| Nitrogen (N) cycling process of wetland not only affected its self-regulation mechanism, but also showed a special kinetics process in the Earch Surface System. Moreover, the special kinetics process was correlated with a series of global environmental problems. Hydrologic process was a decisive factor in the formation and maintenance of special type of wetlands, which constrained the succession of wetland system. Hydrological conditions controlled the spatial patterns of vegetation, the growth of plants and living environment of vegetation. Moreover, some aspects of plant community attributes controlled N cycling. In results, shifts in hydrological conditions may alter N cycling through changing community attributes. In order to provide an insight into the mechanism of hydrologic regime constraining the process of N circulation, patterns of N cycling in plant-soil systems were examined using compartments model in three freshwater wetlands along a water level gradient in the Sanjiang Plain, Northeast China. N storages, N distributions, N fluxes and cycle efficient were measured in Calamagrostis angustifolia wetland (XW), Carex meyeriana wetland (WW) and Carex lasiocarpa wetland (MW). The main results were drawn as following:Orgnic nitrogen was the main form in XW, WW and MW soils. More than 99% of total N storage was orgnic nitrogen in the three wetland soils. The N content and distribution pattern were different among the three wetland soils. Because the water condition and soil type which induced by morphological charictaristic and N physical transference were different in the three wetlands. The vertical distributions and seasonal change characteristics of all kinds of N were different significantly in XW, WW and MW soils. Because the main factors affected N distribution in various soil layers and times were different in the three wetland soils.Total N content in aboveground organs of plants in XW, WW and MW were declined gradually during the growing season. The total N content in roots was in greater fluctuation during the growing season. The total N content in litter was declined gradually with lapse of time and showed a significant decrease: WW>XW>MW. N accumulation amounts in the three wetlands showed a significant decrease: WW>XW>MW. The N accumulation amount in root was the highest in MW. In the three wetlands, the N accumulation amount in organs of plant in growth time showed a decrease: root>leaf>stem, and in autumn showed a different decrease: root>stem >leaf. It is indicted that root was the important N storage. The N accumulation amounts in litter among three wetlands were increased with lapse of time during the growing season.Calico decomposition rates were statistically different among the three wetlands in the upper soil profile and were also different in the lower depth range, which indicats environmental conditions influence on decomposition strongly. Calico decomposition rates were posively correlated with temperature and pH, but were negtively correlated with soil water contents. In this study, the strong influence of litter quality on decomposition was demonstrated by different decomposition rates between calico and litter/ roots in the same environment. Litter decomposition rates were positively correlated with initial N and P concentrations, but were not correlated with other parameters. Root decomposition rates were posively correlated with C/P ratio and N/P ratio, were negtively correlated with initial P concentration, but were not correlated with initial N and C/N ratio. The N contents in Calamagrostis angustifolia, Carex meyeriana and Carex lasiocarpa litter decreased gradually during the decomposition period and showed a significant increase all the time:Carex lasiocarpa litter -2,626.49 g·m-2 and 552.59 g·m-2 respectively.In plant-soil systems, annual N uptake by aboveground biomass showed a significant decrease: WW>XW>MW. Annual N uptake by aboveground biomass was higher in WW, but the difference between XW and MW was not so significant. Annual N transferred from aboveground biomass to litter showed a significant increase: MWMW>XW. Re-translocation of N from aboveground biomass to root in WW was three-fold higher than that in MW, but five-fold higher than that in XW. N uptake by root among the three wetlands showed a significant increase: XWWW>MW.Characteristics of N cycling in plant-soil system among the three wetlands along a water level gradient were different significantly. N outputs from the soil exceeded N inputs in plant-soil system of the three wetlands. Moreover, N release from litter decomposition increased significantly with water level decreasing. A large percent of N uptake from soil was detained in root, and just a small percent of N uptake was used in internal circulation. In addition, annual N used in internal circulation among the three wetlands showed a decrease: WW>XW>MW. A majority of annual N uptake by aboveground plant was returned through litterfall, and a small part was transferred from aboveground to belowground in the three wetlands. With water level increasing, annual N uptake and annual N retention increased, but annual N return, cycle coefficient decreased. These results suggest that the rate of biological cycling was decreased with water level increasing.
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