Influence of varying nitrogen availability on soil microbial growth efficiency

Background/Question/Methods

Microbial Growth Efficiency (MGE) is the partitioning of substrate between growth and maintenance by the soil microbial community.  This parameter has implications for soil C models, and may be flexible as environmental conditions change.  Generally it is expected that increasing N availability in soils will increase MGE.  The goal of this study was to determine the effects of N addition on MGE and microbial turnover.  We incubated grassland soil from Northern Arizona for two weeks with the equivalent of 0, 10, or 400 kg ha-1 of nitrate added.  At the end of the incubation, we measured MGE using the metabolic tracer method and calculated microbial turnover using MGE, respiration, and microbial biomass concentrations.  

Results/Conclusions

Nitrogen addition did not significantly affect the soil MGE (p= 0.28), and across all treatments the average MGE was 0.74 (mg C mg C^-1).  Microbial turnover and respiration was also unaffected by nitrogen addition.  The results of this incubation experiment indicate that the soil microbial community in the Northern Arizona grassland soil is not nitrogen limited and that microbial processes are unaffected even by nitrogen fertilization rates comparable to those used in agriculture.  These results have implications for models that include MGE and turnover.