Mixed Foliar Decomposition And Nutrient Release Of Suitable Tree Species In The Water Level Fluctuation Zone Of The Three Gorges Dam Reservoir

After the Three Gorges Dam operation,a huge land-water interlaced area was formed around the reservoir area,namely the water-level fluctuation zone of the Three Gorges Reservoir.This area is significantly affected by the artificial water level adjustment and has unique characteristics due to hydrological changes.The hydrological rhythm of the artificial regulation of the Three Gorges Reservoir has broken the original natural ecological balance of the water-level fluctuating zone,resulting in a noticeable degradation of the ecological system of the water-level-fluctuating zone.In order to improve the habitat conditions of the degraded water-level fluctuating zone,the researchers selected a variety of flood-tolerant plants to be planted here.Among them,the main suitable tree species planted in the upper part of the water-level fluctuating zone include Taxodium distichum（Linn.）Rich.,Taxodium ascendens Brongn.,Salix matsudana Koidz.,and other flood-tolerant woody plants.In order to make full use of environmental conditions（light,soil fertility,etc.）,improve the soil and enhance the ability to withstand natural disasters,planted forests are generally planted in a mixed manner between rows.After years of periodic flooding,the artificial mixed forests are growing well.The environment of the water-level fluctuation zone has been greatly improved.It is worth noting that the Three Gorges Reservoir is subject to an artificial hydrological rhythm,and the time and degree of water flooding in the water-level fluctuating zone are different from those of natural rivers every year.It is flooded in autumn（late September）and has deep water in winter（a vertical drop of up to 30 m）.The water level did not gradually decrease until the following spring（late January）.Every autumn,when the water level rises,the vegetation is in a period of vigorous growth.A large number of leaves are directly exposed to flooding before they fall.Different types of trees’leaves are flooded and decomposed in the water at the same time.Until the following spring,the water level gradually recedes without decomposition.The leaves may continue to stay in the water to undergo water-flooded decomposition,or they may not be taken away by the water flow and remain on the bank of the reservoir,undergoing drying decomposition after flooding decomposition,and so on.In the huge water-level fluctuating zone,the decomposition and nutrient release of the main suitable tree species in the artificial mixed forest under the special hydrological rhythm of the reservoir area will inevitably have an important impact on the material cycle process of the reservoir area’s ecosystem.In order to clarify the impact of hydrological dynamics in the water-level fluctuation zone on the mixed decomposition and nutrient release of the main suitable tree species,this study selected the single and mixed leaves of Taxodium distichum,Taxodium ascendens,and Salix matsudana in the Ruxi River Basin,Zhong County,Chongqing Municipalities,in the hinterland of the Three Gorges Reservoir area.To explore the decomposition rate,nutrient element release,cellulose and lignin degradation dynamics at different altitudes and different flooding depths.At the same time,16S r RNA high-throughput sequencing was used to understand the characteristics of bacterial community changes in foliage decomposition.The main research results obtained are as follows:（1）The decomposition rate and nutrient release rate of single and mixed leaves of the main suitable tree species of Taxodium ascendens and Salix matsudana under different flooding depths are significantly different in the water-level fluctuating zone of the Three Gorges Reservoir.Foliage type,flooding depth,decomposition time,and their interactions all substantially impact the mass remaining.After one year of decomposition,all samples lost 83～90%,79～85%and 54～67%of their mass in shallow water submergence（SS）,deep water submergence（DS）and unflooded（CK）conditions,respectively,SS and DS treatments significantly promoted leaf decomposition.Compared with the single treatment,the mixed leaves of Taxodium ascendens and Salix matsudana showed greater mass loss under both SS and DS treatments and greater phosphorus（P）release in the SS treatment,and more significant release of nitrogen（N）in the DS treatment.Flooding significantly promoted the decomposition and nutrient release of all foliage types.The decomposition rate of all samples in SS treatment was considerably more significant than its decomposition rate in DS.（2）After one year of decomposition under SS,DS,and CK treatments,the cellulose degradation range of single and mixed leaves of Taxodium ascendens and Salix matsudana was 55.57%-97.76%,and the degradation range of lignin was 79.87%-93.82%.Different foliage types（except lignin degradation rate）,water treatments,decomposition time,and the interaction between water treatments and decomposition time significantly affect cellulose and lignin’s degradation rate.At the early stage of decomposition（the first 30 days）,cellulose and lignin were rapidly degraded in all foliage types.The degradation rates of cellulose and lignin in SS and DS were much higher than those in CK.The leaves of Salix matsudana had the highest cellulose release rate in CK,SS and DS treatments.The leaves of the mixtures had the highest lignin degradation rate under CK and DS treatments,while the leaves of Salix matsudana of SS treatment had the highest lignin degradation rate.The degradation of cellulose and lignin is mainly related to leaf quality（cellulose and lignin concentration,C,N,P content）,decomposition cycle and local environmental factors（temperature,water treatment,dissolved oxygen）.（3）The hydrological environments of continuous flooding（SS and DS）and flooding-drying（SS-CK and DS-CK）significantly promoted the decomposition of single and mixed leaves of Taxodium ascendens and Salix matsudana.Foliage types,decomposition time,hydrological environment,and their interactions substantially affect leaf mass remaining and nutrient retention.The decomposition rate of the leaves of the mixtures was the fastest in the CK,SS,and DS treatments in the first half of the year（0-180 days）.The decomposition rate constants k in the CK,SS and DS groups reached 0.00348,0.00749,and 0.00604 d^-1,respectively.The P release rate of all foliage types was significantly faster than TOC and N.During the decomposition process in the second half of the year（180-360 days）,the decomposition rate of each foliage type in the continuously flooded treatments SS and DS were significantly greater than that in the flooded-drying treatments SS-CK and DS-CK.The decomposition rate of leaves of the mixtures was also the fastest in the DS,SS-CK,and DS-CK groups in the second half of the year;the decomposition rate constant k reaches0.00252,0.00196,and 0.00138 d^-1,respectively.At the end of decomposition,all foliage types are in a nutrient-release state.（4）There are significant differences in the decomposition and nutrient release of single and mixed species of the main suitable tree species of Taxodium distichum,Taxodium ascendens and Salix matsudana at different altitudes in the water-level fluctuating zone of the Three Gorges Reservoir.The third batch of samples placed at175 m was recovered when the water retreated from 175 m（the first sample recovery）,and the mass loss of Salix matsudana was the largest;The mass loss of the same leaf samples at 170 m was significantly higher than that at 175 m when the water retreated from 170 m（the second sample recovery）.The mass loss of the leaves of Salix matsudana was the largest at the same altitude;its mass loss at 170 and 175 m was57.76%and 51.74%,respectively.When the leaf samples of 165,170,and 175 m were recovered at 165 m（the third sample recovery）,it was found that the same leaf sample showed the largest mass loss at 165 m y.The mass loss of Salix matsudana was the largest among the different leaf samples at the same altitude;its mass loss at 165,170,and 175 m was 64.95%,59.11%,and 53.43%,respectively,which was similar to the second sample recover.During each sample recovery,the mass loss of each leaf sample was significantly correlated with leaf type,and altitude and the release rates of C,N,and P of each sample were correlated considerably with leaf type,altitude,and their interaction（except for the C release rate of the third and fifth recovery,the N release rate of the interaction between the altitude and leaf type in the third sample times,and the P release rate of the interaction between altitude and leaf type in the fourth sample times）.（5）The main bacterial community structure of single and mixed leaves of Taxodium distichum,Taxodium ascendens and Salix matsudana at different altitudes is significantly different;the Shannon and Simpson index of the three altitudes of mixed-foliage types were 0.22～0.33 and 0.002～0.008 higher than that of single foliage types,respectively.Among them,Bacteroidetes and Proteobacteria are the most abundant dominant bacteria.At the genus level,the abundance of Bacillus Terrimonas,Flavisolibacter,Sphingosphinobacter are higher.The key bacteria in the decomposition process of different leaves may be different,and the mixing of different leaves may induce specific bacteria with different biological functions.At 165,170 and 175 m,there was a significant correlation between initial TOC TN C/N,N/P and mass loss.The substrate quality of each foliage type was also the main factor affecting the change of bacterial community.In conclusion,leaf decomposition and nutrient release were generally faster in flooding environments than in flooding-drying and non-flooding environments.Still,different foliage types and different flooding depths had other effects on decomposition and nutrient release.The degradation of cellulose and lignin of the refractory substances in leaves is mainly related to leaf quality（cellulose and lignin concentration,C,N,and P content）,the decomposition cycle,and local environmental factors（temperature,water treatment,dissolved oxygen）.The leaves of low-altitude trees suffer from flooding for a longer time and with more significant quality loss.The nutrient release of mixed leaves at all altitudes is not lower than the lowest nutrient release of a single foliage type.The substrate quality of each foliage type and the mixing of different leaves may induce specific bacteria with different biological functions and affect the diversity of bacterial groups.The research results provide a clear perspective for the in-depth understanding of the material circulation process in the water-level fluctuating zone of the Three Gorges Reservoir area and the land-water interlaced zone in other regions and provide a reference for formulating appropriate management and utilization strategies for the fluctuating zone.