Response of soil carbon dioxide efflux to temporal repackaging of rainfall into fewer, larger events in a semiarid grassland

Authors: Roby, Matthew; Scott, Russell - Russ; Biederman, Joel; SMITH, WILLIAM - University Of Arizona; MOORE, DAVID - University Of Arizona

Submitted to: Frontiers in Environmental Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/31/2022
Publication Date: 9/21/2022
Citation: Roby, M.C., Scott, R.L., Biederman, J.A., Smith, W.K., Moore, D.J. 2022. Response of soil carbon dioxide efflux to temporal repackaging of rainfall into fewer, larger events in a semiarid grassland. Frontiers in Environmental Science. 10. Article 940943. https://doi.org/10.3389/fenvs.2022.940943.
DOI: https://doi.org/10.3389/fenvs.2022.940943

Interpretive Summary: Changing rainfall patterns linked to climate warming will alter water availability to plants and soil organisms in semiarid regions. Because soil carbon dioxide release (CO2 efflux) is driven by plant and soil responses to moisture, these changes will likely impact CO2 emissions from semiarid ecosystems. We used a field experiment to examine how simultaneous changes in rainfall size and timing affect soil CO2 efflux at a semiarid grassland in southern Arizona. Plots received equal total rainfall that was repackaged into events with varied size and frequency. We found that repackaging equal rainfall into a few/large events separated by long dry intervals decreased total CO2 efflux over the summer growing season. This reduction was explained by a decrease in the sensitivity of CO2 efflux to soil moisture compared to plots subject to frequent, small events. We also found that changes in plant growth and temperature influenced how CO2 efflux responded to rain events. A model based on soil moisture, temperature, and plant growth explained 85% of the variability in CO2 efflux. These findings demonstrate that soil moisture and plant responses to changes in rainfall size and frequency impact seasonal soil CO2 emissions in semiarid grasslands. These results, coupled with the knowledge that CO2 efflux pulses play an outsized role in dryland carbon exchange, indicate the possibility of future climate-mediated shifts in the carbon cycling of semiarid ecosystems.

Technical Abstract: Changing rainfall patterns will alter soil water availability to plants and microbes and likely impact soil CO2 efflux (Fs) in semiarid ecosystems. However, our understanding of the response of Fs to compound changes in rainfall event size and frequency remains relatively limited. To address this knowledge gap, we examined how compound changes in rainfall size and frequency impact Fs in a semiarid grassland by deploying automated soil chambers at a rainfall manipulation experiment. All plots within the experiment received equal total summer growing season precipitation that was temporally repackaged into regular events of inversely varied size and frequency, with event sizes ranging from 5 to 50 mm and dry intervals ranging from 3.5 to 21 days. We found that repackaging rainfall into few/large events with long dry intervals decreased seasonal cumulative Fs compared to regimes with many/small events. Repackaging influenced key aspects of pulses including mean, maximum, and antecedent (day before irrigation) values of soil moisture and Fs and their rate of decline during drying intervals. Soil moisture explained substantial variation in Fs (R2 > 0.84) for all treatments; however, the sensitivity of Fs to soil moisture decreased in the few/large regime compared to the many/small regime. Dynamics in plant phenology (quantified by plot greenness) and soil temperature interacted with soil moisture to influence the seasonal evolution of Fs pulses and cumulative efflux. Our findings demonstrate that soil moisture and vegetation responses to changes in rainfall size and frequency impact soil CO2 efflux pulses and seasonal emissions in semiarid grasslands. These results, coupled with the knowledge that CO2 efflux pulses play an outsized role in dryland carbon exchange, indicate the possibility of future climate-mediated shifts in the carbon cycling of semiarid ecosystems.