| Since 2002, the implementation of the ecological restoration project in the Yellow River estuary not only supplemented abundant freshwater but also imported large amounts of nitrogen(N) nutrient for wetlands. In this paper, the Phragmites australis wetland in degraded and restored regions were selected as study objects to understand the influences of ecological restoration project(variations of hydrological regime and increases of N import) on the key N biogeochemical processes in wetland ecosystems. Through using acetylene(C2H2) inhibition technique and static-chamber and gas-chromatography methods, the experiments were performed to study the influences of ecological restoration project on the temporal-spatial distributions of N in wetland soil(or water), the effects of hydrological regime on N accumulation, distribution and release of plant, the emission rules of nitrous oxide(N2O) and the production mechanisms before and after wetland restoration. Finally, the N cycling compartment models of atmosphere-plant-soil system were established and the statuses of N balance in different wetland ecosystems were evaluated. The main results were drawn as follows:(i) The vertical distribution of N content in different wetland soils showed high variability in different years. The N contents in different wetland soils had significantly seasonal characteristics and inter-annual variations, which were mainly correlated with the approach of water replenishment and the quality of water during the implementation of ecological restoration project.(ii)The height, density, litter production and biomass of P. australis had significant difference in different restoration phases. The aboveground biomass of P. australis in different restoration phases were in the order of restoration wetland since 2002(R2002)>restoration wetland since2007(R2007) > un-restoration wetland(R0) during 2012-2014. Wetland hydrological regime(water depth and water quality) and import of nutrients were the most important factors influencing the different ecological traits and the aboveground biomass of P. australis in different restoration phases.(iii) The N/P ratio of R0, R2002 and R2007 plants were 8.99±1.92,7.04±1.65 and 9.02±2.45, respectively, which implied that N was the limiting nutrient in different restoration phases and its limitation degree in R2002 was the highest.(iv)The mass loss rates of litters in different restoration phases were in the order of R2002>R2007>R0. Both the litter decomposition rates in R2007(0cm>60cm>30cm>90cm) and R2002(30cm>0cm>60cm>90cm) differed among the four water depths, but the values in R2007 were generally higher that those in R2002 under different hydrological regimes. The approach of water replenishment and the import of nutrient in different restoration phases were important factors affecting the relative decomposition rates of different litters.(v)The variation patterns of N contents, C/N and C/P ratios in litters of P. australis and S. salsa showed significant differences before and after wetland restoration. The vitiation patterns of N contents, C/N and C/P ratios in P. australis litters at the same subzone in different hydrological regimes were generally consistent, but differed significantly among different decomposition subzones. The supply status of nutrient in the decomposition environment and the hydrological regime showed important effects on the variations of N absolute amount in litters. The C/N ratio had important regulation functions to the changes of N in litter in decomposition process.(vi) The N2 O concentrations in different wetland water were in the order of R2002(329.39±212.80nmol·L-1) > R2007(202.68±53.29nmol·L-1). The N2 O saturation in wetland water of R2002 and R2007 were73.86~396.47% and 44.97~118.00%, and the means were 159.71% and 93.18%, respectively,indicating that the former was in saturation status. The TC, TN and TOC contents were positively correlated with N2 O concentration(p<0.05), while Eh was negatively correlated with N2 O concentration(p<0.05). The N2 O emission in R2002 and R2007 were 0.72 ±8.10μmol·m-2·d-1and-3.32±1.83μmol·m-2·d-1, respectively. Water temperature and wind speed were important factors affecting the emissions of N2 O in water and atmosphere interface.(vii) The N2 O fluxes from different wetlands in the growing season were in the order of R2007(0.0054 mg·m-2·h-1) >R0(0.0029 mg·m-2·h-1) > R2002(-0.0006 mg·m-2·h-1). The diurnal variations of N2 O flux in summer and autumn were significantly different among the three wetlands, in the order of R2007(0.0144 mg·m-2·h-1 and 0.0057mg·m-2·h-1) > R0(0.0057mg·m-2·h-1 and 0.0034mg·m-2·h-1) >R2002(0.0032mg·m-2·h-1and-0.0022 mg·m-2·h-1). The light and N2 O concentrations in water were important factors influencing the diurnal variations of N2 O fluxes.(viii) The total N2 O production of wetland soils showed significant difference in different restoration phases, but the whole acted as N2 O source. The N2 O production in restoration wetland was higher than that in un-restoration wetland. The N2 O production was mainly due to the nitrification and nitrifierdenitrification processes, while the denitrification processes showed great weakening effects on N2 O production, which was closely related to the physical and chemical properties of wetland soils in different restoration phases. The non-biological processes made greater contributions to N2 O production and these were mainly due to that iron was reductive; while the Yellow River estuary was an area of high actively iron.(ix) There had different influences on wetland soil processes generating N2 O between temperature and water content, indicating the responses of soil microbial activities to temperature and water content were different. The N2 O production of wetland soils with NO3--N addition was much higher than that with NH4+-N addition. At different levels of NH4+-N addition, N2 O production was mainly due to the nitrification, nitrifier denitrification and non-biological processes, and the denitrification processes showed great weakening effects on N2 O production. Comparatively, the N2 O production in R2002 was mainly due to the nitrifier denitrification and denitrification processes at different levels of NO3--N addition. In R2002 and R2007, the N2 O produced by non-biological processes generally increased with NH4+-N addition, while with NO3--N addition, the effect was just the opposite. The N addition inhibited the non-biological processes generating N2 O in R0, R2002 and R2007.(x) The N cycling compartment models of plant-soil(water)-atmosphere system in different wetlands were established, and the N stocks of different compartments and the N turnovers among compartments were calculated. Based on these quantitative relationships of N cycling paths, the balance of N cycling of plant-soil(water)-atmosphere system in different wetlands was evaluated.From the perspective of N cycling, in order to maintain the stability of ecosystem, the approach of water replenishment with only one time in R2007 was unfavorable for the maintenance of ecosystem stability. Considering the ecological traits of wetland plant, the N supply capacity of soil(water), the function of weakening greenhouse gases, and the N absorption, utilization and cycle coefficients of plant-soil(water)-atmosphere system, it was suggested that, in the next step,the restoration project should adopt the approach of water replenishment in R2002. Also, the wetland restoration project should adopt the approach of less replenishment but with more times,and the restoration project should avoid the period of poor water quality.(xi) The above-mentioned quantitative relationships of N cycling paths and the evaluation of N balance in plant-soil(water)-atmoshpere system in different wetlands will have great theoretical and realistic significances for strengthening the restoration of degraded wetland and the protection and management of restored wetlands in the next step. |
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