A tale of two extremes: Temperature sensitivity of soil respiration from cold and hot climates

Soils represent the largest terrestrial C pool, and the flux of CO2 from soils to the atmosphere is ~ 6-10 times more than anthropogenic emissions (Friedlingstein et al., 2022; Jian et al., 2021). Warming can accelerate soil C loss to the atmosphere, exacerbating warming via a positive feedback between soil C and climate. However, the fate of soil C in a future, warmer world is still highly uncertain, particularly for arctic and boreal (cool-climate) ecosystems and tropical (warm-climate) ecosystems, which represent dominant portions of the global terrestrial C cycle and which are historically understudied (Schuur et al., 2018; Wood et al., 2019). The uncertainty in soil C-climate feedbacks is exacerbated by extreme climatic conditions typical of ecosystems with large carbon (C) stocks, such as many tundra, boreal and tropical forested ecosystems, where temperature responses may be dominated by different underlying controls. Reducing uncertainties in soil C-climate feedback requires systematic synthesis of underlying mechanisms related to soil C turnover in these critical and relatively poorly understood regions.

The relationship between microbial respiration and temperature is typically modeled using Q10 function. Generally, the apparent Q10 of soil respiration is much higher for cool vs. warm-climate ecosystems (Lloyd and Taylor, 1994; Kirschbaum, 1995; Mikan et al., 2002), reflecting biochemical limits to decomposition at low temperatures. However, results from two field warming experiments in the tropics contradict this expectation, both observing extraordinarily high soil respiration responses to in situ warming (e.g., 29 - 244% increase; Nottingham et al., 2020; Wood et al., in revision). Together, these findings suggest that there are many indirect temperature effects on soil C stability and subsequent C flux to the atmosphere at different time scales, thus requiring re-evaluation of various proposed mechanisms that control the sensitivity of soil C across ecosystems with different climate history. It has also become clearer that different assumptions related to the persistence and vulnerabilities of Arctic soil C in biogeochemical models can lead to divergent responses of soil C losses or gains in a future climate(Wieder et al., 2019). Recent observations showed that episodical cold season CO2 emissions make a significant contribution to the annual budget (Natali et al., 2019). Cold season emissions are linked to the period when soil temperatures are poised near 0 °C and soil respiration is extremely sensitive to other factors, such as soil moisture and labile C. Predicting soil respiration across disparate systems and in new future climates will require using mechanistic instead of purely descriptive models. Rather than Q10's, enzymatic processes can be modeled by Arrhenius kinetics, which work well when there are no other limiting factors. However, many other factors affect the supply and binding of substrates to the extracellular enzymes that control soluble C availability to microbes, thus moderating temperature responses through substrate-limitation of enzyme activities (Davidson and Janssens, 2006; Conant et al., 2011; Schmidt et al. 2011). Substrate limitations to extracellular enzymes include freezing, substrate transport, enzyme-substrate binding, and stabilization of soil organic matter (SOM) and enzymes in aggregates and on mineral surfaces (Burns et al. 2013, Fanin et al., 2022, Zheng et al., 2022). In addition, changes in the microbial community and its enzymatic capacities, sometimes referred to as acclimation, can alter enzyme activities (Bradford, 2013, Fanin et al., 2022). Overall, the many direct and indirect temperature effects on rates of biogeochemical processes (e.g., enzyme kinetics), community structure, microbial biomass and activity, water phase and availability, and substrate supply are likely to influence soil C stability and subsequent C fluxes to the atmosphere at different timescales, thus requiring a reevaluation of our assumptions about the mechanisms that control the sensitivity of soil C across ecosystems with different climate histories.

We propose a synthesis of soil respiration data across temperature extremes, i.e., Arctic/Boreal and Tropical regions, to advance our understanding and ability to model soil respiration-temperature relationships. Our overall objective for the proposed work is to reduce uncertainty in soil C-climate feedback by systematically synthesizing underlying mechanisms related to soil C turnover and stabilization. Our data synthesis from sites with high and low temperature extremes will provide generalizable insights for future models. This synthesis closely aligns with the ESS (Environmental System Science) program goal in BER (Biological and Environmental Research) to advance an integrated, robust, and scale-aware predictive understanding of interacting biogeochemical, ecological, hydrological and physical processes that shape ecosystem function.