Amplifying feedback loop between drought, soil desiccation cracking, and greenhouse gas emissions

Abstract

Abstract Approximately two-thirds of the energy imbalance responsible for the rise in global temperatures can be attributed to the increase of carbon dioxide (CO2) released into the atmosphere (1). While the primary anthropogenic source of increased atmospheric CO2 concentration is the combustion of fossil fuels, the largest terrestrial source of CO2 emissions is soil (2) where 80% of the total terrestrial carbon is stored. Approximately 62% of soil carbon is in organic form and readily released as CO2, while the remaining is made up of inorganic carbon (SIC) (3). Here, we postulate that there is an amplifying feedback loop between drought, soil desiccation cracking, and CO2 emission in a warming climate (Fig. 1) – a critical aspect that has been overlooked in the existing literature. Further, we argue that the postulated feedback loop affects the emissions of other greenhouse gases (GHGs), such as methane (CH4) and nitrous oxide (N2O), from soils. The urgent need to recognize and characterize this exacerbating feedback loop is twofold. Firstly, it is widely acknowledged that drought accelerates the oxidation of SOC and, thus, increases CO2 emissions into the atmosphere. Drought-induced soil moisture deficits cause plants to reduce their rates of photosynthesis and respiration, resulting in reduced carbon uptake and increased carbon emissions from the soil. Secondly, drought triggers soil desiccation cracking, substantially increasing the permeability of the soil and the interfacial exchange area between the atmosphere and the soil, which, in turn, can considerably increase CO2 efflux in soil by exposing deeper and older stores of soil carbon. Desiccation cracking threatens earthen infrastructure systems and the natural environment. The problems associated with desiccation cracks are becoming more prevalent as anthropogenic climate change exacerbates the severity and frequency of droughts, heatwaves, and drought-heavy precipitation cycles (4). As the warming trends continue, more (and possibly older) CO2 is released from the soil, which can further contribute to global warming. Thus, a chain of events happens in a cascading manner. Failure to consider the hypothesized feedback loop can result in significant inaccuracies when modeling and predicting GHG emissions from soil. It may also lead to underestimating the overall impact of climate change on critical aspects such as soil health, crop production, and the structural integrity of earthen infrastructure.