Priming of the decomposition of ageing soil organic matter: concentration dependence and microbial control

Johannes Rousk,Paul W. Hill,Davey L. Jones,Carly Stevens

doi:10.1111/1365-2435.12377

Johannes Rousk, Paul W. Hill + Show 2 more

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https://doi.org/10.1111/1365-2435.12377

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Journal: Functional ecology	Publication Date: Dec 16, 2014
Citations: 66	License type: CC BY 4.0

Affiliation: Lund University, Bangor University

Abstract

Summary The amount of carbon (C) stored in soil is an important regulator for the global climate and soil fertility and is the balance between formation and decomposition of soil organic matter (SOM). Decomposition of SOM can be powerfully affected by labile carbon (C) supplements in, for example, the rhizosphere. A stimulation of SOM mineralisation induced by labile C additions is termed ‘priming’, and the mechanisms for this phenomenon remain elusive. The most widely held explanation assigns priming to successional dynamics in r‐ and K‐selected groups within the microbial community; groups which have also been connected with fungal (K‐selected) and bacterial (r‐selected) decomposers. New evidence has also suggested that recently formed SOM is particularly sensitive to priming. We investigated (i) the labile C concentration dependence of SOM mineralisation, (ii) the susceptibility of differently aged SOM to priming and (iii) if priming is due to bacterial or fungal growth dynamics. To create an age gradient of traceable SOM, we spiked a pasture soil using 14C glucose, and subsampled plots 1 day, 2 months, 5 months and 13 months after application (i.e. SOM aged 1 day – 13 months). Glucose (0–4000 μg C g−1) was added in subsequent laboratory experiments, and respiration, SOM mineralisation (14CO2 evolution), bacterial growth rates (leucine incorporation) and fungal biomass (ergosterol) were tracked during ca. 1 week. Mineralisation of SOM aged 2–13 months showed similar labile C concentration dependencies, and priming increased mineralisation of SOM systematically by up to 350%. The glucose treatments induced variable microbial growth responses for differently aged SOM, which were unrelated to the priming effect. That successional dynamics in microbial r‐ and K‐selected groups, or bacterial and fungal decomposers, respectively, underpinned priming was incompatible with the results obtained. An alternative explanation could be that SOM transformation by extracellular enzymes, for subsequent respiration, could be triggered by labile C. In conclusion, labile C primed the mineralisation of 2–13 months aged SOM, and the mechanism for this priming was unrelated to microbial growth dynamics.

Highlights

Understanding the processes that govern the turnover of soil organic matter (SOM) is central to understanding the functioning of soil systems and to predicting plant nutrient supply (Waksman & Gerretsen 1931; Schimel 1995).at a size two or three fold that of the atmosphere (Le Quere et al 2009), the carbon (C) stored in soil represents the largest pool held in terrestrial ecosystems
A stimulation of SOM mineralisation induced by labile C additions is termed ‘priming’, and the mechanisms for this phenomenon remain elusive
The most widely held explanation assigns priming to successional dynamics in r- and K-selected groups within the microbial community; groups which have been connected with fungal (K-selected) and bacterial (r-selected) decomposers

Summary

Introduction

Understanding the processes that govern the turnover of soil organic matter (SOM) is central to understanding the functioning of soil systems and to predicting plant nutrient supply (Waksman & Gerretsen 1931; Schimel 1995).at a size two or three fold that of the atmosphere (Le Quere et al 2009), the carbon (C) stored in soil represents the largest pool held in terrestrial ecosystems. Most efforts directed at describing the exchange processes of the soil C pool rely on dividing SOM into a number of subcomponents, whose individual decomposition rates occur according to first-order kinetics (Manzoni & Porporato 2009; Xu et al 2014). It is assumed that the turnover rates of these subcomponent pools are not interdependent. A symptom of this interdependence, or coupling, between labile C input and soil C output is the frequently noted priming effect (Kuzyakov, Friedlel & Stahr 2000; Blagodatskaya & Kuzyakov 2008; Kuzyakov 2010). The ‘priming effect’ is defined as an increase in decomposition of SOM in response to the input of available C sources (Kuzyakov, Friedlel & Stahr 2000; Ostle et al 2009; Garcia-Pausas & Paterson 2011)

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