Carbon fluxes from plants through soil organisms determined by field 13CO2 pulse-labelling in an upland grassland

The main findings of research into carbon (C) fluxes from plants to soil micro-organisms using in situ (CO2)-C-13 upland grassland at the NERC Soil Biodiversity Thematic Programme field site in Southern Scotland are reviewed. From 1999 to 2003 the site was the focus of a unique and intensive programme of stable isotope tracing of C flux through rhizodeposition to soil microbiota and stable isotope probing of microbial biomarker compounds. We review the findings published to date, and highlight the novel ways in which the pulse-labelling approach has been applied to further understand C fluxes in the rhizosphere and mycorrhizophere in this grassland. The most important achievements from these studies, many of which are the first field measurements of their kind, include: (1) quantification of C flux from recent photosynthate into roots, soil microbial populations and soil respiration over time periods of hours to months; (2) analysis of diurnal control of root exudation and respiration linked to photoperiod and photosynthetic activity; (3) measurements of C flux from plants directed through mycorrhizal fungal networks; (4) establishing the importance C flow from recent photosynthate into soil fungi, revealed by C-13 enrichment of phospholipid fatty acid biomarker molecules (PLFA); (5) detection of the disruptive effects of fungal-feeding microarthropods on (CO2)-C-13 respiration in the mycorrhizosphere; (6) measurement of C-13 enrichment into soil microbial DNA and RNA and the rates of turnover of RNA; (7) identification of soil micro-organisms most enriched with C-13 by sequence analysis of 'heavy' RNA separated by density-gradient centrifugation; and (8) estimates of the effects of liming on C flux into and through upland grassland, and its effects on C cycling by soil micro-organisms. In reviewing all these findings we highlight the strengths and limitations of the in situ C-13 technique. We also explain how the new insights gained from these studies emphasise the complex temporal dynamics of recent photosynthate entering the soil through different pathways and the role of multi-trophic interactions between soil biota in determining the fate of recently fixed carbon in grasslands. (c) 2006 Elsevier B.V. All rights reserved.