Interactions between elevated CO 2 and added N: effects on water use, biomass, and soil 15 N uptake in wheat

The interaction between atmospheric CO 2 (ambient=365 ppmv, elevated=500 ppmv) and fertilizer N (6, 12 and 18 g N m −2 ) was examined in spring wheat ( Triticum aestivum , L.) grown on a 15 N-labelled sandy loam soil. Wheat shoots were pulse-labelled once with 14 CO 2 to trace the photosynthetically fixed C within the soil/plant system. Fertilizer N increased root length and canopy water use efficiency (WUE). At each N level, root length and WUE were higher under elevated than under ambient CO 2 , but no CO 2 -N interaction was observed. The N concentration in roots, but not in shoots, was affected by a CO 2 -N interaction. Fertilizer N increased and elevated CO 2 decreased shoot N concentrations. The root-to-shoot (R/S) ratio decreased with N addition and was affected by interactions between N and CO 2 . Plant biomass C and N increased significantly with increased N addition. The effect of elevated CO 2 on shoots was relatively small regardless of N level, whereas roots experienced a clear CO 2 -N interaction. The 14 C recovered one week after labelling was mainly in the shoot biomass (80%). The activity increased with N addition, reflecting a larger shoot biomass, but the relative distribution of 14 C between shoots, roots, soil and rhizosphere respiration was little affected by CO 2 level and N addition. Soil 15 N contributed 7.1 to 9.0 g N m −2 to shoot N, the total increase (9.3 to 24.3 g N m −2 ) being dominated by contributions from fertilizer N. We conclude that effects of elevated CO 2 on wheat tissue N cannot be compensated for by N additions, and that plant uptake of native soil N is marginally affected by added N and elevated CO 2 .