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Title: CO2-induced alterations in plant nitrate utilization and root exudation stimulate N2O emissions

Author
item WU, KEKE - Sun Yat-Sen University
item CHEN, DIMA - Institute Of Botany - China
item TU, CONG - North Carolina State University
item QUI, YUNPENG - North Carolina State University
item Burkey, Kent
item REBERG-HORTON, CHRIS - North Carolina State University
item PENG, SHAOLIN - Sun Yat-Sen University
item HU, SHUIJIN - North Carolina State University

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2016
Publication Date: 3/1/2017
Citation: Wu, K., Chen, D., Tu, C., Qui, Y., Burkey, K.O., Reberg-Horton, C., Peng, S., Hu, S. 2017. CO2-induced alterations in plant nitrate utilization and root exudation stimulate N2O emissions. Soil Biology and Biochemistry. 106:9-17.

Interpretive Summary: Carbon dioxide is a well known greenhouse gas, and rising carbon dioxide in the atmosphere is expected to enhance plant growth through higher rates of photosynthesis. However, much less is understood about the effects of elevated carbon dioxide on the complex carbon and nitrogen cycles operating in terrestrial ecosystems. A collaboration of North Carolina State University, USDA-ARS, and Chinese colleagues showed that elevated carbon dioxide enhances emission of a second powerful greenhouse gas, nitrous oxide, from the soil through the biological process of denitrification. The emission of nitrous oxide was dependent on the nitrogen source used for plant growth with greater production from fertilizers containing oxidized forms of nitrogen (e.g. nitrate) as compared with reduced forms of nitrogen (e.g. ammonium). A new conceptual model is proposed to explain the complex interactions between elevated carbon dioxide, nitrogen utilization by plants, and the soil microbial processes leading to nitrous oxide production. These findings suggest that fertilizer management in intensive agricultural systems will become more challenging in a future as farmers must optimize plant growth while minimizing nitrous oxide emissions from their fields in a climate where atmospheric carbon dioxide continues to rise.

Technical Abstract: Atmospheric carbon dioxide enrichment (eCO2) often increases soil nitrous oxide (N2O) emissions, which has been largely attributed to increased denitrification induced by CO2-enhancement of soil labile C and moisture. However, the origin of the nitrogen (N) remains unexplained. Emerging evidence suggests that eCO2 alters plant N preference in favor of ammonium (NH4-N) over nitrate (NO3-N). Yet, whether and how this attributes to the enhancement of N2O emissions has not been investigated. We conducted microcosm experiments with wheat (Triticum aestivum L.) and tall fescue. (Schedonorus arundinaceus (Schreb.) Dumort.) to examine the effects of elevated CO2 on soil N2O emissions in the presence of two N forms (NH4-N or NO3-N). Results obtained showed that N forms dominated elevated CO2 effects on plant and microbial N utilization, and thus soil N2O emissions. eCO2 significantly increased the rate and the sum of N2O emissions by three to four fold when NO3-N, but not NH4-N, was supplied under both wheat and tall fescue. While enhanced N2O emission was more related to reduced plant NO3-N uptake under wheat, it concurred with increased labile carbon under tall fescue. In the presence of NO3-N, significantly lower shoot biomass N and 15N, but higher plant biomass carbon:nitrogen ratio, microbial biomass carbon and N, and/or soil extractable carbon indicated that eCO2 constrained plant NO3-N utilization and likely stimulated root exudation. We propose a new conceptual model in which eCO2-inhibition of plant NO3-N uptake and/or CO2-enhancement of soil labile C enhances the N and/or carbon availability for denitrifiers and increases the intensity and/or the duration of N2O emissions. Together, these findings indicate that CO2-enhancement of soil N and labile carbon favors denitrification, suggesting that management of N fertilizers in intensive systems will likely become more challenging under future CO2 scenarios.