The isotope composition of the soil microbial biomass is potentially of great importance as it may reflect fractionation during key steps in ecosystem C and N cycling. In the past, we have shown that the relative C and N availability influences the N isotope composition of the microbial biomass in ecosystems as varied as semiarid woodlands, high elevation deserts, temperate grasslands and tropical rainforests. The C isotope composition of microbial biomass seems to reflect mostly the C source. Thus, the natural abundance microbial stable isotope signatures can be useful to track seasonal variation in C and N availability and source of C for microorganisms in C3-C4 mixed vegetations.

We determined the C and N isotope composition of microbial biomass in soils from eight ecosystems (Great Basin desert, cool desert grassland, grass-dominated piñon-juniper interspaces, meadows in the ponderosa pine and mixed conifer forests, and an aspen and ponderosa pine stand) once a month for one year. We compared the isotope composition of microbial biomass with available C and N and net N mineralization in laboratory and field incubations.

Results/Conclusions

We observed significant variability across seasons in microbial C and N signatures, C and N availability, and net N mineralization for most sites. However, there was not a consistent correlation between N mineralization and microbial isotope composition. These results indicate that the relationship between N cycling, C availability and microbial isotope composition is more complex than previously thought.