title: 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.
authors.affiliation: National Institute for Agro-Environmental Sciences, Tsukuba, Ibaraki, Japan. rota@affrc.go.jp
authors.forename: Rota
authors.initials: R
authors.lastname: Wagai
authors.forename: Ayaka W
authors.initials: AW
authors.lastname: Kishimoto-Mo
authors.forename: Seiichiro
authors.initials: S
authors.lastname: Yonemura
authors.forename: Yasuhito
authors.initials: Y
authors.lastname: Shirato
authors.forename: Syuntaro
authors.initials: S
authors.lastname: Hiradate
authors.forename: Yasumi
authors.initials: Y
authors.lastname: Yagasaki
mesh_terms: D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D012987:Soil; D012988:Soil Microbiology; D013696:Temperature
chemical_list: D012987:Soil