| Photosynthetic carbon (C) fixation of plants is the important C sinks in wetlands, and plays an important significant role in the process of C cycle in wetland ecosystem. It is quite significant to understand the effect of nitrogen (N) input on the photosynthetic fixation of C in the estuarine wetland, and prompted to better understand the process of C and N cycling and its response to environmental changes in estuary areas, but studies on the response mechanism of synthesized C in wetland plants is relatively less. In this paper, native dominant species of Cyperus malaccensis in the Min River estuary which located in the southeast of China, was selected as the research object. By the field investigation and microcosm, effects of simulated nitrogen input on the growth and photosynthetic characteristics, the contents of C, N, the fate of newly synthesized carbon and plasticity index in the C. malaccensis of the Min river estuary were studied. The major conclusions of this study showed as follows:(1) Middle and low levels of N input (6 to 16 g N m-2·a-1) is optimum to increase the fixation of photosynthetic carbon. With the increased of nitrogen supply, the trend of maximum net photosynthetic rate (Pnmax) and light saturation point (LSP) was increased, and the light compensation point (LCP) and dark respiration rate(Rd) was decreased in C.malaccensis. The increase of net photosynthetic rate (Pn) also response to prolong the photosynthesis period by stomatal and non-stomatal factors. In the quickly growth stage of C. malaccensis, N input enhanced aboveground and underground biomass, C, N accumulation and declined C/N significantly. The new synthesized carbon were quickly translocated and distributed unevenly in the plant-soil system, showing an order of shoots> roots> soil. The 813 C values and 13 C fixation of the aboveground and underground in C. malaccensis were much higher than those of without N supply, but there was no significant difference in soil. Distribution proportion of newly photosynthetic carbon in C. malaccensis also changed under different N treatments. The proportion of 13C fixation in the roots and soil increased with the low N supply(<8g N m-2·a-1), but more inclined to distribution to the aboveground under high N treatments.(2) Under different inundation and salinity treatments, N supply showed a significant effect on the process of synthesized C. The daily mean values of Pn and C accumulation improved markedly in the two inundation treatments due to the non-stomatal factors. The photosynthetic C fixation in C.malaccensis enhanced through the growth and photosynthetic characteristics, the contents of C, N adjustment under short-time inundation treatment, however, moderate-inundation treatment is optimum to increase the fixation of photosynthetic carbon for the long-term. The interaction test of salinity and N input showed that N input improve the stomata conductance(Gs) and the rate of transpiration(Tr), so as to increase the daily mean values of Pn. The daily mean values of Pn, aboveground and underground biomass and the photosynthetic carbon fixation under salinity(5) was significantly higher than those in other salinity treatments. The 13C stable isotope labeling also confirmed that under moderate salinity with middle N input, the mean value of 513C and net 13C fixation was significantly higher (P<0.05). The distribution proportion of stem and leave of the newly photosynthetic carbon in C.malaccensis increased under salinity and N input treatments, and the carbon fixation proportion of root and soil decreased.(3) The growth characteristics (aboveground biomass), photosynthetic characteristics (Pn), and the C contents were the main influencing factors which affect the capacity photosynthetic carbon fixation with N input in C.malaccensis of the Min river estuary. Density of the plant, C/N and intercellular CO2 concentration also acted important roles on the capacity photosynthetic carbon fixation in different inundation and salinity treatments. The plasticity index of growth characteristics, photosynthetic characteristics and nutrient content of C.malaccensis improved markedly after the N input with middle and low levels. Compared with moderate-inundation, short-time inundation had higher plasticity index, the similar result also found in the low salinity and moderate N input contrasted with other treatments, which suggest that C.malaccensis has the possibility to expand to high tidal flat and low salinity environment under the condition of continuous N input in the Min River estuary. |
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