Quantifying remobilization of pre-existing nitrogen from cuttings to new growth of woody plants using 15N 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 (delta N-15) from the delta(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 (delta N-15 = 58.5 parts per thousand) or ammonium (delta N-15 = -0.96 parts per thousand). First, the fraction of carry-over nitrogen remaining in the cutting was estimated by isotope mass balance. Then measured delta N-15 values were adjusted for the fraction of pre-existing nitrogen remobilized to the plant. Results: Mean plant delta N-15 prior to correction was 49 parts per thousand and -5.8 parts per thousand under nitrate and ammonium, respectively. Plant delta N-15 was non-linearly correlated to biomass (r(2) = 0.331 and 0.249 for nitrate and ammonium, respectively; P < 0.05) where the delta N-15 of plants with low biomass approached the delta N-15 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 delta N-15 of newly acquired nitrogen averaged 52.7 parts per thousand and -6.4 parts per thousand for nitrate and ammonium-grown plants, respectively; both lower than their source values, as expected. Since there was a greater difference in delta N-15 between the carried-over pre-existing and newly assimilated nitrogen where nitrate was the source, the difference between measured delta N-15 and adjusted delta N-15 was also greater. There was no significant relationship between biomass and plant delta N-15 with either ammonium or nitrate after adjusting for carry-over nitrogen. Conclusion: Here, we provide evidence of remobilized pre-existing nitrogen influencing delta N-15 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.