Effects Of Soil Phosphorus Forms On The Growth And Photosynthetic Characteristics Of Subtropical Seedling

Phosphorus(P)is an essential element for plant growth and development,which is the main limiting nutrient factor of subtropical forest ecosystem productivity in China.There are two main fractions of soil P:inorganic P and organic P.During pedogensis and ecosystem development,the forms and concentrations of soil P have changed,inorganic P content decreased and organic P content increased.Growth and photosynthetic intensity can be used as important indexes to evaluate adaptability of tree species.Our study chose twelve common subtropical tree species,combined with the evolutionary period of tree species and the transformation of soil P forms on time scale,we analyzed tree species adaptability to soil P forms in different evolutionary periods.We wonder if plant evolution is consistent with the changes of soil P forms during pedogenesis?Early evolutionary tree species would have higher adaptability and absorption efficiency for inorganic P,and their growth capacity and photosynthetic performance would be better.Late evolutionary tree species would improve their absorption efficiency of organic P by various morphological,biochemical or symbiotic mechanisms,which promotes plant growing and photosynthesis.In this study,four gradients of P form treatments(deep soil,mixed soil,topsoil,litter)were used for pot experiments.The total P in four soil treatments was kept at the same level,while inorganic P decreasing and organic P increasing,we plant one seedling per pot.According to plant evolutionary periods,12 broad-leaved subtropical trees species were selected(including 4 early evolutionary tree species,3 middle evolutionary tree species and 5 late evolutionary tree species).16 seedlings with the same growth condition were selected for each tree species,including 4 repititions in each treatment,and the seedlings were irrigated with P free nutrient solutions during cultivation.Plant growth(ground diameter and height increment),photosynthetic characteristics(net photo synthetic rate Pn,stomatal conductance Gs,transpiration rate Tr and intercellular CO2 concentration Ci),chemical properties of leaves(relative chlorophyll content SPAD and leaf P content)of each tree species in different P form treatments were measured,and the correlation between indexes was analyzed.Main contributions are as follows:(1)Under different P form treatments,the trend of ground diameter and height increments of same tree species was almost same,but growth trend of tree species was different.There was no significant interaction between different tree species and phosphorus forms.With the rise of inorganic P content in soil treatments,the growth of early evolutionary tree species generally increased;while the growth of middle evolutionary stage tree species generally decreased;ground diameter increments of late evolutionary stage tree species generally increased,while height increments showed irregular change.(2)Soil phosphorus forms had a significant effect on tree species photosynthetic characteristics(p<0.05),and there was a significant interaction between different tree species and phosphorus forms on Pn(p<0.001).Affected by the absorption strategies and photosynthetic characteristics,Pn of different tree species showed diverse change among P form treatments.With organic P increasing in soil treatment,Pn of some tree species like Ilex latifolia gradually increased while Pn of tree species such as Neolitsea sericea showed:mixed soil<litter<topsoil<deepsoil.With increasing of organic P in soil treatments,Pn of early and late evolutionary stage plants generally decreased,while Pn of middle evolutionary stage tree species increased,and Pn was higher in subsoil treatment in each evolutionary stage.The photosynthetic parameters of Gs,Ci and Tr were significantly different(p<0.05).(3)Soil phosphorus forms had significant effect on the leaf P concentration(p<0.05),leaf P generally was higher in litter.The effect of P forms on SPAD in leaves was also different between different species,only some tree species leaf SPAD had significant difference among soil phosphorus forms.(4)There was a significantly positive correlation between Pn,Gs and Tr,while Pn was significantly negative correlated with Ci(p<0.001),indicating that photosynthesis was mainly restricted by non stomatal factors.Pn was significantly positive correlated with ground diameter increment in deep soil and topsoil treatment,while there was no significant correlation between photosynthetic index and growth index in the treatment of mixed soil and litter.