Objective:  The objective of this experiment is to determine how carbon flow from plants into and through microbial biomass (via root exudation) is altered in limed grasslands. We will determine the relative quantity and rates of transfer of rhizosphere carbon to soil fungi and bacterial groups.  We hypothesize that (1) the transfer of 13C below-ground and through the soil system will be accelerated in limed plots and (2) fungi will contain more 13C immediately after pulsing reflecting the importance of mycorrhizal relationships in these grasslands.  However, bacteria will contain more label later, as plant-derived C moves through the soil system. 

Methodology:  Plots at the NERC Soil Biodiversity Sourhope field site will be labeled with 13C using the Stable Isotope Delivery system and sampled at intervals thereafter.  Soils will be collected for a suite of analyses, including:

(1) Determination of label incorporation into soil microbial biomass using the chloroform fumigation extraction technique and subsequent analysis of freeze-dried extracts by GC-IRMS (Gregorich et al. 2000).
(2) Separation and isotopic analysis of microbial phospholipid components derived from soils by the PLFA extraction and analysis procedure combined with compound specific analysis of the lipid fractions by GC-C-IRMS (Gas chromatography-combustion-isotope ratio mass spectrometry (sensu Boschker et al. 1998).  This technique will allow us to determine which groups of microbes respond to changes in soil carbon cycling caused by application of lime to the plots. 
(3) Finally, plant material (roots and shoots) collected from the plots at the time of soil collection and will also be submitted for 13C analysis. 

Justification:  The use of a stable isotope (13C) to label plants and soils in the field, combined with analyses of specific microbial groups for incorporation of this label will allow us to take a unique look at rhizosphere dynamics.  While laboratory investigations using 14C labeling techniques have been successful at elucidating the rates of transfer of photosynthetically-fixed carbon into the rhizosphere, little work has been done under field conditions, and no work has been done that separates the microbial response into functional groups at the scale we propose. 
 

References:

Boschker HTS, Nold SC, Wellsbury P, Bos D, de Graaf W, Pel R, Parkes RJ, Cappenberg TE.  1998.  Direct linking of microbial populations to specific biogeochemical processes by 13C-labelling of biomarkers.  Nature. 392:801-805.

Frostegard A, Baath E, Tunlid A.  1993.  Shifts in the structure of soil microbial communities in limed forests as revealed by phospholipid fatty acid analysis.  Soil Biology & Biochemistry.  25:723-730. 

Gregorich EG, Liang BC, Drury CF, Mackenzie AF, McGill WB  2000.  Elucidation of the source and turnover of water soluble and microbial biomass carbon in agricultural soils. Soil Biology & Biochemistry.  32:581-587.

Stenberg M, Stenberg B, Rydberg T.  2000.  Effects of reduced tillage and liming on soil microbial activity and soil properties in a weakly-structured soil.  Applied Soil Ecology.  14:135-45.

Zelles L, Scheunert I, Kreutzer K.  1987.  Bioactivity in limed soil of a spruce forest.  Biology and Fertility of Soils.  3:211-216.