Temperature sensitivity and enzymatic mechanisms of soil organic matter decomposition along an altitudinal gradient on Mount Kilimanjaro

Abstract

Short-term acceleration of soil organic matter decomposition by increasing temperature conflicts with the thermal adaptation observed in long-term studies. Here we used the altitudinal gradient on Mt. Kilimanjaro to demonstrate the mechanisms of thermal adaptation of extra- and intracellular enzymes that hydrolyze cellulose, chitin and phytate and oxidize monomers (14C-glucose) in warm- and cold-climate soils. We revealed that no response of decomposition rate to temperature occurs because of a cancelling effect consisting in an increase in half-saturation constants (Km), which counteracts the increase in maximal reaction rates (Vmax with temperature). We used the parameters of enzyme kinetics to predict thresholds of substrate concentration (Scrit) below which decomposition rates will be insensitive to global warming. Increasing values of Scrit, and hence stronger canceling effects with increasing altitude on Mt. Kilimanjaro, explained the thermal adaptation of polymer decomposition. The reduction of the temperature sensitivity of Vmax along the altitudinal gradient contributed to thermal adaptation of both polymer and monomer degradation. Extrapolating the altitudinal gradient to the large-scale latitudinal gradient, these results show that the soils of cold climates with stronger and more frequent temperature variation are less sensitive to global warming than soils adapted to high temperatures.

Catalytic power of enzymes decreases with temperature: New insights for understanding soil C cycling and microbial ecology under warming

“…For enzymes that could face low substrate concentration, the substrate affinity (1/ K m ) and its dependence to temperature must be considered. Given that K m increases with temperature, thus slowing down the enzymatic reaction (Equation ; Stone et al., ; Blagodatskaya, Blagodatsky, Khomyakov, Myachina, & Kuzyakov, ), the consideration of this parameter should not qualitatively change our predicted negative temperature effect on E power . The strength of this theory is to show that the temperature responses (transitory and long‐term) of biocatalysed reactions can be better understood by considering the catalytic power of enzymes.…”

Pathways regulating decreased soil respiration with warming in a biocrust‐dominated dryland

“…Here, we addressed the role of soil microbial thermal acclimation as a pathway driving the effects of warming on R S , by investigating extracellular enzymatic activities (β‐glucosidase) and mass‐specific respiration rates ( R mass ). This indirect approach is adequate to identify potential microbial acclimation to the thermal regime (Allison et al., ; Blagodatskaya, Blagodatsky, Khomyakov, Myachina, & Kuzyakov, ; Rinnan, Rousk, Yergeauz, Kowalchuk, & Bååth, ), but cannot be used to establish a causal link with the reduced R S observed under warming. Acclimation to the thermal regime in plants and animals can occur within short‐time spans (from days to weeks) when affecting enzymes of respiratory pathways (Atkin & Tjoelker, ; Hochachka & Somero, ).…”

Leaf and root litter decomposition is discontinued at high altitude tropical montane rainforests contributing to carbon sequestration

“…Differences in climatic conditions, that is, precipitation and temperature, and soil characteristics, that is, soil pH and soil moisture, also influence decomposition patterns (Aerts, ; Fierer et al., ; Scheu et al., ; Wieder, Cleveland, & Townsend, ). However, the sensitivity to variations in these parameters, especially temperature, differs between ecosystems, climatic zones and between different qualities of the litter material (Aerts, ; Blagodatskaya et al., ; Fierer et al., ). We showed in our study that decomposition patterns were similar at the two higher altitudes although temperature, precipitation, soil moisture, and soil pH varied between each of the three study sites.…”