Seasonal changes in the effects of free-air CO2 enrichment (FACE) ion nitrogen (N) uptake and utilization of rice at three levels of N fertilization

Overtime, the relative effect of elevated [CO2] on the photosynthesis and dry matter (DM) production of rice crops is likely to be changed with increasing duration of CO2 exposure, but the resultant [CO2] effects on rice N concentration, uptake, efficiency and allocation remain unclear, especially under different soil N availability. Therefore, we conducted a free-air CO2 enrichment (FACE) experiment at Wuxi, Jiangsu, China, in 2001-2003. A japonica cultivar with large panicle was grown at ambient or elevated (ca. 200 mu mol mol(-1) above ambient) [CO2] under three levels of N:low (LN, 15 g N m(2)), medium (MN, 25 g N m(2)) and high N (HN 35 g N m(2) (2002, 2003)). The MN level was similar to that recommended to local farmers. Averaged across all N levels and years, shoot N concentration (dry base) was lower under FACE by 1.8%, 6.1%, 12.2%, 14.3%, 12.1 %, and 6.9% at early-tillering, mid-tillering, panicle initiation (PI), booting, heading and grain maturity, respectively. Shoot N uptake under FACE was enhanced by 46%, 38%, 6% and 16% on average during the growth periods from transplanting to early-tillering (period 1), early-tillering to mid-fillering (period 2), mid-tillering to PI (period 3) and heading to grain Maturity (period 5), respectively, but slightly decreased by 2% in the period from PI to heading (period 4). Seasonal changes in crop response to FACE in ratio of shoot N uptake during a given growth period to that over the whole season followed a similar pattern to that of shoot N uptake, with average responses of 33%, 26%, -3%, -11% and 10% in periods 1-5 of the growth period, respectively. As a result, FACE increased final aboveground N uptake by 9% at maturity. FACE greatly reduced the ratio of leaf to shoot N content over the season, while allocation of N to steins and spikes showed an opposite trend. FACE treatment resulted in the significant increase in N use efficiency for biomass (NUEp) over the season except at early-tillering and in N use efficiency for grain yield (NUEg) at grain maturity. These results indicate that, in order to maximize grain output in a future hi.-h [CO,] environment, the recommended rates, proportion and timing across the season of N application should be altered, in order to take Full advantage of strong N uptake capacity during the early growth period and facilitate N Uptake after that. (c) 2006 Elsevier B.V. All rights reserved.