| Soil nitrogen and moisture content are important factors that limit plant growth in forest ecosystems.It is of great significance to illustrate the effects of nitrogen application and rainfall exclusion on the three saplings in mid-subtropical zone.This study,taking Castanopsis fargesii of Fagaceae,Cinnamomum camphora of Lauraceae and Schima argentea of Theaceae as the research object,compared the varieties of water physiology characteristics,foliar phosphorus fractions,biomass and its allocation of four treatments including control?CK?,nitrogen application(+N,N application as 60 kg·hm-2·a-1),rainfall exclusion?-W,reducing natural rainfall by 50%?and their interaction(+N-W,N application as 60 kg·hm-2·a-1and reducing natural rainfall by 50%).This study could provide a theoretical basis for predicting the response of the subtropical forest ecosystem to the nitrogen application and rainfall exclusion.The main results are as follows:?1?Under different treatment conditions,there were no significant differences in the relative water content of three five-year-old saplings?P>0.05?.Under the control conditions,there were no significant differences in leaf water potential,transpiration rate,water use efficiency,and leaf hydraulic conductance of the three species?P>0.05?,while there were significant differences in branch specific conductivity and the loss rate of hydraulic conductivity?P<0.05?.?2?Under the condition of nitrogen application,there waere no significant changes in the water potential of the three saplings compared with the control?P>0.05?.There were no significant changes in the transpiration rate of C.fargesii and C.camphora?P>0.05?,while the transpiration rate of S.argentea was significantly increased?P<0.05?.The water use efficiency of leaves of three saplings increased.Under the condition of rainfall exclusion,there were no significant changes in the water potential of C.fargesii and S.argentea?P>0.05?,while the water potential of C.camphora was significantly decreased?P<0.05?.There were no significant changes in transpiration rate of three saplings?P>0.05?.The response of water use efficiency to water reduction varied from species to species:The water use efficiency of C.fargesii saplings was significantly reduced?P<0.05?;there was no significant change in water use efficiency of C.camphora saplings?P>0.05?;the water use efficiency of S.argentea saplings significantly increased?P<0.05?.The branch specific conductivity of S.argentea increased significantly?P<0.05?.Under the interaction of nitrogen application and rainfall exclusion,there were no significant changes in the water potential and water use efficiency of the three saplings compared with the control?P>0.05?.There were no significant changes in the transpiration rate of C.fargesii and S.argentea?P>0.05?,while the transpiration rate of C.camphora was significantly increased?P<0.05?.The branch specific conductivity of C.camphora increased significantly?P<0.05?.The results of the study indicated that the physiological mechanisms of three saplings for nitrogen application and rainfall exclusion were different.?3?The results showed that the proportion of structural phosphorus in the C.camphora saplings was the largest,followed by the C.fargesii and S.argentea.The proportion of metabolic phosphorus in the S.argentea saplings was the largest,followed by the C.fargesii and C.camphora.The order of the proportion of nucleic phosphorus was:C.fargesii>C.camphora>S.argentea.The order of the proportion of residual phosphorus was:C.camphora>C.fargesii>S.argentea.The results indicated that the proportions of the four phosphorus fractions in different species were different.In this study,nitrogen application increased the proportion of structural phosphorus in C.fargesii saplings,reduced the proportion of metabolic phosphorus,and reduced the total phosphorus concentration?P<0.05?.The nitrogen application also significantly increased the photosynthetic phosphorus use efficiency of three saplings?P<0.05?,which was consistent with previous research results.The rainfall exclusion did not change the ratio of each phosphorus fractions to total phosphorus in the leaves?P>0.05?.Under the interaction of nitrogen application and rainfall exclusion,the photosynthetic phosphorus utilization efficiency of three saplings was significantly increased?P<0.05?.The results indicated that nitrogen application and rainfall exclusion had an interactive effect on phosphorus concentration,phosphorus fractions,and phosphorus utilization efficiency of three saplings.?4?The leaf biomass,branch biomass,stem biomass,root biomass,and total biomass of C.camphora saplings were significantly higher than those of C.fargesii and S.argentea?P<0.05?,but there was no significant difference between C.fargesii and S.argentea?P>0.05?.The results of the study indicated that the difference in biomass between tree species was genetically decisive.?5?Under the condition of nitrogen application,the ground meter,plant height,biomass and its allocation of three saplings were significantly larger than the control?P<0.05?and the biomass of C.fargesii increased most.Under the condition of rainfall exclusion,there were no significant changes in the ground meter,plant height,biomass and its allocation of the three saplings compared with the control?P>0.05?.Under the condition of nitrogen application and rainfall exclusion,the effects of interactions on the three saplings differed due to different species,and the growth promotion effect on S.argentea saplings was more significant?P<0.05?.The results showed that nitrogen application promoted the growth of three young trees,but the effect of rainfall exclusion by 50%had no significant effect on the growth of the three saplings.?6?The allocation of biomass of different saplings had different responses to nitrogen application and rainfall exclusion.In general,nitrogen application would reduce the distribution of root biomass of three saplings and increased the distribution of leaf,branch and stem biomass?P<0.05?.Under the condition of rainfall exclusion,there were no significant changes in biomass allocation?P>0.05?. |
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