Abstract:

Objective: To provide theoretical guidance for construction of Ulmus pumila shelterbelt, the adaptive mechanisms of C, N, P allocation of U. pumila seedlings in response to nitrogen deposition and increasing precipitation were studied. Method: A full factorial experimental design of two factors (nitrogen and water), was applied, i.e., four N fertilization levels (0, 5, 10 and 15 g N·m^-2a^-1) and three water treatments (natural precipitation, natural precipitation increased by 50% and 100%) with randomized block design. The C, N, and P contents in leaf, branch, stem, coarse root and fine root of U. pumila seedlings under the different water and nitrogen treatments were measured and their ratios were calculated, and the stability of C, N, and P and their relationships with allometric growth were explored. Result: There were significant interactions between nitrogen addition and increasing precipitation on N content, P content and their ratios in different organs of U. pumila seedlings. With the increase of N addition, C content increased in leaf and fine root, while it was unchanged in branch, stem and coarse root. Meanwhile, N content and N:P ratio increased in all organs, whereas C:N ratio decreased. P content declined in leaf and stem while C:P ratio elevated in leaf, stem and fine root. With the increasing of precipitation, C content reduced in leaf and fine root, while it remained unchanged in branch, stem and coarse root, and P content decreased and C:P and N:P ratios increased in branch and stem. The effects of precipitation increase on the element allocation were various under different nitrogen deposition levels. With the increasing of precipitation, under no N addition condition, N content and N:P ratio in leaf increased whereas C:N ratio decreased, and P content firstly increased and then decreased in leaf, coarse root and fine root whereas C:P firstly reduced and then elevated. Meanwhile, N:P ratio firstly increased and then declined in fine root. However, under 15 g N·m^-2a^-1 nitrogen addition condition, N content decreased, C:N ratio increased and N:P ratio remained unchanged in leaf, while P content declined and C:P ratio enhanced in leaf, coarse root and fine root, N:P ratio gradually elevated in fine root with the increasing of precipitation. C content of different organs followed the following order:stem > coarse root > leaf and fine root, while N and P content followed the order:leaf > fine root > branch, stem, and coarse root. Meanwhile, the coefficient of variation for C content followed the order:branch, stem, and coarse root < leaf and fine root. The variation coefficient of N content was the largest in leaf and coarse root, and the smallest in fine root. The variation coefficient of P content was the largest in stem and the smallest in leaf. The variation coefficient for C:N ratio followed the order:coarse root > leaf, branch, and stem > fine root, whereas the variation coefficients for C:P and N:P ratios followed the order:stem > coarse root > fine root > leaf and branch. The variation coefficient for N:P ratio was greater than those for N and P content. There were significant allometric growth relationships with C and N content in leaf, branch, coarse root and fine root of U. pumila seedlings. C content was negatively related to P content in leaf and coarse root. Furthermore, N content was negatively related to P content in each organ, and the index of allometric growth ranged from -0.534 to -1.224. Conclusion: With the increasing nitrogen deposition and precipitation, N limitation for the growth of U. pumila seedlings changed into N and P joint limitation. Meanwhile, N use efficiency decreased, but P use efficiency enhanced. The stability of N content was the strongest in fine root, while P content and N:P ratio stability were the strongest in leaf. The stability of N and P content were stronger than that of N:P ratio in each organ.

Key words: water and nitrogen coupling, allometric growth relationships, stoichiometry, nutrient limitation, C:N:P