| Urban green space soil is an important component of urban ecosystem and the growth medium of urban vegetation,but with the development of urbanization,the native soil structure of urban green space soil has long changed and cannot provide sufficient fertility for vegetation,and some green space is even contaminated by heavy metals.At present,many research scholars have investigated the soil quality of large cities with more developed economies,but there are fewer studies related to green land soils in small western cities,and some scholars have studied urban green land soils in western China,but few studies have been conducted on fertility and environment at the same time.In order to protect the urban soil ecosystem,a study on the current situation of soil quality of urban green areas in Lhasa can provide scientific basis and theory for the management of urban green area soil in Lhasa.According to the green space classification standard and the actual situation of green space in Lhasa city,105 samples were collected and divided into road green space,park green space,educational green space and residential green space,The physicochemical properties and heavy metal elements of the soil were measured,the improved Nemerow comprehensive evaluation method was selected to analyze the soil fertility quality,and the single factor pollution index method,Nemerow comprehensive pollution index method,ground accumulation pollution index method,potential ecological risk index method,and geostatistical analysis method were used to analyze the soil environmental quality,while the non-parametric independent samples were combined to analyze the variability of indicators among greenfield types,and the multivariate analysis method was used to The analysis of heavy metal sources was carried out.The main conclusions were obtained as follows:（1）The average value of p H of green land soil in Lhasa city is 7.94,which is alkaline,and all four types of green land soils are alkaline.The average value of water content is 12.09%,which is a low level,and the soil water content of different green space types is ranked as:educational green space>park green space>road green space>residential green space.The average value of soil density is 1.48g·cm-3,which exceeds the capacitance limit for normal plant growth,4 types of green space capacity are not in line with the standard restrictions for green planting soil.The average value of organic matter is 19.97 g/kg,which is the lower middle level and lower than the standard limit（>20 g/kg）.Among the different types of green areas,residential green areas belong to the upper middle level,while park green areas,road green areas and educational green areas belong to the lower middle level.The average value of alkaline nitrogen is 19.97 mg/kg,which is low,and it is in the middle to lower level in educational green space,and it is in the low level in park,road and residential green space.The mean value of effective phosphorus was9.11 mg/kg,which was in the middle to lower level,and was in the middle to lower level in road green space and residential green space,and was in the low level in park green space and educational green space.The mean value of potassium potassium was 127.87 mg/kg,which was in the lower middle level,and in the high level in road green space,and in the lower middle level in park,residential and educational green space.The Kruskal-Wallis test showed that greenfield type reached significant differences with effective phosphorus and fast-acting potassium.Spearman correlation results showed that p H was negatively correlated with all fertility factors,positively correlated with capacity,negatively correlated with organic matter and fast-acting potassium,and significantly positively correlated with organic matter,alkaline decomposition nitrogen and effective phosphorus.（2）Fertility evaluation of urban green land soils in Lhasa was conducted by the improved Nemerow integrated index method.The average value of the integrated fertility index was 1.171,which was of medium（average）fertility,with organic matter at a fertile level and alkaline decomposition nitrogen,bulk weight,effective phosphorus and fast-acting potassium at a medium level.Roads,parks and residential green areas are at average fertility level,educational green areas are poor in fertility level,and the soil capacity and fast-acting nutrients in Lhasa urban green areas are limiting factors.（3）The median elemental contents of Cu,Cr,Ni,Zn,Pb,Cd,As and Hg were 1.41,1.02,0.86,1.08,1.09,1.25,0.90 and 0.87 times of the background values of Lhasa city soil,and compared with the background values of Tibetan surface soil,the median elemental contents of Cu,Cr,Ni,Zn,Pb,Cd,As and Hg were 1.42,0.56,0.56,0.95,1.17,1.88,0.96and 3.04 times of the background values of Tibetan surface The median contents of Cu,Cr,Ni,Zn,Pb,Cd,As and Hg were 1.42,0.56,0.56,0.95,1.17,1.88,0.96 and 3.04 times of the background values of Tibetan surface soils.The interpolated distribution maps of Kerry gold showed that the distribution maps of Cu,Zn,Pb,Cd and Hg were similar,and the distribution of Cr,Ni and As elements were similar;Spearman correlation method showed that Cu,Zn,Pb and Cd elements were significantly correlated with each other,which was consistent with the similarity results of spatial distribution maps,indicating that these four elements had the same or similar sources,and Cr,Ni and As were significantly correlated with each other.（4）With the requirement of"green planting soil"as the standard,the single factor pollution index of 8 elements Pi are unpolluted,the comprehensive pollution index of Nemerow is unpolluted,the result of ground accumulation index shows that the average value of 8 elements belong to unpolluted,the average value of potential ecological comprehensive pollution index（19.14）is slight ecological hazard,but far below 150.With the requirement of heavy metal of Lhasa city soil as the standard,except Ni element Pi single factor pollution index mean value is unpolluted,the remaining seven elements belong to slight pollution,Nemerow comprehensive pollution index for light pollution,spatial distribution map shows that the overall comprehensive pollution index for the northern pollution than the southern,ground accumulation index results show that the eight elements mean value is unpolluted,the integrated potential ecological risk index results show that only Hg and Cd elements Eriare moderate ecological hazards,the overall RI value（140.13）of urban green areas in Lhasa city belongs to slight ecological hazards,among which the park green areas are moderate ecological hazards and the other three green areas are low ecological hazards.Cd and Hg are the elements with the highest contribution in all four types of green spaces,indicating that Cd and Hg are the most important ecological risk elements in the study area.（5）The correlation between physicochemical factors and heavy metals showed that p H and bulk weight were negatively correlated with heavy metals,and the increase of organic matter and fast-acting nutrient content could cause the enrichment of soil heavy metals.Both principal component and cluster analysis classified the eight heavy metal elements into three categories,component 1 including Cu,Zn,Pb,Cd,which is inferred to be mainly anthropogenic transport sources,component 2including Cr,Ni,As,which is inferred to be mainly from soil-forming parent material sources,and component 3 being Hg,which may come from a mixture of coal-fired sources and agricultural sources.Therefore,the heavy metal content in Lhasa is mainly influenced by anthropogenic activities,physicochemical content and soil-forming parent material. |
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