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 ((14)C-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.

Figure 1

Rates of reactions mediated by hydrolytic enzymes (a–c) and rates of glucose oxidation to CO₂ (d) as dependent on substrate concentration at 10 and 20 °C for the site located at 2010 m a.s.l. Symbols – experimental data, lines – approximation by Michaelis–Menten kinetics (Eq. 1). Bars show standard deviations of the means (n = 3).

Figure 2

The Q₁₀ values for enzymatic activities (a–c) and glucose oxidation to CO₂ (d) as dependent on substrate concentration at two altitudes. The emphasized sections show the concentration range at which no temperature effects occur (below S_crit) with shading colors corresponding to different altitudes. The Q₁₀ values derived from experimental data are shown as symbols. The model simulations based on experimentally obtained parameters of Michaelis–Menten kinetics (Eqs 1 and 1S) are shown as curves (a–c). For glucose oxidation (d) at 3020 m elevation, non-linear trend was very weakly expressed. Bars show standard deviations of the means (n = 3).

Figure 3

The Q₁₀^total values for hydrolytic enzyme activity at saturating substrate concentrations (A) and the increase in V_max and K_m induced by a temperature increase from 10 to 20 °C for ¹⁴C-glucose oxidation (B) depending on altitude. Symbols – experimentally derived values for Q₁₀^total (B), Q₁₀^Vmax, and Q₁₀^Km (A). Lines are the trend-lines obtained by the best fitting of power (A) and linear functions (B) at P values < 0.05, bars show standard deviations of the means (n = 3).

Figure 4

The values of Q₁₀^Km (a) and Q₁₀^Vmax (b) for hydrolytic reactions and for reactions of glucose oxidation at low and high altitudes. Bars show standard deviations of the means (n = 3).

Figure 5. Relevance of three thermal adaptation mechanisms of SOM decomposition based on parameters of enzyme kinetics.