What is recalcitrant soil organic matter?
Markus KleberDepartment of Crop and Soil Science, Oregon State University, 3017 Agriculture and Life Sciences Building, Corvallis, OR 97331, USA. Email: markus.kleber@oregonstate.edu
Environmental Chemistry 7(4) 320-332 https://doi.org/10.1071/EN10006
Submitted: 20 January 2010 Accepted: 1 June 2010 Published: 20 August 2010
Environmental context. On a global scale, soils store more carbon than plants or the atmosphere. The cycling of this vast reservoir of reduced carbon is closely tied to variations in environmental conditions, but robust predictions of climate–carbon cycle feedbacks are hampered by a lack of mechanistic knowledge regarding the sensitivity of organic matter decomposition to rising temperatures. This text provides a critical discussion of the practice to conceptualise parts of soil organic matter as intrinsically resistant to decomposition or ‘recalcitrant’.
Abstract. The understanding that some natural organic molecules can resist microbial decomposition because of certain molecular properties forms the basis of the biogeochemical paradigm of ‘intrinsic recalcitrance’. In this concept paper I argue that recalcitrance is an indeterminate abstraction whose semantic vagueness encumbers research on terrestrial carbon cycling. Consequently, it appears to be advantageous to view the perceived ‘inherent resistance’ to decomposition of some forms of organic matter not as a material property, but as a logistical problem constrained by (i) microbial ecology; (ii) enzyme kinetics; (iii) environmental drivers; and (iv) matrix protection. A consequence of this view would be that the frequently observed temperature sensitivity of the decomposition of organic matter must result from factors other than intrinsic molecular recalcitrance.
Acknowledgements
I am indebted to Noah Fierer for sharing the data for Fig. 3 and to Joan Sandeno for language edits to the final version of the manuscript. Marco Keiluweit, Peter S. Nico and Dave D. Myrold provided helpful comments on earlier versions of the manuscript. Three anonymous reviewers are acknowledged for their constructive suggestions to improve the manuscript.
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