| After completion of the Three Gorges Dam,the water level operation mode of the“Winter Storage Summer Drainage”caused the water level in the Three Gorges Reservoir Area to fluctuate periodically between 145 m and 175 m above sea level,forming a hydro-fluctuation belt with a vertical drop of 30 m and an area of about 350 km~2.The soil moisture condition of the hydro-fluctuation belt showed a gradient from drought to complete flooding with an annual cycle.The original vegetation in the reservoir area has gradually disappeared due to the inability to endure the drastic changes in the environment,which in turn caused the reservoir area to many severe ecological problems.In order to repair the ecological environment of the Three Gorges hydro-fluctuation belt,artificial vegetation construction is the key to maintain the ecosystem functions and services of the Three Gorges hydro-fluctuation belt.Both Taxodium distichum and Taxodium ascendens are deciduous trees of the genus Taxodium,which are resistant to waterlogging and soil infertility,with fast growth and strong adaptability.They are suitable woody species for vegetation reconstruction in the Three Gorges hydro-fluctuation belt.During the drawdown period of the Three Gorges hydro-fluctuation belt,the T.distichum and T.ascendens can take up nitrogen and phosphorus to protect the water body,and the plant tissues that fall on the surface due to drought and heat in summer can increase soil fertility.But during the flooding period,the T.distichum and T.ascendens are subject to long-term flooding stress,and the decomposition of the underwater plant tissues will release a large amount of nutrients to the water body,and the possibility of eutrophication of the water body will thus increase.In order to explore leaf decomposition and nutrient dynamics of the artificially reconstructed vegetation of T.distichum and T.ascendens leaves in the Three Gorges hydro-fluctuation belt under different water conditions,the present study selected the leaves of T.distichum and T.ascendens planted at the 165-175 m above sea level together with the purple soil of the forest as the research material.Under laboratory conditions,the group with T.distichum leaf addition and with T.ascendens leaf addition was set up.To simulate the characteristics of soil water content in the hydro-fluctuation belt of the Three Gorges Reservoir,five different kinds of water treatments were applied in the group with T.distichum leaf addition and with T.ascendens leaf addition.Five kinds of water treatments included normal growth water condition(CK),light drought water stress(T1),water saturation(T2),2 cm flooding(T3),and10 cm flooding(T4),which were subject to a 180-day decomposition treatment to test the characteristics of leaf decomposition and nutrient dynamics,and the effect of leaf decomposition on soil and water’s physico-chemical properties.The results showed that:(1)Different water treatments can significantly affect the leaf decomposition of the T.distichum and T.ascendens.The foliar mass loss rate of T.distichum and T.ascendens increased with an increase of soil moisture conditions,and the flooding conditions significantly increased the foliar mass loss of the T.distichum and T.ascendens.Under all water conditions,the foliar mass loss of T.distichum and T.ascendens were most obvious on the 10~thh day of decomposition,accounted for 35%(CK),38%(T1),40%(T2),52%(T3)and 52%(T4)of the foliar mass loss of T.distichum,accounted for 39%(CK),36%(T1),43%(T2),53%(T3)and 62%(T4)of the foliar mass loss of T.ascendens.(2)The nitrogen content in the leaves was enriched during the experiment,and the flooding treatment can promote the enrichment of nitrogen in the leaves.The phosphorus content in the leaves was first enriched and then released during the experiment,and the flooding treatment can promote the above mentioned enrichment and release.In the non-flooding treatment,the potassium content of the leaves enriched,while in the flooding treatment the potassium in the leaves released.(3)In the non-flooding environment,the addition of T.distichum and T.ascendens leaves can significantly increase the soil oxidation-reduction potential,while in the flooding environment,the addition of T.distichum and T.ascendens leaves significantly reduced the oxidation-reduction potential of the soil.The addition of T.distichum and T.ascendens leaves in all water treatments reduced the soil pH value,and compared with the non-flooding treatments,the flooding treatment can increase the soil pH value.The addition of leaves can increase the contents of total carbon,total nitrogen,total phosphorus,total potassium,ammonium nitrogen,available phosphorus and available potassium in the soil,but the addition of leaves under flooding reduced the soil nitrate nitrogen content.The effects of foliar decomposition of T.distichum and T.ascendens on soil physico-chemical properties showed that leaf addition of T.distichum and T.ascendens can improve the soil environment.(4)The addition of T.distichum and T.ascendens leaves can significantly reduce the oxidation-reduction potential of water.At the initial stage of foliar decomposition of T.distichum and T.ascendens,a large amount of carbon,nitrogen and phosphorus were released into the water body.In the group with leaf addition,the content of carbon,nitrogen and phosphorus in the water body increased sharply in the initial stage of leaf decomposition,and then gradually decreased,but it was always higher than that in the the group without leaf addition,indicating that the T.distichum and T.ascendens leaves decomposition in the water can increase the pressure of the water environment,which may cause the potential risk of eutrophication of the water in the reservoir area.Compared with the leaves of T.ascendens,the foliar decomposition of T.distichum in flooding environment release more phosphorus,which were 1.09 times(T3)and 1.48 times(T4)respectively.In summary,except that in the flooding treatment T.distichum and T.ascendens leaves addition reduced the soil nitrate nitrogen content,the foliar decomposition of the T.distichum and T.ascendens under different water conditions increased the nutrient content of the soil to some extent.At the same time,the decomposition of the T.distichum and T.ascendens leaves in the flooding environment released a large amount of carbon,nitrogen and phosphorus into the water body,increasing the risk of eutrophication of the water body in the Three Gorges Reservoir Area.Therefore,it is necessary to take appropriate measures to collect the leaves of the artificially reconstructed T.distichum and T.ascendens forest stand before flooding in the hydro-fluctuation belt of the Three Gorges Reservoir so as to minimize the nitrogen and phosphorus load in the water caused by the decomposition of T.distichum and T.ascendens leaves in the reservoir area as much as possible. |
