Temporal Changes in Soil and Biomass Nitrogen for Irrigated Wheat Grown under Free‐Air Carbon Dioxide Enrichment (FACE)

Increasing atmospheric CO 2 concentrations are expected to increase plant production and demand for N and other nutrients. The objectives of this investigation were to characterize and quantify the temporal trends in soil mineral N and aboveground biomass N during the growing season of wheat ( Triticum aestivum L.) with adequate N, ambient and elevated CO 2 , and two levels of water stress. The free‐air CO 2 enrichment (FACE) technique was used to enrich the air from 370 to 550 μmol mol −1 CO 2 . Spring wheat was planted in late December of 1992 and 1993 and harvested at the end of May. Each main plot (CO 2 level) was split into two irrigation treatments to replace 100 and 50% of the potential evapotranspiration. Soil and plant samples were taken for N analysis six times each year. Elevated CO 2 lowered soil mineral N concentrations in the top 0.3 m of soil as much as 40% and increased aboveground biomass N by as much as 16% compared with the ambient treatment. Before anthesis, irrigation level had little effect on either soil mineral N or aboveground biomass N, but at harvest in 1992–1993 and at dough stage in 1993–1994 deficit‐irrigated plots had higher soil mineral N ( p < 0.05) and lower aboveground biomass N than plots that received adequate irrigation. There was little variation in the concentrations of N in the aboveground biomass among treatments within a sampling date. The data suggest elevated CO 2 may lead to rapid N uptake, which could result in increased early vegetative growth.