Department of Forest Sciences, University of British Columbia, 2424 Main Mall, Vancouver, BC V6T1Z4, Canada.
Plant Methods. 2013 Jul 12;9(1):27. doi: 10.1186/1746-4811-9-27.
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 (δ15N) from the δ15Nof 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 (δ15N = 58.5‰) or ammonium (δ15N = -0.96‰). First, the fraction of carry-over nitrogen remaining in the cutting was estimated by isotope mass balance. Then measured δ15N values were adjusted for the fraction of pre-existing nitrogen remobilized to the plant.
Mean plant δ15N prior to correction was 49‰ and -5.8‰ under nitrate and ammonium, respectively. Plant δ15N was non-linearly correlated to biomass (r2 = 0.331 and 0.249 for nitrate and ammonium, respectively; P < 0.05) where the δ15N of plants with low biomass approached the δ15N 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 δ15N 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 δ15N between the carried-over pre-existing and newly assimilated nitrogen where nitrate was the source, the difference between measured δ15N and adjusted δ15N was also greater. There was no significant relationship between biomass and plant δ15N with either ammonium or nitrate after adjusting for carry-over nitrogen.
Here, we provide evidence of remobilized pre-existing nitrogen influencing δ15N 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.
对于自然丰度下氮同位素组成的测量，先前重新分配到新植物生长中的氮的残留会导致测量的同位素组成（δ15N）偏离新获得的氮的δ15N。为了解决这个问题，提出了一种两步法来量化和校正用硝酸盐（δ15N=58.5‰）或铵盐（δ15N=-0.96‰）生长的白柳无性繁殖体中重新分配的氮。首先，通过同位素质量平衡估计留在插条中的残留氮的分数。然后，对测量的δ15N 值进行调整，以校正重新分配到植物中的先前存在的氮的分数。
校正前植物的平均 δ15N 值分别为硝酸盐和铵盐下的 49‰和-5.8‰。植物的 δ15N 与生物量呈非线性相关（r2分别为硝酸盐和铵盐下的 0.331 和 0.249；P<0.05），其中生物量低的植物的 δ15N 接近先前存在的氮的 δ15N。无论大小如何，大约 50%的插条氮没有重新分配。新生长中残留氮的比例在不同来源之间没有差异，但范围在 1%至 21%之间，并且取决于植物生物量，在较小程度上取决于插条的大小。硝酸盐和铵盐生长的植物新获得的氮的平均 δ15N 值分别为 52.7‰和-6.4‰；都低于其源值，这是预期的。由于硝酸盐作为氮源时，残留的先前存在的氮和新同化的氮之间的 δ15N 差异更大，因此测量的 δ15N 和调整后的 δ15N 之间的差异也更大。调整后，用铵盐或硝酸盐生长的植物的生物量和 δ15N 与氮之间没有显著关系。
在这里，我们提供了白柳 L.无性繁殖体中重新分配的先前存在的氮影响新生长的 δ15N 的证据。提出了一种简单但近似的校正方法，可以解释植物中重新分配的部分。通过仔细采样来量化先前存在的氮，可以更准确地确定植物中氮同位素分馏的变化。
