The energetic and chemical signatures of persistent soil organic matter
A large fraction of soil organic matter (OM) resists decomposition over decades to centuries as indicated by long radiocarbon residence times, but the mechanisms responsible for the long-term (multi-decadal) persistence are debated. The current lack of mechanistic understanding limits our ability to accurately predict soil OM stock evolution under climate and land-use changes. Using a unique set of historic soil samples from five long-term (27–79 years) bare fallow experiments, we demonstrate that despite wide pedo-climatic diversity, persistent OM shows specific energetic signatures, but no uniform chemical composition. From an energetic point of view, thermal analyses revealed that combustion of persistent OM occurred at higher temperature and provided less energy than combustion of more labile OM. In terms of chemical composition, persistent OM was H-depleted compared to OM present at the start of bare fallow, but spectroscopic analyses of OM functional groups did not reflect a consistent chemical composition of OM across sites, nor substantial modifications with bare fallow duration. The low energy content of persistent OM may be attributed to a combination of reduced content of energetic C–H bonds or stronger interactions between OM and the mineral matrix. Soil microorganisms thus appear to preferentially mineralize high-energy OM, leaving behind material with low energy content. This study provides the first direct link between long-term persistence of OM in soil and the energetic barriers experienced by the decomposer community.
KeywordsCarbon cycling Long-term bare fallow Rock–Eval 6 NEXAFS TG-DSC
The INSU EC2CO program, ADEME and the ESF (MOLTER program) are acknowledged for financial support. We thank Rothamsted Research and the Lawes Agricultural Trust for access to archived samples and the BBSRC for support under the Institute National Capabilities programme grant (BBS/E/C/00005189). Related information and data can be found in the electronic Rothamsted Archive (era.rothamsted.ac.uk). The Danish contribution was financially supported by The Ministry of Food, Agriculture and Fisheries. NEXAFS data were acquired at the beamline11ID-1 at the CLS, which is supported by the NSERC, the CIHR, the NRC and the University of Saskatchewan. Special thanks go to Tom Regier for his expert support on the SGM-beamline at CLS. We also thank the reviewers for their helpful comments.
- Beleites C, Sergo V (2014) hyperSpec: a package to handle hyperspectral data sets in R. R package version 0.98-20140523. InGoogle Scholar
- Chessel D, Dufour AB, Thioulouse J (2004) The ade4 package - I: One-table methods. In: R News. vol 1. p 5–10Google Scholar
- De la Rosa JM, González-Pérez JA, González-Vázquez R, Knicker H, López-Capel E, Manning DAC, González-Vila FJ (2008) Use of pyrolysis/GC-MS combined with thermal analysis to monitor C and N changes in soil organic matter from a mediterranean fire affected forest. Catena 74(3):296–303CrossRefGoogle Scholar
- Lefèvre R, Barré P, Moyano FE, Christensen BT, Bardoux G, Eglin T, Girardin C, Houot S, Katterer T, van Oort F, Chenu C (2014) Higher temperature sensitivity for stable than for labile soil organic carbon—Evidence from incubations of long- term bare fallow soils. Glob Change Biol 20(2):633–640CrossRefGoogle Scholar
- Lopez-Capel E, Sohi SP, Gaunt JL, Manning DAC (2005) Use of thermogravimetry-differential scanning calorimetry to characterize modelable soil organic matter fractions. Soil Sci Soc Am J 69(1):136–140Google Scholar
- R Core Team (2013) R: A language and environment for statistical computing. In: R foundation for statistical computing, Vienna, AustriaGoogle Scholar
- Swift MJ, Heal OW, Anderson JM (1979) Decomposition in Terrestrial Ecosystems. Blackwell Scientific Publications, OxfordGoogle Scholar