How interactions between microbial resource demands, soil organic matter stoichiometry, and substrate reactivity determine the direction and magnitude of soil respiratory responses to warming

Abstract

Recent empirical and theoretical advances inform us about multiple drivers of soil organic matter (SOM) decomposition and microbial responses to warming. Absent from our conceptual framework of how soil respiration will respond to warming are adequate links between microbial resource demands, kinetic theory, and substrate stoichiometry. Here, we describe two important concepts either insufficiently explored in current investigations of SOM responses to temperature, or not yet addressed. First, we describe the complete range of responses for how warming may change microbial resource demands, physiology, community structure, and total biomass. Second, we describe how any relationship between SOM activation energy of decay and carbon (C) and nitrogen (N) stoichiometry can alter the relative availability of C and N as temperature changes. Changing availabilities of C and N liberated from their organic precursors can feedback to microbial resource demands, which in turn influence the aggregated respiratory response to temperature we observe. An unsuspecting biogeochemist focused primarily on temperature sensitivity of substrate decay thus cannot make accurate projections of heterotrophic CO2 losses from diverse organic matter reservoirs in a warming world. We establish the linkages between enzyme kinetics, SOM characteristics, and potential for microbial adaptation critical for making such projections. By examining how changing microbial needs interact with inherent SOM structure and composition, and thus reactivity, we demonstrate the means by which increasing temperature could result in increasing, unchanging, or even decreasing respiration rates observed in soils. We use this exercise to highlight ideas for future research that will develop our abilities to predict SOM feedbacks to climate.

Lasting effect of soil warming on organic matter decomposition depends on tillage practices

“…Thus, thermal adaptation of soil respiration involved weaker response of mass-specific respiration to temperature change (Atkin and Tjoelker, 2003). Our results were supported by previous findings demonstrating that long-term warming can change the mechanisms of SOM turnover (Billings and Ballantyne, 2013).…”

“…Second, warming may increase microbial activity, leading to acceleration of SOM decomposition. This acceleration may come from long-term (Billings and Ballantyne, 2013) or temporary thermal adaptation of the microbial community to the warmer conditions (Bradford, 2013). An increasing number of studies have shown that warming changes microbial community structure, and warming increases the rate of enzymaticallycatalyzed reactions up to a temperature optimum (Wallenstein and Weintraub, 2008;Burns et al, 2013).…”

Temperature sensitivity of soil enzymes along an elevation gradient in the Peruvian Andes

“…The response of enzymatic SOM degradation to temperature also depends on half-saturation (K m ) constants, which may respond to temperature in a manner that offsets SOM losses associated with V max responses (German et al 2012). Enzyme-specific thermal sensitivities have important implications for our understanding of the cycling of different nutrients and organic matter of different complexity under elevated temperatures (Allison et al 2010;Billings and Ballantyne 2012). While more controlled experiments are required to resolve the mechanism involved, the apparent site-acclimation of specific enzyme Q 10 responses has important ecological consequences.…”

Temperature Dependence of Soil Respiration Modulated by Thresholds in Soil Water Availability Across European Shrubland Ecosystems

“…Other studies have also attempted to provide mechanistic explanations for the temperature dependence of SR (Davidson and others, 2006;von Lützow & Kögel-Knabner, 2009). In a review, Billings & Ballantyne (2013) examined the mechanisms that are linked to SR, and reported that temperature-induced changes in microbial community structure, microbial metabolic rates and catalytic rate of exo-enzymes may lead to a decline of SR as a response to an increase in the soil temperature.…”