The Effects Of N On Competition Between Of P. Cathayana And P. Beijingensis

In recent years, nitrogen deposition is increasingly serious,resulting in the gradually increasing of content of nitrogen in the soil environment. N deposition would change the nitrogen cycle in the ecosystem, and affect competition pattern between interspecies. Nitrogen is known as life elements of plant, and changing of N level in soils directly affects the growth and physiological characteristics of plants. As an important afforestation tree species in China, supply of nutrients for poplars seriously restricts growth and yield, and also changes competitive relationship between species. Therefore, we used Populus cathayana and P. beijingensis shared extensive overlap regions as experimental material to study growth, photosynthetic efficiency, pigment content, characteristics of nitrogen metabolism and nutrients, biomass accumulation and distribution of both tree species in the condition of intraspecific competition and interspecific competition. Our aims were to know the difference of competitiveness between P. cathayana and P. beijingensis under different nitrogen levels, to provide theoretical basis for the prediction of interspecific competition of plants and the change of community structure. The main results are as follows:(1) Nitrogen fertilizer significantly promoted morphological growth of both P. cathayana and P. Beijingensis, especially for P. Beijingensis. Under conditions of interspecific competition (QB/N), the height of P. Beijingensis was 183.60 cm and increased by 12.64% as compared to that of P. Beijingensis in QB treatment. Under such conditions, the basal diameter was 14.40 mm, increased by 24.14%. But for P. cathayana, height and basal diameter of P. cathayana increased only by 8.93% and 7.49%, respectively.No matter in intraspecific competition or interspecific competition conditions, both the height and basal diameter of P. Beijingensis were higher than those of P. cathayana. When exposed to nitrogen fertilizer, total leaf area of P. Beijingensis increased 107.74% in intraspecific competition (QB/N) and increased 63.07% in intraspecific competition (BB/N), when compared to the controls. In addition, specific leaf area (SLA) of both tree species reduced slightly due to nitrogen deposition.(2) Nitrogen fertilizer promoted biomass accumulation of both tree species. Under the conditions of nitrogen deposition, both leaf dry weight (LW) and total biomass (DMA) of P. Beijingensis were higher than those of P. cathayana. Under the conditions of QB/N, P. Beijingensis'stem dry weight (SW) was higher than P. cathayana's, while there was no significant difference between them (BB/N and QQ/N). For DMA, nitrogen fertilizer increased by 22.22% and 30.63% under QB/N and QQ/N for P. cathayana, respectively, when compared to the controls, whereas an increase with 123.26% and 92.66% occurred, respectively for the P. Beijingensis. When comes to the underground biomass (RW),in addition to the QB, irrespective each kind of competitive condition pattern, P. Beijingensis' biomass are slightly higher than those of P. cathayana, though the difference was not significant.(3) The gas exchange rates of both tree species increased with different degrees when exposed to nitrogen application. The net photosynthetic rate (Pn) of P. cathayana were significantly higher than that of P. Beijingensis in intraspecific competition conditions, whereas both stomatal conductance (Gs) and transpiration rate (Tr) of P. Beijingensis was significantly higher than those of P. cathayana in QB/N. Under the treatment of QB/N, intercellular carbon dioxide concentration (Ci) of P. Beijingensis was higher than that of P. cathayana, but under other conditions, there is no significant difference between the species.(4) Nitrogen fertilizer increased pigmentscontent of both P. cathayana and P. Beijingensis under different competitive pattern. In addition to the chlorophyll b (Chl b) and total chlorophyll (TChl) under the conditions of QB, other pigments content and total chlorophyll of P. cathayana were significantly higher than P. Beijingensis. Because there was a similar change among chlorophyll a (Chl a), chlorophyll b (Chl b) and Caro, there is no significant effect of N deposition on the ratio of chlorophyll a and chlorophyll b (Chl a/b), and Caro/chlorophyll (Caro/Chla+b) of the two species.(5) Nitrogen fertilizer promoted the activity of related enzymes of nitrogen metabolism in leaves of P. cathayana and P. Beijingensis. In all competitive conditions, GS, NAD-GDH, AlaAT and AspAT activities in P. cathayana are higher than those in P. Beijingensis. Except for GS activity in interspecific competition conditions (QB/N, QB) and in intraspecific competition conditions (QQ,QB) and the NAD-GDH in QB,activities of rest of the nitrogen metabolism enzyme in P. cathayana were highly significant higher than those in P. Beijingensis. These related enzymeactivity in both species under interspecific competition pattern were significantly higher than those under intraspecific competition conditions.(6) Nitrogen fertilizer induced an increase in total nitrogen (N) and total phosphorus (P) in both P. cathayana and P. Beijingensis with a different degree, while there was no significant variations in carbon (C).Under nitrogen fertilizer conditions, content of total N in leaves of P. Beijingensis increased by 40.30% and 36.67% in the QB/N and BB/N, respectively, when compared to the controls, whereas in P. cathayana,an increase of 16.67% and 36.72%,respectively, occurred. For total Pcontent of blade, P. Beijingensis exhibited an increase by 6.41% and 15.18%, respectively, whereas in P. cathayana there was an increase by 7.04% and 16.43%, respectively. In nitrogen fertilizer conditions, the content of total N in leaves of P. Beijingensis are higher than those in P. cathayanain the interspecific competition, but under two conditions, there is no significant difference between the species. In the interspecific competition conditions, the content of total Pin leaves of P. Beijingensis are higher than those in P. cathayana, but an inverse occurred under the intraspecific competition conditions. Under the N treatment conditions, leaves C/N in both species were significantly lower than that in control conditions. When compared with intraspecific competition, P. Beijingensis C/N in the interspecific competition decreased more significantly. The change of the C/P is similar to the C/N changes. Similarly, there was no significant effect of N deposition on N/P in both species.In summary, nitrogen fertilizer significantly affected the photosynthetic rate of both species, activities in related of nitrogen metabolism, as well as the nutrient and biomass accumulation. The changes in the gas exchange rate, photosynthetic pigment and related enzyme activities of nitrogen metabolism suggested that P. cathayana should be more adaptive to environment in lack of nutrient, but P. Beijingensis would gain an advantage in interspecific competition, due to the higher photosynthetic rate, leaf area, and above ground biomass in P. Beijingensis when compared to P. cathayana under the rich-nitrogen conditions.