Linking temperature sensitivity of soil organic matter decomposition to its molecular structure, accessibility, and microbial physiology

Abstract

Temperature sensitivity of soil organic matter (SOM) decomposition may have a significant impact on global warming. Enzyme-kinetic hypothesis suggests that decomposition of low-quality substrate (recalcitrant molecular structure) requires higher activation energy and thus has greater temperature sensitivity than that of high-quality, labile substrate. Supporting evidence, however, relies largely on indirect indices of substrate quality. Furthermore, the enzyme-substrate reactions that drive decomposition may be regulated by microbial physiology and/or constrained by protective effects of soil mineral matrix. We thus tested the kinetic hypothesis by directly assessing the carbon molecular structure of low-density fraction (LF) which represents readily accessible, mineral-free SOM pool. Using five mineral soil samples of contrasting SOM concentrations, we conducted 30-days incubations (15, 25, and 35 °C) to measure microbial respiration and quantified easily soluble C as well as microbial biomass C pools before and after the incubations. Carbon structure of LFs (<1.6 and 1.6-1.8 g cm(-3) ) and bulk soil was measured by solid-state (13) C-NMR. Decomposition Q10 was significantly correlated with the abundance of aromatic plus alkyl-C relative to O-alkyl-C groups in LFs but not in bulk soil fraction or with the indirect C quality indices based on microbial respiration or biomass. The warming did not significantly change the concentration of biomass C or the three types of soluble C despite two- to three-fold increase in respiration. Thus, enhanced microbial maintenance respiration (reduced C-use efficiency) especially in the soils rich in recalcitrant LF might lead to the apparent equilibrium between SOM solubilization and microbial C uptake. Our results showed physical fractionation coupled with direct assessment of molecular structure as an effective approach and supported the enzyme-kinetic interpretation of widely observed C quality-temperature relationship for short-term decomposition. Factors controlling long-term decomposition Q10 are more complex due to protective effect of mineral matrix and thus remain as a central question.

Total, cold and hot water extractable organic carbon in soil profile: impact of land-use change

“…Taking into consideration significant change of electrical conductivity in arable compared to forest soil (Tables 1-2), it could be hypothesized that not only labile organic matter content, but the whole mineralization potential of soil has decreased. The most labile fraction of SOM -water extractable organic matter -consists of the substances with low activation energy barrier, so a depletion of WEOC pool might change the temperature sensitivity of soil (Wagai et al, 2013). Therefore the investigation of carbon dioxide emission rates from studied forest and arable soils is an important issue for future research.…”

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

“…1). No-Till has been observed to strongly increase soil microbial biomass and activity, and to release more CO 2 during incubation relative to tilled soil (Wagai et al, 2013). However, the findings of higher CO 2 efflux due to warming under no-till compared to tillage treatment measured in the laboratory should be cautiously extended to field conditions.…”

Comparing models of microbial–substrate interactions and their response to warming

“…Limited sites may play a role if the substrate has a high volume-to-surface ratio, or if the substrate is associated with minerals (Davidson and Janssens, 2006;Gillabel et al, 2010;Conant et al, 2011;Davidson et al, 2012Davidson et al, , 2014Cotrufo et al, 2013;Wagai et al, 2013;Benbi et al, 2014;Wieder et al, 2014a;Tang and Riley, 2015). Our implementation of limited substrate causes a surplus of free enzymes that compete for binding to substrates, similar to the Langmuir adsorption isotherm theory (Vetter et al, 1998;Schimel and Weintraub, 2003;; and see Appendix B, "Model with limited available substrate"), leading to diminishing depolymerisation returns and a REV model formulation.…”