Examining N-limited soil microbial activity using community-level physiological profiling based on O2 consumption

Abstract

Nitrogen-limited soil microbial activity has important implications for soil carbon storage and nutrient availability, but previous methods for assessing resource limitation have been restricted, due to enrichment criteria (i.e., long incubation periods, high substrate amendments) and/or logistical constraints (e.g. use of radioisotopes). A microtiter-based assay of basal and substrate induced soil respiration based on O2 consumption may be a rapid, ecologically relevant means of assessing N limitation. The present study evaluated this approach by examining 1) the extent and duration of N limitation on soil respiratory activity following different levels of N fertilization in the field, and 2) the relationship between N-limited activities and growth under the assay conditions. Fertilization rate and the time since fertilization had significant impacts on the degree of N limitation of soil microbial activity. The highest fertilization rate showed the earliest and most persistent reduction in N limitation, as would be predicted from the higher concentration of extractable inorganic soil N observed with this treatment. Bacterial growth under the assay conditions, as estimated by quantitative-PCR of 16S rRNA genes, was less than twofold in soils demonstrating a rapid respiratory response (i.e. peak within 6-8 h of initiating incubation) to up to fourfold in soils demonstrating a slower respiratory response (i.e., peak response after 14 h of incubation). Increased respiratory response with N amendment was usually associated with increased cell growth, although for rapidly responding soils some C sources showed N-limited use without growth. This was likely due to exhaustion of the relatively low levels of available C amendment before growth was detected. The method appears useful for assessing N-limited microbial growth, and it may be effective as a rapid indicator of bioavailable soil N. It may also be a tool to evaluate the complexity of N limitation among various metabolic pathways found in soil microbial communities, particularly if linked to dynamics in community structure and gene activation.