Microbial exudates: Incorporating an overlooked precursor to stable organic matter into the concept of carbon use efficiency
Abstract
Efficient microbial growth has been a central focus of determining how microbial activity in soil helps to build stable organic matter. Processes like microbial exudation, however, have been regarded as relatively unimportant C fluxes even though such materials could form the same types of stable interactions with organic and mineral surfaces as biomass. We explored the magnitude and timing of microbial exudation by tracing 13C-glucose movement in a forest soil among the pools of respired CO2, dissolved organic carbon, microbial biomass, and exudates (defined operationally as unextractable soil 13C). CUE (= 13MBC/(13MBC + 13CO2)) was 0.7 within 6 h of amendment and minimal tracer remained in DOC indicating rapid microbial uptake of substrate. However, focusing on 13MBC and 13CO2 alone ignored the largest pool of tracer; 75% was unextractable from soil and presumably in the form of stabilized microbial exudates since glucose was fully extractable in the absence of microbial processing. Factoring the mass of exudates into calculation of CUE (= 13MBC/(13MBC + 13CO2 + exudates)) reduced estimates to 0.2 and indicate low growth efficiency despite relatively high capacity to store glucose as either MBC or stabilized exudates ( 90% of added glucose). We compared our results to other published tracer experiments (N = 11) and confirm that a wide variety of soils incubated with glucose between 0.6 to 97 days also exhibit relatively low CUE of growth after accounting for exudate production (average = 0.19 +/- 0.11 s.d.) but relatively high capacity to stabilize C as MBC and exudates (average = 61% of added glucose +/- 21% s.d.). Microbial exudation therefore appears to be an overlooked avenue for generating the types of precursor compounds that retain C in soil. Future work exploring the variety of metabolites that compose exudates and the conditions affecting their production will be important. To do this, we suggest utilization of total stabilized substrate (either intra- or extracellular) per unit of microbial substrate uptake as a meaningful estimate of microbial contributions to SOM formation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.B34B..02G
- Keywords:
-
- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCESDE: 0439 Ecosystems;
- structure and dynamics;
- BIOGEOSCIENCESDE: 0463 Microbe/mineral interactions;
- BIOGEOSCIENCESDE: 0470 Nutrients and nutrient cycling;
- BIOGEOSCIENCES