Characteristics Of Plant-Soil Stoichiometry,C And N Stable Isotopes After Forest Conversion

Conversion of natural secondary forest to plantation is a common management practice in subtropical China.However,the impacts of forest conversion on plant growth,soil fertility and stability have not been well quantified.In order to understand the characteristics of stoichiometry,carbon and nitrogen stable isotope composition in forest ecosystem,and to provide scientific theoretical basis for ecological restoration and sustainable development after forest conversion,three forest types were selected in this study.The relationship between aboveground leaf and litter,and belowground fine root and soil nutrients include their stoichiometric ratio and stable carbon(δ¹³C)and nitrogen(δ¹⁵N)isotopes were investigated after the Pinus massoniana secondary forest（PM-SF）was converted to Pinus elliottii plantation（PE-P）and Cunninghamia lanceolata plantation（CL-P）in Caijiaqiao National Forest Farm.The comprehensive effects of forest conversion on plants and soil were examined.The results are as follows.（1）The effect of forest conversion on plantδ¹³C andδ¹⁵N values were very significant.Theδ¹³C values of leaves,litter and fine roots in Pinus massoniana secondary forest were higher than those in the Pinus elliottii plantation and Cunninghamia lanceolata plantation.Theδ¹³C value in fine root was the highest while the lowestδ¹³C value was in litter.The values ofδ¹⁵N in leaves and litter of Pinus massoniana secondary forest were higher than those of the two kinds of plantations,and the value ofδ¹⁵N in fine root was lower than that in the two kinds of plantation.The foliar and litter TN,TN/TP and fine root TN/TP and TC/TP of the plantations were lower than those of the secondary forest after forest-type conversion,while the foliar TC/TN and fine root volume,SSA,SRL,TN and TP of the plantations were higher than those of the secondary forest.（2）Theδ¹³C value of soil did not change significantly after forest conversion,while increased significantly for theδ¹⁵N value of soil.The soil organic carbon turnover was higher in the secondary forest than in the two kinds of plantations（the absolute value of regression slopeβdecreased）,and the soil stability was stronger.Except for SWC,p H and SOC/TN,there were significant negative correlations between soilδ¹³C and other factors,and significant negative correlations between soilδ¹⁵N and other factors except p H and SWC,and a positive correlation between soilδ¹⁵N and SWC.The contents of SOC/TN,SOC and TN in soil decreased after forest conversion.The soil NO₃^--N of secondary forest was significantly higher than those of plantations,the positive correlations of soil NH₄⁺-N,NO₃^--N with SOC,DOC,TN,MBC,MBN.（3）Based on the PCA analysis of plant and soil factors,it was found that the differences of plant characteristics after forest conversion were mainly reflected in the stoichiometry related to leaf and litter N and P,and the changes of fine root nutrients and forms,while the differences of soil factors were mainly between Pinus massoniana secondary forest and Cunninghamia lanceolata plantation,and the influencing factors were more complex.（4）After forest conversion,the values ofΔδ¹³C andΔδ¹⁵N of Pinus massoniana secondary forest were higher than those of plantations.The relationship between plant and soil was discussed by RDA analysis.The results showed that the positive correlations of plant leafδ¹³C withΔδ¹³C and soil NO₃^--N,the positive correlations of leafδ¹⁵N with soil SOC,TN and SOC/TN,and the negative correlations of soilδ¹⁵N withΔδ¹⁵N and leaf TN/TP.The positive selection of RDA ordering variables showed thatΔδ¹³C was mainly determined byδ¹³C in plant leaves,whileΔδ¹⁵N was affected by the interaction between plant and soilδ¹⁵N.After comprehensive analysis,it revealed that the change ofΔδ¹³C was mainly affected by plant factors,and the change ofΔδ¹⁵N was mainly affected by soil factors.Above all,the fractionation of carbon and nitrogen isotopes by plants and soil was affected by many factors,such as forest type,interaction between plant and soil,environment and human disturbance.The plant growth of plantations was better,and the competition was more fierce than the secondary forest Different forest types,stand structure and management measures after forest conversion might lead to the loss of soil nutrient and to destroy soil stability,thus reducing the soil fertility of the forest ecosystems,which would have a great impact on plants and soil in a short time.