Photosynthetic Carbon Allocation And Their Responses To Coupling Factors Of Soil Nutrient And Water In Typical Plantation Saplings In Sub-tropical Region

Plants photosynthetic carbon assimilation is an important part of terrestrial ecosystem carbon cycle.Soil water and nutrients are important environmental factors affecting photosynthetic carbon assimilation,migration and distribution.However,most previous studies were limited to the effects of a single environmental factor on photosynthetic carbon migration and distribution of plants in farmland and grassland ecosystems,and the studies on photosynthetic carbon distribution of trees and their responses to water and nutrient in forest ecosystems were relatively lacking.China's plantation area is about 69 million hectares,which is the largest in the world.Sub-tropical region is the mainly distribution area of artificial forest in China,which plays an important role in the potential of forest carbon sink.Therefore,it is of great significance to study photosynthetic carbon distribution and environmental factors responses of plants in typical sub-tropical plantations in China.In this study,the effects of coupled factors of tree species,soil water and nutrients on the assimilation,migration and distribution pattern of photosynthetic carbon in Chinese fir,Phoebe,Eucalyptus and Masson pine?2-year saplings?of four typical plantations in sub-tropical region were studied by using ¹³C stable isotope pulse-labeling technique.The main research results are as follows:?1?The net photosynthetic rate and chlorophyll fluorescence parameters of different tree species were significantly different.The order of the net photosynthetic rate of Saplings leaves was Eucalyptus(3.19 mol m^-2 pine(0.33 mol m^-2s^-1).There were significant differences in Fo,Fm,Fv,Fv/Fm,Fv/Fo and Fv'/Fm'of saplings leaves.Soil water significantly affected the net photosynthetic rate of leaves,and there was a significant interaction between tree species and water,but fertilization did not significantly affect the net photosynthetic rate and chlorophyll fluorescence characteristics of saplings leaves.?2??¹³C values of saplings leaves of 4 species of sub-tropical plantations all reached a peak value at 2h and then began to decline,while ¹³C values of roots and soil gradually increased and reached a maximum value at15d.Tree species significantly affected the?¹³C values of leaves,roots and soil of saplings trees,and the?¹³C values of leaves,stems,roots and soil of Eucalyptus were higher than those of other tree species,indicating that Eucalyptus saplings had a higher photosynthetic carbon assimilation rate and migration ability.With the increase of soil water,stem,root and soil?¹³C values of saplings in each plantation showed an increasing trend,while?¹³C values of leaves showed no significant difference in different soil water gradients.The effect of soil nutrients on?¹³C value of each plant organ and soil was not significant.?3?The three factors of tree species,soil water and nutrients significantly affected the content and distribution ratio of ¹³C in saplings leaves,stems,roots and soil.The proportion of photosynthetic carbon to soil increased with the growth of saplings trees.Under different water conditions,the content of ¹³C in leaves,stems,roots and soil of the four saplings increased with the increasing of water.With the increase of soil moisture,the ¹³C distribution ratio of saplings leaves and stems increased,while the¹³C distribution ratio of soil decreased.The increase of soil nutrients can improve the ¹³C content of salings leaves and stems and the ratio of leaves to soil ¹³C when the first sample is taken.?4?The effects of tree species,soil water and nutrients on roots,stems,leaves and total biomass of saplings were significantly different.The order of total biomass was Chinese fir>Eucalyptus>Masson pine>Phoebe.The biomass of saplings increased with the increasing of soil water,and the biomass allocation proportion of leaves increased with the increasing of water,while the biomass allocation proportion of roots decreased with the increasing of water.?5?Tree species,soil water and nutrients had significant effects on the content and accumulation of C in leaves,stems,roots and soil of saplings trees.The accumulation of C in roots,stems and leaves of four plantations saplings were increased with the increasing of water,but there was no significant difference on the accumulation of C in soil.Soil fertilization can improve the accumulation of plant and soil C to a certain extent.In summary,we found that there were significant species differences in photosynthetic carbon assimilation capacity,migration and distribution pattern of different saplings species in sub-tropical plantations.At the same time,the environmental factors of soil water and nutrients can change the photosynthetic carbon assimilation,migration and distribution pattern of plants.Compared with soil water,the effect of soil nutrient availability on photosynthetic carbon assimilation and distribution pattern of plants is relatively small.In addition,tree species,soil moisture and nutrients have certain interactive effects on photosynthetic carbon migration and distribution of plants,so reasonable soil water and fertilizer management should be carried out for different tree species to improve the carbon sink potential of sub-tropical plantation.