13C and 15N natural abundance of the soil microbial biomass

Stable isotope analysis is a powerful tool in the study of soil organic matter formation. It is often observed that more decomposed soil organic matter is C-13, and especially N-15-enriched relative to fresh litter and recent organic matter. We investigated whether this shift in isotope composition relates to the isotope composition of the microbial biomass, an important source for soil organic matter. We developed a new approach to determine the natural abundance C and N isotope composition of the microbial biomass across a broad range of soil types, vegetation, and climates. We found consistently that the soil microbial biomass was N-15-enriched relative to the total (3.2 parts per thousand) and extractable N pools (3.7 parts per thousand), and C-13-enriched relative to the extractable C pool (2.5 parts per thousand). The microbial biomass was also C-13-enriched relative to total C for soils that exhibited a C3-plant signature (1.6 parts per thousand), but C-13-depleted for soils with a C4 signature (-1.1 parts per thousand). The latter was probably associated with an increase of annual C3 forbs in C4 grasslands after an extreme drought. These findings are in agreement with the proposed contribution of microbial products to the stabilized soil organic matter and may help explain the shift in isotope composition during soil organic matter formation. (c) 2006 Elsevier Ltd. All rights reserved.