Ammonium and nitrate acquisition by plants in response to elevated CO2 concentration: the roles of root physiology and architecture

We examined changes in root system architecture and physiology and whole-plant patterns of nitrate reductase (NR) activity in response to atmospheric CO2 enrichment and N source to determine how changes in the form of N supplied to plants interact with rising CO2 concentration (CO2]). Seedlings of Betula alleghaniensis Britt. and Pinus strobus L., which differ in growth rate, root architecture, and the partitioning of NR activity between leaves (Betula) and roots (Pinus), were grown in ambient (400 microl l(-1)) and elevated (800 microl l(-1)) CO2] and supplied with either nitrate (NO3-) or ammonium (NH4+) as their sole N source. After 15 weeks of growth, plants were harvested and root system architecture, N uptake kinetics, and NR activity measured. Betula alleghaniensis responded to elevated CO2] with significant increases in growth, regardless of the source of N. Pinus strobus showed no significant response in biomass production or allocation to elevated CO2]. Both species exhibited significantly greater growth with NH4+ than with NO3-, along with lower root:shoot biomass ratios. Betula showed significant increases in total root length in response to elevated CO2]. However, root N uptake rates in Betula (for both NO3- and NH4+) were either reduced or unchanged by elevated CO2]. Pinus showed the opposite response to elevated CO2], with no change in root architecture, but an increase in maximal uptake rates in response to elevated CO2]. Nitrate reductase activity (on a mass basis) was reduced in leaves of Betula in elevated CO2], but did not change in other tissues. Nitrate reductase activity was unaffected by elevated CO2] in Pinus. Scaling this response to the whole-plant, NR activity was reduced in elevated CO2] in Betula but not in Pinus. However, because Betula plants were larger in elevated CO2], total whole-plant NR activity was unaffected.