B113-0013
Kinetic Properties of Microbial Exoenzymes Vary With Soil Depth but Have Similar Temperature Sensitivities Through the Soil Profile

Wednesday, 16 December 2020
Poster
Ricardo J Eloy Alves1, Ileana A Callejas1,2, Gianna Marschmann1, Maria Mooshammer3, Hans Singh1,4, Bizuayehu Whitney1,5, Margaret S Torn1 and Eoin Brodie3,6, (1)Lawrence Berkeley National Laboratory, Climate and Ecosystem Sciences Division, Berkeley, CA, United States, (2)University of California Los Angeles, Civil and Environmental Engineering, Los Angeles, CA, United States, (3)University of California Berkeley, Environmental Science, Policy, and Management, Berkeley, CA, United States, (4)University of California Berkeley, Molecular and Cell Biology, Berkeley, CA, United States, (5)University of California Berkeley, Plant and Microbial Biology, Berkeley, CA, United States, (6)Lawrence Berkeley National Laboratory, Berkeley, CA, United States
Abstract:
Current knowledge of the mechanisms and responses of soil organic matter (SOM) turnover to warming is mainly limited to surface soils, although over 50% of global soil carbon is contained in sub-soils. Deep soils have different physicochemical properties and nutrient inputs, which may select for distinct microbial functional traits and lead to different SOM dynamics and temperature responses. We hypothesized that kinetic and thermal properties of microbial exoenzymes, which mediate SOM depolymerization, vary with soil depth, reflecting adaptations to distinct substrate and temperature regimes. We determined the Michaelis-Menten kinetics of three ubiquitous enzymes involved in carbon (C), nitrogen (N) and phosphorus (P) acquisition at six soil depths down to 90 cm at a temperate coniferous forest, and their temperature sensitivity based on Arrhenius and Macromolecular Rate Theory (MMRT) models over six temperatures between 4-50°C. Maximum enzyme reaction rates (Vmax) and substrate turnover (Kcat) decreased with depth for all enzymes, although following different depth patterns, whereas their affinities increased, indicating adaptation to lower substrate availability. Catalytic efficiencies also decreased with depth, except for the P-acquiring enzyme, suggesting that microbial populations in deeper soils have different C and N demands, and possibly different life strategies that favor cellular maintenance over maximal resource acquisition and growth. Vmax, Kcat and catalytic efficiency increased consistently with temperature, indicating higher SOM decomposition potential with warming, although their temperature sensitivity was similar through the soil profile based on both Arrhenius and MMRT models. Our results indicate that enzyme kinetics may represent intrinsic traits of soil microbiomes at depth, and that assumptions of constant enzyme kinetics in current whole-soil profile SOM models should be re-evaluated. Moreover, depth-dependent differences in kinetics of distinct enzymes may also, directly or indirectly, affect predictions of emergent properties, such as soil respiration and SOM turnover. Similar to previous observations of soil respiration, we show that soil warming also has a uniform positive effect on enzyme-mediated SOM decomposition through the whole soil profile.
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