Drought and Elevated CO2 Impacts N-Cycling Microorganisms in a Soybean Agroecosystem.

Engil Isadora Pujol Pereira1*, Kate Scow2, Andrew A.B. Leakey3 and Johan Six1 1 Plant Sciences Dept., Univ. of California, Davis, One Shields Ave, 95616 Davis, CA.

2Land, Air, and Water Resources Dept., , Univ. of California, Davis, One Shields Ave, 95616 Davis, CA.3Univ. of Illinois, Urbana, IL.

IntroductionCarbon dioxide (CO2) is one of the most

important anthropogenic greenhouse gas and its atmospheric concentration is projected to increase from the current 384.78 ppm (September, 2009, Mauna Loa Observatory) about 550 ppm by 2050 (IPCC, 2007).

In addition, drought-affected areas have expanded and the severity of droughts is expected to increase, which will have adverse impacts on agriculture, water supply, energy production, and health (IPCC, 2007). Since the nitrogen (N) cycle is driven by biological processes, it is very likely to be affected by the climate change.

ObjectivesTo expand the current understanding of the links

between microbial community functioning and the impacts of elevated CO2 and drought on N cycling processes, this study aims to investigate the changes in N transformers population and mineral N pool size .

Hypotheses-Bacterial activities are reduced under water stress conditions. Thus, it is expected drought to reduce the abundance of the N-transformers population.

-Since the stomatal conductance is reduced under elevated CO2 concentration (long et al., 2004), which decreases overall plant water demand, elevated CO2 is expected to mitigate the stresses caused by drought.

-This study took place in a soybean (Glycine max) agroecosystem at the University of Illinois Soybean Free Air Concentration Enrichment (SOYFACE) experiment.

-Atmospheric levels of CO2 were increased to 550 ppm, ~180 ppm above ambient conditions.

-This study was carried out during the 2008 growing season randomized complete block design with factorial treatments of elevated CO2 and drought (n = 4) (Fig.1).

-Moderate drought is achieved by an automated system which controls the extension and retraction of awnings, blocking about 20% of the rainfall (Fig. 2);

-Soil samples (0-15cm depth) were collected at full pod plant phenological stage and cooled to 4oC and transported overnight on ice to UC Davis where the following parameters were assessed:

Material and Methods

Fig.1: Layout of plots at the SoyFACE facility.

Fig.2: Automated system for the drought treatment.

Retractable awning

Fig.4: Nitrifiers and denitrifiers gene abundance

Results

Fig.3: Soil Moisture, total N, nitrate and ammonium content.

(a) Soil moisture (b) Total N

(c) Nitrate (d) Ammonium

(a) Nitrifiers (b) Denitrifiers

•Gravimetric soil moisture;•Total soil N;•Mineral N (Nitrate and Ammonium);•Nitrifiers and denitrifiers gene abundance (amoA and nosZ respectively).

Material and Methods (cont.)- The drought treatment decreased 25% of the gravimetric soil moisture (Fig.1a); -Total N and mineral nitrogen (NO3 and NH4) were not affected by the treatments (Fig. 1);-The amoA gene abundances was reduced by drought (p=0.02) (Fig.2);- Elevated CO2 reduced the stress caused by drought on the nitrifier population abundance.- Although not statistically significant (p=0.07), drought reduced the denitrifier bacterial population.

Conclusion

Results (cont.)

We would like to thank Charlotte Decock and Chau Minh Khoi for collecting the soil samples. We also would like to thank Haegeun Chung for her support and guidance during the soil processing and microbial analysis.

This study was supported by the National Science Foundation through Research Grant 0543218.

Acknowledgements

- Intergovernmental Panel on Climate Change (IPCC).2007. Climate Change 2007: The Physical Science Basis. Geneva, Switzerland. 18 pp.- Long S.P., Ainsworth E.A., Rogers A. & Ort D.R. (2004) Rising atmospheric carbon dioxide: plants FACE the future. Annual Review of Plant Biology 55, 591–628.

References

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-Although changes were observed under drought conditions, the total soil N was not affected. It is possible that no changes were observed due to the large size of the total soil N pool.

- Since bacteria abundance usually show fast responses to stresses, it is possible that changes on the mineral N content would be observed in a later period of the growing season. However, at this point, only the nitrifiers and denitrifiers abundances were responsive to such stress.

-Elevated CO2 tend to ameliorate the effect of drought on the abundance of N-transformers and soil moisture.

* Corresponding author: isapereira@ucdavis.edu