Citation ClassicsPriming effects: Interactions between living and dead organic matter
Introduction
It is amusing that our highly cited review on the mechanisms of priming effects (Kuzyakov et al., 2000) originated from a rejected research proposal designed to investigate interactions between carbon (C) pools in soil. In preparing the proposal, we – Jürgen Friedel, Karl Stahr and myself – thoroughly reviewed the available literature on priming effects (PEs), summarized earlier suggested mechanisms, and developed some new hypotheses. The topic was exciting and we were convinced that it was important and would provide a new direction of research. In other words, it had the potential to initiate a new way of thinking about the interactions between biotic and abiotic components, living and dead organic matter. We overcame our disappointment after the rejected proposal and decided to extend what started out as a conventional literature review and discuss suggested approaches to priming effect (PE) quantification and methods for identifying mechanisms.
The paper’s citation success is a result of a number of factors and not just because of the sexy word ‘priming’. Looking back it is clear that we achieved at least some of the prerequisites necessary to generate an appealing (and therefore highly cited) paper.
- 1)
The review was timely – as shown by the fact that the next development step in PE studies was at least partly based on approaches suggested and opinions expressed in our paper. Despite the fact that the phenomenon was discovered 84 years ago (Löhnis, 1926) by studying the effect of legume green manure on mineralization of humus N and that the term ‘priming effect’ was suggested by Bingemann et al. in 1953, it remained largely unrecognized until the 1980s and 1990s. The review by Jenkinson et al. (1985) raised the importance of the inter-relationships between the pools in soil, but was focused on N and mainly related to abiotic processes of isotopic exchange with added mineral 15N. As described below, the new view expressed in our 2000 paper on the interactions between biotic and abiotic pools challenged the conservative picture on independent turnover of individual pools (including microbial biomass), which at that time had been incorporated into most models of C and N dynamics (reviewed by Molina and Smith, 1998, Smith et al., 1997, Smith et al., 1998, Manzoni and Porporato, 2009). To us, the phenomenon of PE suggested new and alternative explanations for the many reports of changes in SOM decomposition after modifications in the pool composition.
- 2)
The paper was of interest to a large number of soil biologists, ecologists and biochemists. This is because many research groups, both then and now, investigate C and N dynamics, nutrient availability for plants, turnover of SOM pools, C availability and stability, and the dependence of C dynamics and turnover on microbial biomass. A review linking these topics, therefore, was appealing and (rewardingly for us) stimulated studies related to understanding the mechanisms of soil functioning for C sequestration and N provision for plants.
- 3)
The isotopic approaches recommended to study PEs were becoming available to a broad research community. We stated that using isotopes was necessary to unambiguously measure the priming effect. This is because it is the only way to separate C and N from various sources. Isotopes were first applied in soil science in the 1940s but even back then studies focused on the interactions between added and already existing pools (Broadbent, 1947, Bingeman et al., 1953, Halam and Bartholomew, 1953). In the early 1990s isotopes began to be applied more widely in soil science and the approaches suggested in our review could be adopted easily by many groups.
Last, but not least, our paper not only provided an overview, summary and systematization of studies up to 2000 but also went beyond the ‘state of the art’ and suggested PE mechanisms as well as providing an outlook on further development. The stimulation of further research has been the most exciting outcome of our paper.
In the last ten years studies on priming effects have become an important (and often controversial) part of soil ecology research, especially in Germany (e.g. Hamer and Marschner, 2005, Blagodatskaya et al., 2007, Dilly and Zyakun, 2008), France (Fontaine et al., 2004, Guenet et al., 2010), the USA (Cheng, 2009, Rasmussen et al., 2007), the UK (Bol et al., 2003a, Nottingham et al., 2009, Paterson et al., 2009) and Italy (Mondini et al., 2006). More than 300 papers have discussed the topic and Soil Biology & Biochemistry is home to a high number of these studies. We reviewed recently the mechanisms of real and apparent priming effects and their dependence on soil microbial biomass and community structure (Blagodatskaya and Kuzyakov, 2008). Therefore, in this article I have elected to look back over the decade since the review was published and suggest directions for future studies.
Section snippets
Is priming a real process in natural soils or is it an artifact of adding glucose?
Some doubt the existence of priming effects in what they call ‘real soil’. They are sceptical and believe that PEs are merely artifacts arising when we add glucose (or other easily-degraded C sources). In fact, the decomposition of most natural polymers releases monomeric sugars into the soil, and the addition of soluble bioavailable substances is, therefore, not artificial. In other words, as polysaccharides (and especially cellulose) are the most common polymer in plant litter (reviewed by
Requirements for priming effect experiments
I expect the topic of PE to be highly relevant to future studies in basic and applied soil biology, ecology, and C and N cycling. PE mechanisms, the drivers and the microbial groups involved are important in recognizing how interactions function between living and dead organic matter. In applied studies, this emphasizes the importance of comparing the processes of C stabilization, SOM turnover, and nutrient release under natural and agricultural ecosystems, and especially nutrient acquisition
Field experiments
It is important to relate PE studies, most of which have been done under controlled laboratory conditions, to the conditions prevailing in the field. This is the complicated issue of scale-up and there is no doubt that compiling budgets of added labeled substrates (a prerequisite for PE calculations) or any other C inputs to the soil under field conditions are much more difficult than under controlled conditions. The ‘artificial’ addition of easily-available substrates can, of course, be done
Acknowledgements
I am deeply appreciative to John Waid and Richard Burns for the invitation to compile this update of priming effects, and especially to Rainer Joergensen, who recognized, as Editor of the manuscript ten years ago, the high potential of the paper despite some critical views of the reviewers. I also express my appreciation to Evgenia and Sergey Blagodatsky along with many colleagues and friends for discussions and ideas that ‘primed’ our studies.
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