Urbanization Influences On Nitrogen Pollution And Cycling At The Forest-Atmosphere-Surface Water Interface Along Two Coastal Rivers In Southeastern China

Sustainable provision of many vital ecosystem services in coastal areas has been increasingly attracting people from non-coastal areas.Consequently,increasing numbers of coastal areas globally are currently experiencing rapid urbanization,which has often been linked to alterations in environmental nitrogen?N?pollution.Given that N saturation will have particularly profound consequences in coastal ecosystems and their associated services,it is important to understand how it responds to increasing urbanization.Although previous studies have investigated the influence urbanization has on the patterns of N saturation in different ecosystems worldwide,the findings from these studies revealed conflicting patterns?increase,decrease or no clear patterns?across urbanization gradients.In this regard,understanding potential mechanisms governing N pollution and cycling responses to increasing levels of urbanization across coastal ecosystems is of the highest ecological and societal significance.The aim of this study was to examine the influence of urbanization on N pollution and its biogeochemical cycling at the forest-atmosphere-surface water interface along the urban Bailongjiang River?BJR?and suburban Wulongjiang River?WJR?,which are the two branches of Fuzhou section of the Minjiang River Estuary in southern China.Specifically,the study aimed to;First,explore the patterns of N availability and cycling in the subtropical forests along the coastal BJR and WJR with the aid of stable isotope technique.In this objective,different soil and foliar N variables were analysed including soil and foliar N concentrations and isotopes as well as soil N cycling rates.Soil total N concentrations,inorganic N concentrations?ammonium and nitrate?,N cycling rates?net N mineralization and net nitrification?and soil?¹⁵N values were significantly greater in BJR forests than in WJR forests.While foliar?¹⁵N values in BJR forests were more than2.5 times higher compared to those in WJR forests,foliar N concentrations were not substantially different between the urban BJR and suburban WJR forests.Along the Bailongjiang and Wulongjiang rivers,foliar?¹⁵N was positively related to soil?¹⁵N,and the latter variable in turn was positively related to soil nitrate concentrations,net N mineralization and net nitrification rates but not to soil ammonium concentrations.Second,investigate the responses of forest soil microbial communities and their activities to urbanization-induced alterations in soil N availability along the BJR and WJR ecosystems.Soil microbial biomass carbon?MBC?,microbial biomass N?MBN?,MBC/MBN,microbial respiration rate?SR?,metabolic quotient?q CO₂?as well as soil C and N availability were among the soil variables measured in this objective.Although measurement of the soil C availability was not the primary focus of this study,its inclusion in this objective provided additional useful information concerning the relationships between urbanization,soil nutrients and soil microorganisms.Soil MBC,MBN and SR values decreased considerably from the urban BJR forest towards the suburban WJR forest and from the 0-10 cm soil depth towards the 10-20 cm soil depth.Forests of the BJR and WJR were not significantly different in their soil MBC/MBN and q CO₂ values.Soil MBC and MBN values increased with increasing soil C and N availability,and these four soil variables ultimately were positively related to SR values across two rivers and depths.Third,determine whether urbanization contributes to changes in patterns and dominance of different N forms in bulk precipitation?BP?and throughfall precipitation?TP?across seasons?summer,autumn,winter and spring?in subtropical forest ecosystems along the Bailongjiang and Wulongjiang rivers.Concentrations of dissolved inorganic N?DIN?,dissolved organic N?DON?and consequently total dissolved N?TDN?in precipitation along the urban BJR forests were greatly enhanced than in precipitation along the suburban WJR forests.Ratios of ammonium-N?NH₄⁺-N?to nitrate-N?NO₃^--N?in precipitation were 0.77 in BJR forests and 1.14 in WJR forests.DON not only contributed up to 38%of N to the TDN pool but also related positively to all other precipitation N forms across seasons and rivers.Generally,N concentrations were considerably lower in BP compared with TP and in summer precipitation than in precipitation across the other seasons.Soil TN and?¹⁵N and foliar?¹⁵N but not TN values were positively related to precipitation TDN values.Last,assess the patterns and controls of N pollution in urban BJR and suburban WJR surface waters across different seasons.Surface water DIN,DON and TDN concentrations increased significantly from the Wulongjiang River to the Bailongjiang River.Concentrations of different N forms in summer surface waters were often much lower compared to those in winter and spring surface waters.Ratios of surface water two rivers and four seasons.Contributions of NH₄⁺-N,NO₃^--N and DON concentrations and WJR surface waters.Throughout the four seasons,concentrations of the assessed N forms in surface waters of the two rivers were positively related to each other.In addition,N concentrations in BJR and WJR surface waters increased with increasing N concentrations in precipitation and soils along these rivers.This study suggests that?a?rapid urbanization can potentially cause forest N saturation and river N eutrophication in coastal areas;?b?soil N availability plays an important role in determining patterns of soil microorganisms and their activities in urban riparian forests;?c?urban forests across coastal areas are increasingly becoming atmospheric N deposition hotspots,especially nitrate deposition;?d?observed DON contribution to the TDN in both precipitation and river waters necessitates DON inclusion in urban N pollution studies and global N budgets in the future;?e?both forest and river ecosystems in urbanized coastal areas may be at increased risk of experiencing the detrimental ecological effects of urbanization-induced atmospheric N deposition enhancement;and?f?further studies are needed to deepen our scientific understanding of how N pollution and cycling in coastal ecosystems respond to increasing urbanization.