Revisiting Why Plants Become N Deficient Under Elevated CO2: Importance to Meet N Demand Regardless of the Fed-Form

An increase in plant biomass under elevated CO2 (eCO(2)) is usually lower than expected. N-deficiency induced by eCO(2) is often considered to be a reason for this. Several hypotheses explain the induced N-deficiency: (1) eCO(2) inhibits nitrate assimilation, (2) eCO(2) lowers nitrate acquisition due to reduced transpiration, or (3) eCO(2) reduces plant N concentration with increased biomass. We tested them using C-3 (wheat, rice, and potato) and C-4 plants (guinea grass, and Amaranthus) grown in chambers at 400 (ambient CO2, aCO(2)) or 800 (eCO(2)) mu L L-1 CO2. In most species, we could not confirm hypothesis (1) with the measurements of plant nitrate accumulation in each organ. The exception was rice showing a slight inhibition of nitrate assimilation at eCO(2), but the biomass was similar between the nitrate and urea-fed plants. Contrary to hypothesis (2), eCO(2) did not decrease plant nitrate acquisition despite reduced transpiration because of enhanced nitrate acquisition per unit transpiration in all species. Comparing to aCO(2), eCO(2) remarkably enhanced water-use efficiency, especially in C-3 plants, decreasing water demand for CO2 acquisition. As our results supported hypothesis (3) without any exception, we then examined if lowered N concentration at eCO(2) indeed limits the growth using C-3 wheat and C-4 guinea grass under various levels of nitrate-N supply. While eCO(2) significantly increased relative growth rate (RGR) in wheat but not in guinea grass, each species increased RGR with higher N supply and then reached a maximum as no longer N was limited. To achieve the maximum RGR, wheat required a 1.3-fold N supply at eCO(2) than aCO(2) with 2.2-fold biomass. However, the N requirement by guinea grass was less affected by the eCO(2) treatment. The results reveal that accelerated RGR by eCO(2) could create a demand for more N, especially in the leaf sheath rather than the leaf blade in wheat, causing N-limitation unless the additional N was supplied. We concluded that eCO(2) amplifies N-limitation due to accelerated growth rate rather than inhibited nitrate assimilation or acquisition. Our results suggest that plant growth under higher CO2 will become more dependent on N but less dependent on water to acquire both CO2 and N.