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United States Department of Agriculture

Agricultural Research Service

Research Project: SOIL AND CROP MANAGEMENT SYSTEMS FOR IMPROVED NATURAL RESOURCE QUALITY AND EFFICIENCY

Location: North Central Agricultural Research Laboratory

Title: Applying an oxygen-based respiratory assay to assess soil microbial responses to substrate and N availability

Authors
item Lehman, R
item Garland, Jay -
item Osborne, Shannon

Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 20, 2012
Publication Date: December 26, 2012
Citation: Lehman, R.M., Garland, J.L., Osborne, S.L. 2012. Applying an oxygen-based respiratory assay to assess soil microbial responses to substrate and N availability. Applied Soil Ecology. 64(2013):127-134.

Interpretive Summary: Documented approaches for measuring soil microbial activities and their controlling factors under field conditions are needed to advance understanding of soil microbial processes for numerous applications. We manipulated field plots with carbon (C) and nitrogen (N) additions to test the capability of an activity assay to measure: (1) respiration of native soil C; (2) activities that are consistent with carbon substrate availability in the field; and, (3) N limitation of activities that are consistent with nitrogen availability in the field. The activity assay utilizes a microplate platform (BD Oxygen Biosensor System, BDOBS) containing an oxygen-sensitive fluorescent ruthenium dye adsorbed in an inert silicon base within each microwell with no other unknown or reactive additives. Respiratory (oxygen-consuming) activities measured with BDOBS have previously been shown to occur within short (6 – 8 h) incubation periods using low carbon substrate concentrations that minimize potential measurement artifacts. With one exception, we found that respiration of native soil C in the BDOBS assay responded to the field treatments in a similar manner to CO2 flux measured in the field. Patterns of respiration with low concentrations of added amino acid or carbohydrate substrate were consistent with field treatments. Respiratory response to added N in the assay was consistent with field N treatments and was related to soil extractable N concentrations. These data significantly extend and support the capability of the BDOBS respiration assay to evaluate in situ soil activities and examine factors that limit these activities.

Technical Abstract: Documented approaches for measuring soil microbial activities and their controlling factors under field conditions are needed to advance understanding of soil microbial processes for numerous applications. We manipulated field plots with carbon (C) and nitrogen (N) additions to test the capability of a respiratory assay to: (1) measure respiration of endogenous soil C in comparison to field-measured CO2 fluxes; (2) determine substrate-induced respiratory (SIR) activities that are consistent with substrate availability in the field; and, (3) report N availability in the field based on assay responses with and without added N. The respiratory assay utilizes a microplate containing an oxygen-sensitive fluorescent ruthenium dye. Respiratory activities measured with this approach have previously been shown to occur within short (6 – 8h) incubation periods using low substrate concentrations that minimize enrichment during the assay. Field treatments were conducted in a randomized full-factorial design with C substrate (casamino acids, glucose, or none) and inorganic N (±) as the treatment factors. With one exception, we found that respiration of endogenous soil C in the assay responded to the field treatments in a similar manner to CO2 fluxes measured in the field. Patterns of SIR with low concentrations of added amino acid or carbohydrate substrate (200 µg C g-1 soil) were consistent with field treatments. The ratio (Nratio) of carbohydrate respiration with added N (25 µg N g-1 soil) to the same without N in the assay was significantly (p<0.05) decreased by field N amendment. The carbohydrate Nratio exhibited a logarithmic relationship (r=0.64, p<0.05) with extractable inorganic soil nitrate and ammonium concentrations. These data significantly extend and support the capability of this oxygen-based respiratory assay to evaluate in situ soil activities and examine factors that limit these activities.

Last Modified: 9/21/2014