Contents And Isotopes Of Nitrogen In Lichens For Indicating Atmospheric Nitrogen Deposition In The Beijing-Tianjin-Hebei Area

Since the industrial revolution,the emission of anthropogenic active nitrogen has increased exponentially,resulting in a significant increase in atmospheric nitrogen deposition from point to regional scales,which in turn triggers a series of ecological and environmental problems.Hence,it’s critical to analyze the flux and source of atmospheric nitrogen deposition for formulating the emission reduction policy of reactive nitrogen pollutants and evaluating the ecological effects of nitrogen deposition.Previous observation networks,model simulations and other methods have provided the information of nitrogen deposition flux at points and regions.However,due to the complex chemical forms of atmospheric nitrogen deposition and the high cost of direct observation,many areas can only obtain information on the deposition rate or the lower-resolution deposition flux in the region through sparse observation points and limited observation time series.There are many uncertainties in the flux and source contribution of regional nitrogen deposition.Lichen has been recognized as an indicator plant for air pollution.There are also a few studies abroad that use lichen nitrogen content and nitrogen isotopes to evaluate the levels of nitrogen pollution and main sources,but there are no reports in China.In particular,few studies have analyzed in detail the effects of growth substrates,species types,and geographical environment gradients on the nitrogen content and nitrogen isotope records of lichens,resulting that how to quantitatively reconstruct the atmospheric nitrogen deposition flux and source contribution is still a key issue.Besides,whether the nitrogen content and isotopes of lichens can respond to the the levels and sources of nitrogen deposition in areas with high nitrogen pollution is still uncertain.Based on the above background and questions,this study investigated lichen carbon（C）,nitrogen（N）,phosphorus（P）,and sulfur（S）content and nitrogen isotope(δ¹⁵N)values in a high pollution area in northern China.We analyzed the change characteristics and main controlling factors of these indicators with the growth substrate and related environmental factors,and explored the nutrient status and limitation of lichens and the mechanism of nitrogen isotope changes.Moreover,combining with the quantitative relationships between lichen N contents and wet N deposition fluxes,S contents and wet sulfate deposition,as well as the quantitative relationships between lichenδ¹⁵N values and the ratios of ammonium（NH₄⁺）to nitrate（NO₃^-）in the wet deposition based on previous studies,we evaluated the fluxes and changes of N and S deposition in the northern forest area of Beijing-Tianjin-Hebei（BTH）region.Beside,we established a new method to reconstruct site-basedδ¹⁵N values of NHx and NOy deposition using lichenδ¹⁵N and then to differentiate the relatively contributions of major N emission sources.The main results are as follows:1.The C,N,and P contents of lichens in the study area showed significant positive correlation（p<0.01）,showing that the lichen symbionts could fix C,N,P and assimilate C,N,P during photosynthesis even under high nitrogen deposition and there was a mutually promoting coupling relationship between nutrient elements.The high lichen N contents,C/P values,and N/P values and the low P contents indicated that the high nitrogen absorption and accumulation of lichens can respond to changes in nitrogen deposition levels.Lichen C contents and C/N values were not significantly different in different growth substrates.N,P contents,C/P,N/P values,andδ¹⁵N values were significantly different in different growth substrates.According to the general linear model,the growth substrate had less explanation for the lichen element content and stoichiometric ratio compared with nitrogen deposition.According to the results of multiple stepwise linear regression,the lichen N contents were significantly related to the MAP（P<0.01）.In addition,the N contents andδ¹⁵N values were not significantly related to total nitrogen deposition,indicating that precipitation became a key environmental factor affecting the metabolic activity of lichens in a nitrogen-rich environment,which was consistent with the unique physiological characteristics of lichens as poikilohydric plants.Besides,the monthly variations of the N contents of lichens were not significant,which indicated lichen N contents could be used to estimate the flux of nitrogen deposition.However,the lichenδ¹⁵N values in the cold season were higher than that in the warm season,reflecting the seasonal differences in the emission of active nitrogen from coal burning,microorganisms and volatile sources,which indicated thatδ¹⁵N values could be used to interpret the source contributions and spatial variations of nitrogen deposition.2.Lichen N（1.0-3.9%,2.5±0.6%）and S（0.09-0.33%,0.21±0.06%）decreased significantly with distances from the polluted center area.The N contents were significantly higher than that of forest lichens in Europe and the United States,and were comparable to that in cities and farmland.Estimated wet inorganic N and sulfate deposition（29.0±6.1 kg-N/ha/yr and 25.8±7.9 kg-S/ha/yr,respectively）based on integrated relationships in previous studies were comparable with existing observations in the study area.These results indicate that lichen N and S are still sensitive to deposition under high pollution and applicable to estimate deposition levels.In addition,the high N and S contents in lichens and the wet nitrogen and sulfate deposition levels above the forest threshold indicated that the forests in northern BTH region were facing the risk of acidification.3.Lichenδ¹⁵N varied between-12.1--4.1‰and averaged-7.1±2.0‰among our study sites,and relatively more stable deposition levels of NH₄⁺（11.5±5.1 kg-N/ha/yr）than NO₃^-（17.6±8.3 kg-N/ha/yr）were identified along the studying urban-rural transect.Based on isotope mass balance model,reconstructedδ¹⁵N values of NH_x and NO_y deposition averaged-12.8±1.0‰and-3.2±0.5‰,respectively.Further source contribution analyses revealed significant contributions of volatilization ammonia（66±3%）from fertilization and waste to NH_x deposition and non-fossil fuel N oxides（62±7%）from biomass burnings and microbial N cycles to NO_y deposition.These results provided an effective and low-cost assessment of regional N pollution,which is rather difficult to be obtained through direct samplings of atmospheric deposition.The Beijing-Tianjin-Hebei region of China is a hotspot with high nitrogen emissions and high deposition fluxes in the world.These findings have enriched the application of lichens to indicate atmospheric nitrogen and sulfur deposition in polluted areas.Moreover,theδ¹⁵N values of lichens were used for the first time to estimate theδ¹⁵N values of NH_x and NO_y in bulk deposition and then differentiate the relative contribution of different pollution sources based on the isotope mixing model.These enrich the tracing theory of lichenδ¹⁵N value,and provide a basis for biological and ecological effects for the prevention and control of atmospheric nitrogen pollution on a regional scale.Moreover,this study enriches the C,N,P content of primary plant lichens and their element stoichiometry characteristics under high background nitrogen deposition,providing a data basis for understanding the community structure of protected lichens and plant protection.