Spatial heterogeneity in the relocation of added 13C within the structure of an upland grassland soil

A pulse of (CO2)-C-13 was added to the above ground vegetation in an upland grassland to determine the effects of faunal diversity on the flux of carbon to the Surface horizons of the soil. Faunal diversity was manipulated by liming and biocide treatments for three years prior to the pulse addition. The relocation of C-13 within roots and rhizosphere soil was determined by analysis of samples of bulk soil and of specific features identified on soil thin sections on four dates after the addition of the (CO2)-C-13 pulse. Analysis of bulk soils showed only a small enrichment in C-13 and no significant effects of the treatments. Analysis by isotope ratio mass spectrometry of the products of in situ laser combustion of root material and aggregates formed from faunal excrement showed that the distribution of the newly photosynthesised C-13 is very localised, with large spatial variability in soil and root delta C-13 at scales of less than I mm. delta C-13 values ranged from the natural abundance level of around -28 parts per thousand to -4.9 parts per thousand in roots and to -8.4 parts per thousand in aggregates. The small pulse and large spatial variability masked any effects of the liming and biocide treatments in these soils. However, the variability in the relocation of newly photosynthesised carbon may help to explain the large spatial variability found in bacterial numbers at the sub-mm scale within soils and emphasises the importance of the accessibility of substrates to decomposers in undisturbed structured soils. (c) 2005 Elsevier Ltd. All rights reserved.