Progress 03/15/03 to 03/14/05
Outputs
The primary objective of NRI support to M.A. Williams was to assess whether microbial compatible solute production could be a contributor to the flush of C that occurs in tallgrass prairie soil during the wetting of dry soil. In addition to a comparison between a drying-wet up and continuously moist treatment, we studied the dynamics of this treatment overlaid on a 12-year field irrigation treatment. Hence, we wanted know if any changes in the susceptibility of the microbial community had occurred during this 12-year period where the ecosystem was virtually unconstrained with respect to water. We assessed whether the degree of drying influenced the post wet-up concentration of compatible solutes and carbon-dioxide production. Furthermore, we assessed whether specific microbial community members responded to the flush through growth on a 13C-labeled glucose tracer, or if specific microogorganisms were killed during dilution stress. Growth directly after dilution stress
was measured as incorporation of 13C into phospholipid fatty acid biomarkers. Cell death was assessed using diglycerides, putative biomarkers for cell death via the incorporation of 13C into diglyceride fatty acids formed from the enzymatic cleavage of the phosphodiester bond of PLFA after cell death. As hypothesized, carbon-dioxide production during the 48-h period following the wetting of dry soil was positively correlated (r= 0.91) to the degree of water potential reduction at 33, 500, 1500, 3000, and 6000 kPa. CO2 production in the long-term irrigated soil was enhanced on average by 18% compared to the control soil at the two driest water potentials when a drying and wetting treatment was induced. Furthermore, the production of total soluble organic C was significantly greater at 6000 then 3000 kPa, and 3000 kPa compared to the lower water potentials (1500, 500, and 33 kPa). Proline and several carbohydrate compatible solutes were the dominant contributors to the extractable C
pool directly after induction of dilution stress at the two lowest water potentials. Trehalose, a compatible solute produced during physiological stress, and many other compatible solutes were noted, and data is currently be analyzed. The ratio of tracer 12/13C in the fungi to bacterial biomass was significantly greater in the irrigated compared to the control soil, consistent with our hypotheses based on greater fungal:bacterial biomass (Williams and Rice, 2004). The post wet-up utilization of C by the active microbial community was different from that in the continuously moist soil, with the largest reductions in C utilization occurring in the gram-negative members during dilution stress. Overall, the data from this project has provided some conclusive evidence for the sources of the C flush following wetting of dry soil, and thus has implications on C cycling and nutrient dynamics, and plant N availability in agroecosystems.
Impacts
We will be able to answer questions that soil microbiologists have posed for decades regarding the importance of wetting and drying on soil microbial communities. Besides a better understanding of the potential mechanisms of nutrient flux and cycling, we will be able to answer questions about the importance of water availability on microbial community composition. The techniques we are using have not been used often, and thus will provide a novel view of wetting and drying processes, a phenomenon important to almost all terrestrial soils. Lastly, we expect that the results will provide even more data to that will point us in the direction to ask more questions regarding microbial community and nutrient dynamics, and micorbial community composition.
Publications
- No publications reported this period
|
Progress 03/14/03 to 10/31/03
Outputs
We are utilizing state-of-the-art techniques to probe questions about soil microbial community response to wetting and drying in soil. Samples from Konza Prairie Biological Station LTER were collected in September 2003, and experiments are expected to be under way during November and December. After the experimental manipuluations on soil have been completed. The lipids from these sample will be stored in UHP N2 in the freezer until analysis of PLFA and diglyceride fractions by GC. Thus far we have tested a methodology proposed to characterize which members of the microbial community are active and killed during wetting of dry soil. While interesting preliminary results show that gram-positive microorganisms are more active after osmotic shock than gram-negative type organisms, we are having trouble directly characterizing which members are moribund after osmotic upshock. The proposed methodology to assess moribund organisms entailed the extraction of diglyceride
fatty acids. We are now in the process of measuring the whole diglyceride molecule by liquid chromatography-mass spectrometry to further assess which extraction and separation protocol is most appropriate for diglycerides. At present, results suggest that extremely low concentrations of diglycerides exist in soil, and no change in diglyceride concentration occur with cell death. This result suggests preliminarily, that diglyceride biomarkers of microbial functional groups may not be good indicators of cell death. Another methodology for assessing which members of the microbial community are moribund will be submitted with the next round of USDA-NRI Soils and Soil Biology proposals in December or January 2004. We have also succeeded in extracting DNA from soils that have remained consistently moist (but not saturated) during the growing season for 15 years. We expect changes in the microbial community to occur as a result irrigation if wetting and drying is selective factor controlling
the composition of microbial community. Nuclear magnetic resonance (NMR) experiments to assess microbial compatible solute formation will be conducted in collaboration with scientists at the Carbohydrate Research Center and a Soil Chemist who specializes in NMR. This work will likely be conducted and finished in February or March 2003.
Impacts
We will be able to answer questions that soil microbiologists have posed for decades regarding the importance of wetting and drying on soil microbial communities. Besides a better understanding of the potential mechanisms of nutrient flux and cycling, we will be able to answer questions about the importance of water availability on microbial community composition. The techniques we are using have not been used often, and thus will provide a novel view of wetting and drying processes, a phenomenon important to almost all terrestrial soils. Lastly, we expect that the results will provide even more data to that will point us in the direction to ask more questions regarding microbial community and nutrient dynamics, and micorbial community composition.
Publications
- No publications reported this period
|