Study On Photosynthetic Characteristics And Greenhouse Gas Emissions Of Saline-alkaline Phragmites Australis Wetlands In Western Songnen Plain

In order to explore the photosynthetic adaptation strategies and greenhouse gas emission strategies of Phragmites australis in saline-alkaline wetlands in Western Songnen Plain,the light response curves and CO₂ response curves of five habitats of reeds(wet reeds,xerophytic reeds,sandy reeds,abandoned farmland reeds and paddy field drainage reeds)at different stages of life history were measured,and the characteristics of greenhouse gas emissions were monitored and modeled.Data fitting was carried out.The conclusions were as follows:(1)The population characteristics of P.australis were:when the habitat of reed changed in saline-alkaline wetland,the coverage,density,plant height,leaf,biomass and the spikes of reed changed significantly.In the growth period,the coverage of P.australis in five habitats was as follows:sandy type<abandoned farmland type<wet type<paddy field type<xerophytic type,the population coverage and density of xerophytic type of P.australis in five habitats was the largest.The population coverage and density of sandy type of P.australis was the smallest in the five habitats.Moreover,the reason why the xerophytic type of P.australis had the largest population density was that the dry environment was more conducive to the asexual propagation of reed,that was,the growth and extension of its stolon.The growth of sandy type of P.australis was not as good as that of the other four habitats because of its weaker soil water holding capacity.With the transition of environment from wet to dry,the number of spikes of P.australis decreased.This indicated that the breeding strategy of P.australis responded to the water deficient environment.After entering the breeding period,the numbers of spikes of xerophytic type and sandy type were still the smallest among the five habitats.Because the population density of sandy type of P.australis was very small,there was no competition for light,so it was more likely to maximize the photosynthesis by increasing leaf width.Therefore,sandy type of the P.australis had the largest leaf width,and also the largest leaf length among the five habitats,that was,the largest leaf area.Xerophytic type of the P.australis had the smaller leaf area because of its largest density and coverage.Due to the large plant density,the higher the density of P.australis was,the greater possibility of shading each other and the smaller the leaf area would be.The diameter and thickness of stem of xerophytic type of the P.australis were also the smallest.The plant height and stem diameter of wet type of P.australis were the highest,while the biomass of P.australis in the drainage area of paddy field was the largest,indicating the morphological adaptation to water and nutrition in these two habitats.The population characteristics of P.australis in the above five habitats indicated the growth response and adaptive strategy of P.australis to different environment.(2)In terms of the photosynthetic characteristics of P.australis:due to the dark light environment,abandoned farmland type of P.australis had the largest apparent quantum efficiency(AQE)and the highest photosynthetic efficiency for low light during the growth period.The paddy field type of P.australis had the lowest dark respiration rate,the highest photorespiration rate,maximum photosynthetic rate(P_max),photosynthetic capacity(Pn),biomass,maximum carboxylation rate(V_cmax)and maximum electron transfer rate(J_max).With the transition from growing season to breeding season,the light insensitivity of P.australis increased,and the dark respiration rate decreased,which were more conducive to biomass accumulation.Besides,after entering the breeding season,P_max and Pn of P.australis in five habitats had the same change trend,and the trend was consistent with the change of V_cmax and J_max,that was,when P_max and Pn showed a decline,V_cmax and J_max also showed a decline,and vice versa.Therefore,V_cmax and J_max could explain the change of total photosynthetic rate to some extent.As for the factors affecting photosynthetic characteristics of P.australis,the population density of P.australis was negatively correlated with the apparent quantum efficiency(AQE).The higher the density was,the larger the shade area of reed leaves would be,and the lower the utilization efficiency of low light would be.Moreover,coverage and density of population could indirectly affect the light energy acquisition of reed leaves,and then affected the photosynthetic rate.(3)In terms of diurnal dynamics of greenhouse gas emissions of P.australis habitats:1)In the growth period,only the sandy type showed the absorption sink of CO₂,and the other habitats showed the emission source of CO₂.Besides,the highest CO₂ emission peak was found in the reed habitat of abandoned farmland,indicating that it had a higher CO₂ emission capacity.The highest CO₂ absorption peak was found in sandy type habitat,indicating that it had a higher CO₂ absorption capacity.In terms of CH₄ emission characteristics,only wet type of reed habitats showed the emission source in daily change of CH₄ emission,and the other four types of reed habitats showed the absorption sink of CH₄.As for the variable coefficient of daily CH₄ emission,the results showed that the variation of diurnal emission characteristics of CH₄ was higher in the drainage area of paddy field and the xerophytic type field,and there was no significant difference between these two habitats,indicating that the CH₄ emission rate of the two habitats was not stable.The variability of CH₄ emission of wet type habitat was the smallest,it was 8.17%,indicating the stable CH₄ emission characteristics.In terms of N₂O emission characteristics,there was no significant difference in N₂O emission among the three habitats:sandy type,abandoned farmland type and paddy field type.Moreover,the daily N₂O emission variability of the three habitats was relatively larger than others,that indicated that the daily N₂O emission rate was unstable.In addition,paddy field type habitat not only had the largest N₂O emission peak,but also the largest N₂O absorption peak,and the N₂O emission peak and absorption peak of wet type habitat were the smallest among the five habitats.The daily N₂O emission variability of the xerophytic type habitat was the smallest among the five habitats,it was only 2.62%,indicating that it had relatively stable N₂O emission capacity;2)In the breeding period,only the sandy type habitat showed a emission source of CO₂ among the five reed habitats,and the other four reed habitats showed the CO₂ absorption sink.The wet type reed habitat had the stronger absorption capacity of CO₂.In terms of CH₄emission characteristics,there was no significant difference between sandy type and abandoned farmland type habitats,and the CH₄ emission rate of xerophytic type habitat was the largest,showing a strong CH₄ release capacity.In terms of N₂O emission characteristics,after entering the breeding period,the wet type,sandy type and paddy field type reed habitats still showed the absorption of N₂O,while other two reed habitats showed the emission of N₂O.Besides,the ability of abandoned farmland type reed habitat as N₂O emission was stronger than that of xerophytic type reed habitat,and there was significant difference between them.In addition,the correlation analysis between the characteristics of P.australis population and characteristics of greenhouse gas flux showed that the coverage and density of P.australis population had a significant positive correlation with the CH₄ emission(r=0.95,r=0.94),that was,the greater the coverage and density were,the greater the CH₄ emission rate would be.