title: Stable isotope switching (SIS): a new stable isotope probing (SIP) approach to determine carbon flow in the soil food web and dynamics in organic matter pools.
abstract: BACKGROUND
  Recent advances in stable isotope probing (SIP) have allowed direct linkage of microbial population structure and function. This paper details a new development of SIP, Stable Isotope Switching (SIS), which allows the simultaneous assessment of carbon (C) uptake, turnover and decay, and the elucidation of soil food webs within complex soils or sedimentary matrices.
  
  
  METHODS
  SIS utilises a stable isotope labelling approach whereby the (13)C-labelled substrate is switched part way through the incubation to a natural abundance substrate. A (13)CH(4) SIS study of landfill cover soils from Odcombe (Somerset, UK) was conducted. Carbon assimilation and dissimilation processes were monitored through bulk elemental analysis isotope ratio mass spectrometry and compound-specific gas chromatography/combustion/isotope ratio mass spectrometry, targeting a wide range of biomolecular components including: lipids, proteins and carbohydrates.
  
  
  RESULTS
  Carbon assimilation by primary consumers (methanotrophs) and sequential assimilation into secondary (Gram-negative and -positive bacteria) and tertiary consumers (Eukaryotes) was observed. Up to 45% of the bacterial membrane lipid C was determined to be directly derived from CH(4) and at the conclusion of the experiment ca. 50% of the bulk soil C derived directly from CH(4) was retained within the soil.
  
  
  CONCLUSIONS
  This is the first estimate of soil organic carbon derived from CH(4) and it is comparable with levels observed in lakes that have high levels of benthic methanogenesis. SIS opens the way for a new generation of SIP studies aimed at elucidating total C dynamics (incorporation, turnover and decay) at the molecular level in a wide range of complex environmental and biological matrices.
authors.affiliation: Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.
authors.forename: P J
authors.initials: PJ
authors.lastname: Maxfield
authors.forename: N
authors.initials: N
authors.lastname: Dildar
authors.forename: E R C
authors.initials: ER
authors.lastname: Hornibrook
authors.forename: A W
authors.initials: AW
authors.lastname: Stott
authors.forename: R P
authors.initials: RP
authors.lastname: Evershed
mesh_terms: D001419:Bacteria / Q000737:chemistry* / Q000378:metabolism; D002247:Carbon Isotopes / Q000032:analysis* / Q000378:metabolism; D020387:Food Chain; D008401:Gas Chromatography-Mass Spectrometry; D007553:Isotope Labeling / Q000379:methods*; D012987:Soil / Q000737:chemistry*; D012988:Soil Microbiology
chemical_list: D002247:Carbon Isotopes; D012987:Soil