Quantifying remobilization of pre-existing nitrogen from cuttings to new growth of woody plants using (15)N at natural abundance

BACKGROUND: For measurements of nitrogen isotope composition at natural abundance, carry-over of pre-existing nitrogen remobilized to new plant growth can cause deviation of measured isotope composition (δ(15)N) from the δ(15)Nof newly acquired nitrogen. To account for this problem, a two-step approach was proposed to quantify and correct for remobilized nitrogen from vegetative cuttings of Populus balsamifera L. grown with either nitrate (δ(15)N = 58.5‰) or ammonium (δ(15)N = −0.96‰). First, the fraction of carry-over nitrogen remaining in the cutting was estimated by isotope mass balance. Then measured δ(15)N values were adjusted for the fraction of pre-existing nitrogen remobilized to the plant. RESULTS: Mean plant δ(15)N prior to correction was 49‰ and −5.8‰ under nitrate and ammonium, respectively. Plant δ(15)N was non-linearly correlated to biomass (r(2) = 0.331 and 0.249 for nitrate and ammonium, respectively; P < 0.05) where the δ(15)N of plants with low biomass approached the δ(15)N of the pre-existing nitrogen. Approximately 50% of cutting nitrogen was not remobilized, irrespective of size. The proportion of carry-over nitrogen in new growth was not different between sources but ranged from less than 1% to 21% and was dependent on plant biomass and, to a lesser degree, the size of the cutting. The δ(15)N of newly acquired nitrogen averaged 52.7‰ and −6.4‰ for nitrate and ammonium-grown plants, respectively; both lower than their source values, as expected. Since there was a greater difference in δ(15)N between the carried-over pre-existing and newly assimilated nitrogen where nitrate was the source, the difference between measured δ(15)N and adjusted δ(15)N was also greater. There was no significant relationship between biomass and plant δ(15)N with either ammonium or nitrate after adjusting for carry-over nitrogen. CONCLUSION: Here, we provide evidence of remobilized pre-existing nitrogen influencing δ(15)N of new growth of P. balsamifera L. A simple, though approximate, correction is proposed that can account for the remobilized fraction in the plant. With careful sampling to quantify pre-existing nitrogen, this method can more accurately determine changes in nitrogen isotope discrimination in plants.