Studies On Effects Of Municipal Sewage Sludge On Forestland

In order to investigate the effects of advanced anaerobic digestion municipal sewage sludge emerging in recent years on plain ecological forest in Beijing and provide a scientific theoretical basis for the promotion of municipal sewage sludge in forestry,a randomized block experiment was designed with six application levels to measure tree growth,leaf chlorophyll and nutrient content,basic physicochemical properties,nutrient content and accumulation of heavy metals in forest soil of Pinus tabulaeformis,Ulmus pumila,Sophora japonica and Ginkgo biloba,four of main afforestation species in Beijing.The main research results obtained are as follows:(1)The crown growth rate of Pinus tabulaeformis was significantly increased after fertilization and reached the maximum when fertilizer rate was 15kg/plant.The DBH and height growth of Ulmus pumila was significantly increased after fertilization,which reached the maximum when fertilizer rate was 60kg/plant.A significant increase was found in DBH and crown growth of Sophora japonica,and the increase range was highest when fertilizer rate was 45kg/plant.The annual growth rate in DBH of Ginkgo biloba increased with fertilizer application rate increasing,and reached the maximum when fertilizer rate was 75kg/plant.(2)Fertilization had no significant effect on chlorophyll synthesis in needles of Pinus tabulaeformis within one year.In July,which trees grows vigorously,the contents of chlorophyll-a and chlorophyll-b in leaves of Ulmus pumila,Sophora japonica and Ginkgo biloba in fertilization treatments were higher than the control,which reached the maximum when fertilizer rate were 60kg/plant in Ulmus pumila and 75kg/plant in Sophora japonica and Ginkgo biloba.There were a negative relation between chlorophyll content and the ratio of chlorophyll a to b in leaves of Ulmus pumila and Sophora japonica.(3)The nitrogen content in needles of Pinus tabulaeformis was significantly increased after fertilization,and reached the maximum when fertilizer rate was 45kg/plant.A negative impacts was found on phosphorus accumulation in leaves of Ulmus pumila and Sophora japonica when fertilizer rate exceeded 45kg/plant and 30kg/plant respectively.Fertilization had no significant effect on potassium accumulation in needles or leaves of four tree species.(4)Fertilization had significant effects on soil physical properties of Ulmus pumila and Sophora japonica forests.The soil bulk density decreased and the soil maximum,capillary and field water capacity and soil porosity of Ulmus pumila forest were increased after fertilization.And the soil capillary and field water capacity of Sophora japonica forest were increased after fertilization.The soil structure was improved after fertilization and it was most beneficial when the fertilization amount was 75kg/plant.The soil electrical conductivity of each tree species forest was within the normal range after fertilization,which had no negative effect on tree growth.(5)The soil pH and cation exchange capacity of each tree species forest were not significantly changed after fertilization.Fertilization had significant effects on the content of soil organic matter and available nitrogen of Ulmus pumila forest and it was most beneficial when the fertilization amount was 75kg/plant,while the content of soil organic matter of Sophora japonica forest decreased significantly when fertilizer rate was 30 and 45 kg/plant.(6)PCA was used to determine the optimum application rate of municipal sewage sludge for the tree growth of various tree species:Pinus tabulaeformis 45kg/plant,Ulmus pumila 60kg/plant,Sophora japonica 60kg/plant,Ginkgo biloba 75kg/plant.(7)The heavy metals Cu,Zn,Cd and Hg in soil of each tree species forestland did not accumulate significantly.However,the Cd content in soil of Pinus tabulaeformis,Ulmus pumila and Ginkgo biloba forestland exceeded soil risk screening value.Evaluation results of soil heavy metal single factor pollution index method and the Nemero comprehensive index method showed that the warning or mild pollution level and it is necessary to strengthen monitoring.Evaluation result of potential ecological risk index method showed that the four heavy metals had lower risk of comprehensive ecological hazard to forestland soil of Pinus tabulaeformis,Ulmus pumila,Sophora japonica and Ginkgo biloba.