Spatial Variation Patterns Of Nutritional And Heavy Metal Elements In Sympatric Poplar And Moss Samples And Ecological Indicators

Poplar has significant ecological and economic value.It is an important timber species in China and also has key roles in maintaining water and soil,conserving water sources,sequestrating carbon,and maintaining the integrity and stability of forest ecosystem functions.Mosses are the middle and lower groups of higher plants,without vascular tissue and true roots;stems and leaves compose a single or a few layers of cells,and there is either no waxy cuticle or only an underdeveloped cuticle on the surface of the plant body,mainly on the non-soil surface.The absorption of water and nutrients from the environment by moss renders them an effective indicator organism for monitoring air pollution.Poplar plantations are“purifiers”and“indicators”of air pollution,whereas mosses are more sensitive to air pollution than poplars,and have a different way of nutrient absorption than poplars.Therefore,here we explored the spatial pattern of poplar and moss in Suqian City,and their indications to the health of the ecological environment.We investigated Haplocladium microphyllum（Hedw.）Broth.,（hereinafter referred to as“moss”）tissue and Populus×canadensis‘Nanlin895’（hereinafter referred to as“poplar”）in terms of their leaf ecological stoichiometry（ES）characteristics,involving carbon（C）,nitrogen（N）,phosphorus（P）,and potassium（K）.We determined the nitrogen isotopes(δ¹⁵N)present in plant samples and the leaves of heavy metal elements in moss tissue and poplar leaves,including cadmium（Cd）,chromium（Cr）,copper（Cu）,nickel（Ni）,lead（Pb）,vanadium（V）,and zinc（Zn）.Comparing the C,N,P,K,and heavy metal content of poplar leaves and moss tissues revealed differences in nutrient uptake and heavy metal enrichment and spatial variability of the two species.Based on the N concentration andδ¹⁵N in moss tissues,this species could be used to monitor the atmospheric N deposition levels,possible pollution sources and potential ecological risks in Suqian City;in contrast,the heavy metal content in moss tissue could be used to assess the level of heavy metal pollution in the atmosphere and its possible pollution sources in the study area.The main aim of the study was to provide a theoretical basis for the prevention and management of environmental pollution in Suqian City.The main findings were as follows:（1）The C and P content in moss tissues was higher than in poplar leaves,whereas the N and K content was lower.N:P<14,indicating that the growth of moss and poplar is restricted by N availability.There was a significant positive correlation between C content in moss tissue and soil C content,between N content in poplar leaves and soil N content,and between P content and soil P content（all,P<0.05）.The phenotypic plasticity index（PPI）of moss tissue was 0.5-0.76,and whereas that of poplar leaves was 0.5-0.71.Thus,moss tissues had higher plasticity compared with poplar leaves.The coefficients of variation（CV）of moss tissues and poplar leaves were K>P>N>C,with C showing weak variability and N,P and K showing moderate variability.The homeostasis index（H）indicated that N,P,C:N,C:P,C:K,and N:P of moss tissue were“strictly homeostatic”,C and P:K were“homeostatic”,K and N were“homeostatic”,and K was“weakly homeostatic”,in poplar leaves,C,N,P,K,C:N,C:P,C:K,N:K,and P:K were“strictly homeostatic”,and N:P was“weakly homeostatic”,Thus poplar homeostasis was stronger than that of moss.In general,both species had adapt to changes in the environment by adjusting their own element balance,with moss having a unique ecological chemical adaptation mechanism,maintaining a high P content,low N:P,and a wider plastic adaptation strategy,resulting strong adaptability.（2）The average N concentration of moss tissue was 1.6±0.39%,whereas that of poplar leaves was 1.94±0.44%.The N content of moss tissue and poplar leaves showed a low spatial distribution in the northern sampling sites and a high distribution in the southern sites,the concentration of N in poplar leaves was significantly higher than that in moss tissue.The average value ofδ¹⁵N in moss tissue was-4.89±0.29‰,whereas that in poplar leaves was+1.22±0.46‰.The values ofδ¹⁵N in moss tissue and poplar leaves were low in the northwestern sampling sites and high in the southern sites,which was consistent with the spatial distribution pattern of leaf N content.There was no significant correlation between N concentration in moss tissue and soil N concentration,whereas was a significant correlation between N concentration in poplar leaves and soil N concentration（P<0.05）,indicating that the N concentration in moss tissue was mainly derived from atmospheric N deposition,whereas that in poplar leaves was affected by soil and soil N concentration,indicating a combined effect of atmospheric N deposition.The range of variation ofδ15N in moss tissue was-8.38‰to+1.32‰,indicating that agricultural activities and urban wastewater and waste were the main sources of atmospheric N deposition.The NH₄⁺-N and NO₃^--N in the atmospheric wet N deposition in Suqian City accounted for 57%and 15%of the TN deposition,respectively,and the NH₄⁺-N/NO₃^--N ratio was 4.36,further indicating that agricultural activities are the main contributor to the atmospheric N deposition in Suqian City.In terms of the N concentration in moss tissue,atmospheric N deposition was 16.75 kg N ha^-1 yr^-1,with 43%of areas exceedeing the critical load of land N deposition(20 kg N ha^-1 yr^-1),with hot spots mainly concentrated in Sihong County.（3）The Cd content of moss tissue was similar to that of poplar leaves,whereas that of the other six heavy metals was significantly higher than that of poplar leaves(P<0.05.The bioconcentration factor（BCF）showed that both moss tissue and poplar leaves showed strong enrichment for Cd,whereas the enrichment factor for Cr was the lowest.The BCF of moss tissue was 1-100 times that of poplar leaves.Pearson correlation analysis showed that the heavy metals in moss tissue and poplar leaves were mainly from atmospheric deposition.There were differences in the concentration of heavy metals in moss tissue and poplar leaves in the same sampling area,mainly because of the structure of plant leaves and the physiological and biochemical characteristics of leaf cells.（4）Spatial analysis showed that most of the high-concentration heavy metal elements occurred in the Sucheng and Suyu Districts.Pearson correlation analysis showed that Cd-V was significantly negatively correlated（P<0.05）,and Cr-Ni,Cr-Pb,Cu-Pb,Cu-Zn,and Ni-V were significantly positively correlated（P<0.01）,indicating that they came from the same source.Principal component analysis（PCA）and positive definite matrix factor（PMF）models indicated that heavy metals in Suqian City were mainly derived from natural soil dust,industrial activities,traffic emissions and agricultural activities.Contamination factor（CF）analysis showed that Cd was moderately polluting;Cu was suspected to be polluting;Cr,Ni,Pb,V and Zn were mildly polluting.The pollution load index(₄0)))indicated that Cd,Cr,Cu,Ni,Pb,V,and Zn in the study area were moderately polluting to unpolluting,moderately polluting or moderately to highly polluting,with the most polluted area being northwest Suqian City.The potential ecological hazard index(⁴))of a single heavy metal showed that except for Cd,which was a high ecological hazard,the remaining elements were all light ecological hazards.₉0))and potential ecological hazard index（RI）indicated moderate to uncontaminated levels of Cd,Cr,Cu,Ni,Pb,V,and Zn throughout the study area.In conclusion,moss had higher ecological adaptability than poplar,and was more sensitive to atmospheric N and heavy metal deposition,which could be used as a supplementary means to assess regional pollution,while poplar was affected by both leaf cell structure and soil,indicating that the effect was not as good as moss.