Soil‐carbon preservation through habitat constraints and biological limitations on decomposer activity

Abstract

We review recent experimental results on the role of soil biota in stabilizing or destabilizing soil organic matter (SOM). Specifically, we analyze how the differential substrate utilization of the various decomposer organisms contributes to a decorrelation of chemical stability, residence time, and carbon (C) age of organic substrates. Along soil depth profiles, a mismatch of C allocation and abundance of decomposer organisms is consistently observed, revealing that a relevant proportion of soil C is not subjected to efficient decomposition. Results from recent field and laboratory experiments suggest that (1) bacterial utilization of labile carbon compounds is limited by short-distance transport processes and, therefore, can take place deep in the soil under conditions of effective local diffusion or convection . In contrast, (2) fungal utilization of phenolic substrates, including lignin, appears to be restricted to the upper soil layer due to the requirement for oxygen of the enzymatic reaction involved. (3) Carbon of any age is utilized by soil microorganisms, and microbial C is recycled in the microbial food web. Due to stoichiometric requirements of their metabolism, (4) soil animals tend to reduce the C concentration of SOM disproportionally, until it reaches a threshold level. The reviewed investigations provide new and quantitative evidence that different soil C pools underlie divergent biological constraints of decomposition. The specialization of decomposers towards different substrates and microhabitats leads to a relatively longer persistence of virtually all kinds of organic substrates in the nonpreferred soil spaces. We therefore propose to direct future research explicitly towards such biologically nonpreferred areas where decomposition rates are slow, or where decomposition is frequently interrupted, in order to assess the potential for long-term preservation of C in the soil

Pore-scale investigation on the response of heterotrophic respiration to moisture conditions in heterogeneous soils

“…3 Effects of soil porosity and structure on simulated soil heterotrophic respiration (HR) rate as a function of saturation degree (S) diffusion in aqueous phase has minimal effect on HR rate and OC are mainly degraded locally in this study. The finding is consistent with the observation where bacterial utilization of soil organic carbon is limited by short-distance transport process (Ekschmitt et al 2008). In contrast, Fig.…”

Altered carbon turnover processes and microbiomes in soils under long-term extremely high CO2 exposure

“…SOM degradation is generally dependent on the physical and biochemical capabilities of the soil biota 10 , and is mediated by a multitude of lignin and carbohydrate metabolizing enzymes underlying biotic and abiotic controls, which we expected to be significantly altered by CO 2 degassings 11 . Assessing the metabolic potential and activity of soil communities as drivers for changes in C cycling has been a focus of ecological research for decades, but largely remains a challenge.…”

Persistence of soil organic matter as an ecosystem property

“…We therefore cannot extrapolate the initial stages of litter decomposition to explain the persistence of organic compounds in soils for centuries to millennia-other mechanisms protect against decomposition. Perhaps certain compounds require co-metabolism with another (missing) compound, or microenvironmental conditions restrict the access (or activity) of decomposer enzymes (for example, hydrophobicity, soil acidity, or sorption to surfaces 18 ).…”

“…The soil volume occupied by micro-organisms is considerably less than 1%: this occupied volume is distributed heterogeneously in small-scale habitats, connected by water-saturated or unsaturated pore space 18 . The availability of spatially and temporally diverse habitats probably gives rise to the biodiversity that we see in soil, but this fragmentation of habitat may restrict carbon turnover.…”