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Title: CO2 ENRICHMENT AND SOIL NITROGEN EFFECTS ON WHEAT EVAPOTRANSPIRATION AND WATER USE EFFICIENCY

Author
item Hunsaker, Douglas - Doug
item Kimball, Bruce
item Pinter Jr, Paul
item Wall, Gerard - Gary
item LAMORTE, ROBERT - CONSULTANT
item Adamsen, Floyd
item LEAVITT, STEVE - UNIV OF AZ, TUCSON
item THOMPSON, TOM - UNIV OF AZ, TUCSON
item BROOKS, TALBOT - MAC, UNIV OF AZ, TUCSON

Submitted to: Agriculture Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2000
Publication Date: 6/15/2000
Citation: Hunsaker, D.J., Kimball, B.A., Pinter Jr, P.J., Wall, G.W., Lamorte, R.L., Adamsen, F.J., Leavitt, S.W., Thompson, T.L., Brooks, T.J. 2000. Co2 enrichment and soil nitrogen effects on wheat evapotranspiration and water use efficiency. Agriculture Forest Meteorology. (104)2:85-100.

Interpretive Summary: The atmospheric carbon dioxide (CO2) level is expected to rise and possibly double within the next century. A body of research indicates that this will increase the growth and yield of many important agricultural crops. However, it is not clearly known what impact higher CO2 will have on crop water consumption (evapotranspiration) and, thus, on water supply demands. This paper presents results on crop water use determined for wheat grown i a natural agricultural field environment with both CO2 enrichment (enriched 50% above present-day levels) and non-enrichment. Studies spanning two seasons indicated that the seasonal water consumption for the CO2-enriched wheat decreased about 4% under well-watered and well-fertilized conditions. Under well-watered and severely limited fertility conditions, the consumptive water use for the CO2-enriched wheat decreased about 1%. However, these small changes in crop water use, when coupled with the significant yield enhancement for the CO2-enriched wheat, resulted in much higher overall water use efficiency. This implies that high wheat yields, by present-day measure, could be produced with slightly less water if atmospheric CO2 rises by 50%. This work will help wheat growers who use irrigation develop water management strategies for the future, give insight to planners of future water supplies on expected crop water consumption, and provide information to present-day crop modelers for improving our ability to predict the impact of increased CO2 on agricultural production.

Technical Abstract: Evapotranspiration (ET) and water use efficiency (WUE) were evaluated for two spring wheat crops, grown in a well-watered, subsurface drip-irrigated field under ambient (about 370 micro/mol mol-1 during daytime) and enriched (200 micro/mol mol-1 above ambient) CO2 concentrations during the 1995-96 and 1996-97, Free-Air CO2 Enrichment (FACE) experiments in central Arizona. .The enriched (FACE) and ambient (Control) CO2 treatments were replicated i four, circular plots, each 25 m in diameter. Two soil nitrogen (N) treatments, ample (High N) and limited (Low N), were imposed on one-half of each circular plot. Wheat ET, determined using soil water balance procedures, was significantly greater in High N than Low N treatments starting in late March (anthesis) during both years. Differences in ET between CO2 treatments during the seasons were generally small and not statistically significant, however, there was a tendency for the ET to be lower for FACE than Control under the High N treatment. The reduction in the cumulative seasonal ET due to FACE averaged 3.7 and 4.0% under High N and 0.7% and 1.2% under Low N in the first and second years, respectively. However, WUE (grain yield per unit seasonal ET) was significantly increased for the FACE treatment under both soil N treatments. For the High N treatment, the WUE was 19% and 23% greater for FACE than Control and for the Low N treatment the WUE was 12% and 7% greater for FACE than Control in the two years, respectively.