Abstract:

The plasticity response of fine root morphology and stoichiometric characteristics to phosphorus (P) addition in Casuarina equisetifolia forests at different development stages was investigated, providing a scientific basis for targeted fertilization and nurturing of C. equisetifolia at different development stages under the limitation of low phosphorus in coastal sandy soil. The C. equisetifolia plantations at young (5 a), middle-aged (12 a) and mature (25 a) stages were selected as the research objects in Pingtan Island, Fujian Province, and the phosphorus addition experiment was carried out by using the inner growth method. After the fine roots growing in the inner growth cores for a year, the fine root morphological characters and stoichiometric characteristics were measured by the functional division method. The results showed that: 1) adding phosphorus significantqly increased the root length and surface area, the total root length and total surface area increased by 7.79 times and 8.69 times, respectively, the specific root length and specific surface area increased by 15.1% and 19.5%, respectively; 2) among different root functional groups, the plastic response of fine root morphology to P addition was significantly different. The morphological plasticity was dominated by absorbing roots, and the response of absorbing roots to P nutrient resources was more sensitive. P addition increased the P content in absorbing roots by 56.0%, decreased the C/P and N/P by 18.7% and 38.7%, and decreased the tissue density by 31.4%; 3) for the change of soil P nutrient resources, different forest ages of C. equisetifolia adopted different plastic response strategies. After P addition, the young forest mainly obtained P element by changing its own physiological characteristics through the variation of stoichiometric characteristics of absorbing roots. The fine root morphology and stoichiometric characteristics of the middle-aged forest had no significant variation. The mature forest mainly obtained P element by changing the absorbing root morphology and expanding the root absorption area. The present study demonstrates that the addition of exogenous P can coordinate the internal relationship between the fine root morphology and stoichiometric characteristics of C. equisetifolia. Moreover, our results show that the fine roots of C. equisetifolia at different ages selectively invest in acquiring P resources. Thus, to effectively fertilize and nurture C. equisetifolia, targeted tending schemes should be adopted according to the forest's age.

Key words: phosphorus addition, forest age, absorptive fine root, transport fine root, fine root morphology, fine root stoichiometry, Casuarina equisetifolia