Effects of repetitive submergence on the accumulation and release of nutrient elements in Pinus elliottii seedlings

Pinus elliottii is an evergreen coniferous tree. It is considered a potential species for ecological restoration in the Three Gorges Reservoir Area (TGRA). To classify the effects of different degrees of flooding stress in winter on nutrient accumulation in Pinus elliottii after experiencing early drought stress in summer, simulated water treatments of deep submergence (DS) and moderate submergence (MS) were imposed after the summer drought period. The results indicated that the survival rate of seedlings was 95.3%, and the accumulation trend of the flooded plants was rapid at an average rate of 1.99 ± 0.33% in the early stage of flooding (stage I: 0-7 days), a rapid release rate in the second stage (stage II: 7-60 days), and an average rate of only 0.07 ± 0.04% in the later stage (stage III: 60-150 days). After 150 days of flooding, the leaves of Pinus elliottii released an average of 7.156 ± 0.4 g kg^-1 of organic carbon, 8.839 ± 0.6 g kg^-1 of nitrogen, 0.781 ± 0.1 g kg^-1 of phosphorus, and 2.985 ± 0.3 g kg^-1 of potassium of macroelement content, and an average of 0.201 ± 0.03 g kg^-1 manganese, 0.147 ± 0.04 g kg^-1 iron, 0.002 g kg^-1 copper, and 0.023 g kg^-1 of zinc of microelement contents. Our results also demonstrated that after 150 days of flooding, the C/N, N/P, and C/P ratios of the nutrient element content of Pinus elliottii in the water-level fluctuation zone of the TGRA were 0.810%, 11.32%, and 9.16%, respectively. The absorption and release of nutrients under water flooding are generally divided into three stages: first, the early storage stage (the first stage: 0 to 7 days, optional), then the rapid release (the second stage: 7 to 60 days), and the later stage slow release phase (third stage: 60 to 150 days). Water flooding reduced the contents of C, N, P, and K and affected the absorption of nutrient elements in the plant. At the same time, soluble Mn^{2 +} and Fe^{2 +} over absorbed during flooding could cause toxicity to leaf tissues. At the same time, Pinus elliottii selected to reduce Cu in leaves to ensure that the root has a strong redox capacity and improve nitrogen utilization, thereby preventing the long-term flooding of toxic cations and acid substances. Taken together, our results conclude that increased drought stress can reduce the ability of Pinus elliottii seedlings to withstand flooding stress; the seedlings of Pinus elliottii can maintain their growth by accumulating certain nutrient elements under submerged conditions, which implies that this species would be a suitable candidate for reforestation in the TGRA because of its tolerance to submergence.