Current uptake of 15N-labeled ammonium and nitrate in flooded and non-flooded black spruce and tamarack seedlings

We investigated the effects of flooding for three weeks on physiological responses and uptake of NH4+ and NO3- by black spruce (Picea mariana (Mill.) BSP.) and tamarack (Larix laricina (Du Roi) K. Koch) seedlings fertilized with labeled ((NH4)-N-15)(2)SO4 or (KNO3)-N-15 in a growth chamber experiment. Flooding reduced photosynthesis (A), transpiration (E), water use efficiency (WUE), and current uptake of NH4+ and NO3- in both species. Under flooding, there were no significant differences between the two species in uptake of either NH4+ or NO3- at the whole-plant level but black spruce had higher translocation of NH4+ to the shoots than did tamarack. Under non-flooded conditions, black spruce seedlings exhibited higher uptake of both NH4+ and NO3- than did tamarack and demonstrated preferential uptake of NH4+ (19.7 mg g(-1)dw) over NO3-(12.3 mg g(-1)dw after three weeks). In contrast, non- flooded tamarack seedlings had equal uptake of NH4+ (4.96 mg g(-1)dw) and NO3- (4.97 mg g(-1)dw). We hypothesize that the ability of tamarack to equally exploit both (NH4+)-N-15 and (NO3-)-N-15 would confer an advantage over black spruce, when faced with limitations in the availability of different forms of soil nitrogen.