AUTHOR=Jing Hang , Zhou Haoxiang , Wang Guoliang , Xue Sha , Liu Guobin , Duan Mengcheng 

TITLE=Nitrogen Addition Changes the Stoichiometry and Growth Rate of Different Organs in Pinus tabuliformis Seedlings

JOURNAL=Frontiers in Plant Science

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.01922

DOI=10.3389/fpls.2017.01922

ISSN=1664-462X

ABSTRACT=<p><bold>Background:</bold> Nitrogen (N) deposition could influence plant stoichiometry and growth rate and thus alter the structure and function of the ecosystem. However, the mechanism by which N deposition changes the stoichiometry and relative growth rate (RGR) of plant organs, especially roots with different diameters, is unclear.</p><p><bold>Methods:</bold> We created a gradient of N availability (0–22.4 g N m<sup>-2</sup> year<sup>-1</sup>) for <italic>Pinus tabuliformis</italic> seedlings for 3 years and examined changes in the carbon (C):N:phosphorus (P) ratios and RGRs of the leaves, stems, and roots with four diameter classes (finest roots, &lt;0.5 mm; finer roots, 0.5–1 mm; middle roots, 1–2 mm; and coarse roots, &gt;2 mm).</p><p><bold>Results:</bold> (1) N addition significantly increased the C and N contents of the leaves and whole roots, the C content of the stems, the N:P ratios of the leaves and stems, and the C:P ratio of the whole roots. (2) In the root system, the C:N ratio of the finest roots and the C:P ratios of the finest and finer roots significantly changed with N addition. The N:P ratios of the finest, finer, and middle roots significantly increased with increasing amount of N added. The stoichiometric responses of the roots were more sensitive to N addition than those of the other organs (3) The RGR of all the organs significantly increased at low N addition levels (2.8–11.2 g N m<sup>-2</sup> year<sup>-1</sup>) but decreased at high N addition levels (22.4 g N m<sup>-2</sup> year<sup>-1</sup>). (4) The RGRs of the whole seedlings and leaves were not significantly correlated with their N:P ratios at low and high N addition levels. By contrast, the RGRs of the stems and roots showed a significantly positive correlation with their own N:P ratio only at low N addition level.</p><p><bold>Conclusion:</bold> Addition of N affected plant growth by altering the contents of C and N; the ratios of C, N, and P; and the RGRs of the organs. RGR is correlated with the N:P ratios of the stems and roots at low N addition level but not at high N addition level. This finding is inconsistent with the growth rate hypothesis.</p>