Kandelia candel is one of major mangrove species for coastal conservation and restoration in China. Understanding the stoichiometry of nutrient elements of K. candel growing in complicated living environment can help characterize plant growth status and guide ecological restoration in different biogeochemical environments. In this study, the influences of two hydrologic factors (waterlogging time and salinity) on stoichiometry characteristics of K. candel in the different organs (leaf, stem and root, respectively) were studied using the automatic tidal-simulation device in sand culture of greenhouse. The contents of nutrient elements including N, P, K, Na, Ca, Cl, Mg, Al, Fe, Zn, Cu, B, Mn, Ni, S, Pb and Cr were determined in the waterlogging and salinity treatments, respectively.
The result of the waterlogging treatment showed waterlogging had an significant effect on the distribution characteristics of elements in different organs of K. candel. The contents of N, P, K, Ca, Mg and Zn in leaf were higher than those in stem and root. In contrast, the contents of Cl, Al and Fe in root were higher than those in leaf and stem. The stoichiometry characteristics of seedling of K. candel, however, showed different patterns under salinity and waterlogging treatments. The content of B, N, K, Ca, Cl, Mg, Na, Zn, Cu differed significantly in seedlings when hydrological stress varied.
The result of salinity treatment showed the contents of Na, Cl increased, K, Ca, Mg decreased in all organs when the salinity increased. However, the difference of salt content (Na, Cl, K, Ca, Mg) among three organs decreased when the salinity increased, which showed convergent effect in organs. The total content of salt (K, Ca, Cl, Mg, Na)displayed no obvious variation with the extending waterlogging time, except in leaf where the salt content (except K) increased. When the salinity increased, the content of Zn decreased, Mn increased in leaf and Fe increased in both leaf and root. With the extending waterlogging time, Fe increased in leaf, Cu decreased first, then increased in all three organs.
The element ratios were also determined in waterlogging and salinity treatments, respectively. The N∶P ratio was 3-11 in waterlogging treatment and the N∶P ratio was 6-15 in salinity treatment, which were lower than the results of previous research that demonstrated N limited the growth of seedlings and P had no limit growth of seedlings. Our result showed N limitation enhanced in seedlings when salinity stress enhanced. The N limitation was obvious in salinity treatment than that in waterlogging treatment.
Significant correlations between waterlogging time and the contents of nutrient elements in three organs (leaf, stem and root) were observed in K. candel, respectively. In leaf, the order of significant positive correlation between waterlogging time and the contents of nutrient elements was Fe> Na> Cl> Mg= B> P> N while the order of significant negative correlation was K< K∶Cl. In stem, the order of significant positive correlation between waterlogging time and the contents of nutrient elements was K> Fe. In root, the order of significant positive correlation between waterlogging time and the contents of nutrient elements was K∶Cl> K> N.
Significant correlations between salinity and the contents of nutrient elements in three organs (leaf, stem and root) were also observed in K. candel, respectively. In leaf, the order of significant positive correlation between salinity and the contents of nutrient elements was Ca=Na > Cl while the order of significant negative correlation was K∶Cl=Zn=Ca Na> Mn> Mg. In root, the order of significant positive correlation between salinity and the contents of nutrient elements was Cl> Na> P> Mn while the order of significant negative correlation was K∶Cl= K< Ca.