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

Increasing atmospheric CO2concentrations 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 aestivumL.) with adequate N, ambient and elevated CO2, and two levels of water stress. The free‐air CO2enrichment (FACE) technique was used to enrich the air from 370 to 550 μmol mol−1CO2. Spring wheat was planted in late December of 1992 and 1993 and harvested at the end of May. Each main plot (CO2level) 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 CO2lowered 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 CO2may lead to rapid N uptake, which could result in increased early vegetative growth.