperature, modulated by canopy structure and phenology that differed greatly between the aspen and conifer forests. Forest type differences were also observed in the radiocarbon source partitioning results, where microbial respiration was more consistent and dominant in the conifer stand (ranging between 62–70 ± 5%) over the summer months. Whereas in the aspen stand, microbial respiration was more seasonally variable (47, 73, 29 ± 8% in July, August, September, respectively) and the contribution from root respiration was greater.