Nitrogen Flows and Fate in Urban Landscapes Executive Summery
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12 Chapter: Nitrogen flows and fate in urban landscapes Authors: A. Svirejeva-Hopkins and S. Reis G. Nardoto J. Magid J.-M. Mouchel S.Barles Lex Bouwman Ipek Erzi Marina Kousoulidou Clare Britton Executive Summery . Nature of the problem (science/management/policy): Significant modification of the N cycle is taking place in and around urban areas. Although they take only ~2% of land, but due to their dense population settlement structure, transportation networks, energy use and altered surface characteristics, the change the cycle by first pumping nitrogen from outside (also indirectly in the form of fertiliser needed to produce food to provide for the rapidly growing urban population) , and then dispersing it in the form of pollution through water and air to other ecosystems on much larger areas than their own. Approaches: In order to quantify the fluxes of Nitrogen in and out of a city we use the mass- balance approach, that allows to characterise a city either as a source of nitrogen (i.e. emitting large amounts of N as liquid and solid household waste, automobile exhausts, air pollution from power plants; or a sink of it (importing food, fossil fuel etc., and having fewer emissions to the air and water. Paris metropolitan area is our case study, since it represents a typical European historical capital. Key findings/state of knowledge: Paris metropolitan area went from being a sink of Nitrogen in the 18th and 19th century to the source at present stage. Major changes in city functioning occurred before 1950, but especially the last decades are characterized by an unprecedented amplification of those changes. The major part of N output is attributed to the atmospheric emissions from transport and energy, second place belongs to the air emissions from incineration of solid waste, the third to the ones from the functioning of water treatment plants. The balance number does not take into account atmospheric fertilization, fossil fuel inputs, the Nitrogen stored in landfills, accumulated in sewage sludge. Major uncertainties/challenges: The following uncertainties need better understanding: 1. The mechanisms of dry- deposition processes in urban systems with patchy vegetation; 2. High NOx emissions and the increase in travel distance of smaller particles coming from modern engines; 3. Complex patterns of air flow in the dense build-up areas 4. Urban soil N dynamics, since 1 soil represents a major sink of N in natural ecosystem, but what happens in urban soils due to smooth surfaces (roads etc.) has not been studied much; 4. Factors that control denitrification in urban landscapes are related to the presence of water bodies and green areas within a city, but those areas differ from natural landscapes. They have lower density of biomass and altered decomposition rates. The derival of city-specific coefficients for the quantification of fossil fuel inputs, which would allow us to use national statistics data on urban per capita basis, is needed. Recommendations (research/policy): If we aim to organise sustainable (in relation to N cycle) life in cities of Europe, we have to increase recycling of food and water, minimise household waste either through reusing sewage waters or technologically improving treatment plants, reduce travel by car as much as possible and this eventually could turn urban area from being a source to neutral or even sink state. Minimising food imports through growing some organic foodstuff under controlled management practices and avoiding mineral fertilzers within city’s area will also help. We recommend regional adaptation measures, specifically tailored to individual ecosystems of Europe. Structure: 12.1. Introduction. 12.2 . Physiography and Urban Demography of Europe. 12.2.1. Physiography of the region 12.2.2. Urban Demography of Europe 12.2.3. The City as an ecosystem. 12.3. N-fluxes and the city sub-systems’ functioning 12.3.1. Urban air quality due to combustion of fossil fuels in stationary and mobile sources 12.3.2. Human food sub-system 12.3.3. Sewage system: N in liquid and solid fractions of urban waste. 12.3.4. Urban green and urban soils 12.4. Paris Metropolitan Area (PAM)-case study. 12.4.1. Evolution of the city - Historic development of PAM with regard to its N fluxes 12.4.2 . Nitrogen balance for PAM. 12.5. Conclusions, Uncertainties and the future of sustainable cities . 12. 1 Introduction. In this chapter we ask and will try to answer the following questions: what are the specifics of urban landscapes in Europe and how is N cycle different in urban systems? What are the important issues concerning Nitrogen management in cities of European region? As an example and a fair illustrative case study we choose the city of Paris for a more in depth quantification of terrestrial and atmospheric fluxes. 2 But first of all, why is a dedicated chapter on Nitrogen fluxes in urban systems is required for the ENA? Urbanization is considered one of the most powerful and characteristic anthropogenic forces on Earth in the 21st century. Although, currently, cities occupy 2-2,5 percent of the Earth’s land surface, they are home to over 50 percent of the world’s population. The number of city dwellers grew from a mere 14% in 1900 and is estimated to increase to 60 per cent by the year 2030 (UNCHAS 2002). However, more than 95% of the net increase in the global population will be in cities of the developing world, which will soon approach 80% urbanization level of most industrialized nations today (World Urbanization Prospects (2006). In addition, large cities are continuing to grow to become mega-cities, but again most of them are typical for the developing world, while urbanization processes in Western Europe have reached the so-called stagnation period, and are giving an example of saturated growth. This is however accompanied by the expansion of medium sized cities (~1 mil). The dynamics of European urbanization, the differences between Western and Eastern Europe’s urban patterns and the implications for the terrestrial, water and atmospheric parts of the Nitrogen cycle are discussed in greater detail in the following sections. Nitrogen cycle, being one of the main cycles in “ Biosphera Machina ”, is densely interconnected with the main driving carbon cycle and also with water and oxygen cycles. Urbanization related disturbances in the main driving cycles of the Biosphere: the global carbon cycle and the nitrogen cycle, induced by sprawling urban human activities, lead to global, regional and local environmental problems, i.e. global warming, photochemical smog, stratospheric ozone depletion, soil acidification, nitrate pollution of surface and ground water, eutrophication of the coastal waters. Even though in some cities, urban population might stabilize or even slightly decrease, the actual urban areas are expected to continue their expansion in the 21 st century, accompanied by growing energy production, increased food demand, expanding transportation and industrialization. The demands of high production to feed the urban population alter land cover, biodiversity, and hydrology, both locally and regionally; and urban waste discharge affects local to global biogeochemical cycles and climate. Although agricultural production is by far the largest cause of the doubling in the amount of reactive nitrogen entering the biospheric cycle compared to pre-industrial conditions (Smil (1999), today more than half of the crops produced in rural areas are consumed in urban zones. Transportation and industry are concentrated in urban centers, making them point sources of greenhouse and trace gases, such as NO, NO 2, O 3, HNO 3. (Pataki et al.,2006). Air and in particular water pollution influences nutrient cycling and primary production in adjacent ecosystems, especially since major European cities are located along rivers and coastlines. Nitrogen in solid wastes generated in cities also eventually enters air and 3 water, affecting biogeochemical cycles, while the extent of influence depends on the vectors by which materials are carried (either landfiled or incinerated, for example). In addition, it is important to study urban areas in a context of ENA, since two of 5 identified key threads in ENA, namely water quality and air quality (particulate matter) are important characteristics of urban landscapes’ functioning, and because pollution generation by cities is of increasing concern when urbanization outpaces societal capacity to implement pollution-control measures. WHO has also identified particulate pollution as the most important contributor to ill health within Europe. On top of it, climate change induced heat waves exacerbate the effect of aerosols, changing the size of particles and their travel distance, and are more severe in cities. They impact health and increase mortality of densely settled urban inhabitants. In other words, high densities of people in cities make them focal points of vulnerability to any global environmental change. Particulate matter is typical urban pollutant and although in Western Europe it rarely exceeds 50 µg/m3, in Central and Eastern European cities levels are much higher – often exceeding 100 µg/m3.8. (M. Ezzat et al, 2004). Therefore, it is important to assess the current situation and to forecast the dynamics of Nitrogen biogeochemical functioning of urban landscapes. This chapter will discuss the past and current situation and would touch upon the future trends of the development of Europe’s urban systems that are related to the Nitrogen cycle. 12.2 . Physiography and Urban Demography of Europe. 12.2.1. Physiography of the region. Europe’s location at the heart of the land hemisphere represents maximum efficiency for contact with the rest of the world. Almost nowhere in Europe is far from the water and it has historically been a place of contact between peoples and cultures resulting in the circulation of goods and ideas. The first cities of European civilization were built and developed here.