Assessment of nitrogen and phosphorus flows in agricultural and urban systems in a small island under limited data availability
Introduction
Cities are centres of resource consumption and waste production. Urban systems have been compared to organisms or ecosystems that have a metabolism. Kennedy et al. (2007) defined this urban metabolism as the technical and socio-economic processes that occur in cities, resulting in growth, production of energy and waste. It has been suggested that this metabolism of cities is mainly linear or throughput oriented, but should be changed to a more circular approach in which resources are used efficiently and reused as much as possible (Girardet, 2004, Agudelo-Vera et al., 2012). In particular, the reuse of nutrients such as N and P from urban areas has been suggested as an option that makes it possible to reduce environmental pressures from nutrient losses. Reuse of these nutrients is crucial because the fossil fuel based energy used for production of N-fertilizer via the Haber-Bosch process is approximately 37–45 kJ/gN (Maurer et al., 2003). The global energy requirement for this process is equal to about 1% of the world's total annual energy supply (Smith, 2002). P-fertilizer is obtained from mining phosphate rock, which is a finite and non-renewable resource that is estimated to be depleted in the next 50 to 400 years (Cordell et al., 2009, Sattari et al., 2012, Scholz et al., 2013, Reijnders, 2014).
Cities rely on their hinterlands for food production. The word hinterland is originating from German and literally means the “land behind” and is defined as the region, economically tied to an urban area (Baccini and Brunner, 2012). In the present globalised economy, this urban hinterland is extended to the entire globe. Therefore, it is hard to progress towards a so-called circular or reuse oriented city system where resources, such as the non-renewable P, can be continuously recycled. For example, cities rely on imported food for human consumption, and fertilizers containing N and P for agricultural production (e.g. P is mainly sourced in Morocco and China) (Ma et al., 2010, van Dijk et al., 2016). By recycling these resources locally from domestic waste and wastewater and reusing them in nearby agricultural production, the potential loss of N and P can be reduced and the production and mining of nutrients reduced. Progressing towards this circular system is further challenging, because of the large number of agents involved in this system change; these actors include: food supplier, waste producer, and farmers at the local scale (Fernandez-Mena et al., 2016).
The problems that cities face are even more amplified on small islands (Deschenes and Chertow, 2004). They represent physically constrained systems with unique challenges that are characterized by small size, insularity, remoteness, proneness to natural disasters, social isolation, and external dependency (Briguglio, 1995, Méheux et al., 2007, Saint Ville et al., 2015). Because of limited resource availability, most resources in small islands have to be imported for a large part of their domestic needs (Krausmann et al., 2014). Furthermore, the terrestrial ecosystems have a limited buffering capacity as there are few or no surface water systems to attenuate pollution with N and P, before entering the marine ecosystem. In the marine ecosystems, elevated N and P concentrations cause eutrophication. This can lead to decreased water transparency, extinction of fish species, death of coral reefs, change of the zooplankton community and the emergence of toxic phytoplankton species (Pinto-Coelho and Bezerra-Neto, 2005, Howarth and Marino, 2006, Martinelli et al., 2006). In addition, the leaching of nutrients will threaten the quality of the small island's groundwater lenses (Dillon, 1997). This makes small islands highly vulnerable to both global economic change and domestic environmental degradation. Hence, the concept of reusing N and P to protect the marine ecosystem and to achieve self-sufficiency in food production is especially appealing to small islands (Douglas, 2006, Forster et al., 2011).
A key requirement for development and planning of reuse is a good understanding of the resource flows through urban systems and their hinterlands. This requires data about the urban system, its hinterland and its sub-systems (Billen et al., 2012). Billen et al. (2012) investigated the issue of closing nutrient cycles in different cities and indicated the necessity to connect urban and hinterland systems. However, the data for closing the nutrient cycles is often not readily available, in particular when investigating the interlinkages between cities and their hinterland. Montangero et al. (2007) indicated that one of the constraints to the quantification of N and P flows is related to the difficulty of obtaining adequate data. A number of studies, therefore, aim to provide methods to conduct Material or Substance Flow Analysis under uncertain or limited data situation (Huang et al., 2007, Montangero and Belevi, 2008, Do-Thu et al., 2011, Espinosa and Otterpohl, 2014). In these studies, the methodology of Material Flow Analysis (MFA) and Substance Flow Analysis (SFA) has been adapted to assess urban water management in Kun Ming City, China (Huang et al., 2007), to optimise nutrient management in environmental sanitation systems in the urban context of Hanoi City, Vietnam (Montangero and Belevi, 2008), to assess nutrient management in the rural area of Hoang Tay and Nhat Tan communes, Vietnam (Do-Thu et al., 2011), and to assess urban water and wastewater management system in the city of Tepic, Mexico (Espinosa and Otterpohl, 2014). The methodology applied in these studies relies on the maximum use of incomplete local data, and the use of data retrieved from literature or expert judgement. However, the limitation of these studies is that the agricultural system component is not or not well described, because it was not included in the system boundaries or because data was very difficult to obtain.
SFAs have been used to quantify the loss of N and P flows at different spatial scales, but have not been applied to small islands to couple urban-agricultural systems. For example, the flows of N and P related to agricultural systems have been studied at global (Liu et al., 2008, Bouwman et al., 2009), national (Antikainen et al., 2005, Chen et al., 2008, Smit et al., 2010, Ott and Rechberger, 2012, Senthilkumar et al., 2012, Cooper and Carliell-Marquet, 2013, Smit et al., 2015), or city level (Schmid Neset et al., 2008, Li et al., 2011, Wu et al., 2014). Moreover, the SFA and MFA methods have been applied to study N and P flows related to sanitation systems in urban areas of developed countries (Belevi, 2002, Sokka et al., 2004, Meinzinger et al., 2007) and developing countries (Huang et al., 2007, Meinzinger et al., 2009).
The objective of this study is to develop an SFA approach for the assessment of coupled agricultural and urban systems under limited data availability in a small island. The island of St. Eustatius in the Caribbean was used as a case study. The developed approach aims to provide useful information for policy makers to improve nutrient (N and P) management by identifying the source of the nutrient losses and stocks that are potentially available for agricultural production.
Section snippets
Description of the study area
St. Eustatius is a small tropical island in the Caribbean and is since 10th October 2010 officially a special municipality of the Netherlands. Formerly, St. Eustatius was part of the Netherland Antilles, which was a constituent country of the kingdom of the Netherlands. The island has a total area of 21 km2 and a population of 3897 people in 2013 (CBS, 2014). Geologically, the island has mountain-like areas in the south and north (Fig. 1). The south is characterized by the 600 meter-high dormant
Overall balance
St. Eustatius receives a total input flow of 65,304 ± 8% kg N/year and 3861 ± 11% kg P/year, with a total output flow of 59,890 ± 10% kg N/year and 356 ± 20% kg P/year (Fig. 3, Fig. 4). Therefore, a net stock change of 5414 ± 67% for N and 3505 ± 12% for P takes place annually. The natural input flows to the system are associated with the N-biological fixation (41,430 ± 12% kg N/year) and atmospheric deposition (1591 ± 27% kg N/year and 105 ± 25% kg P/year). The main anthropogenic N and P inputs to the system are via imported
Comparison with other SFA studies
Comparison of the results with other SFA studies shows that the results for St. Eustatius are not well comparable (Table 3). Only for the Net stock for P are the values comparable. These differences indicate the specific characteristics of small islands such as St. Eustatius. The very low agricultural input and the very low imported mineral fertilizer (N&P) provide evidence for the subsistence agriculture on the island. Even compared to cities such as Bangkok, which have a relatively high
Conclusion
The SFA approach developed in this study is considered as a first step to analyse the actual problems related to nutrient management. As a next step, it allows for the identification of critical intervention points and mitigation strategies for reducing N and P nutrient taking urban-rural development policies on the island into account. Moreover, the results indicate that most N and P loss in St. Eustatius is through erosion/run-off, leaching and gas emission. Accumulation of N and P takes
Acknowledgement
This research was carried out within the project of TripleP@Sea project of Wageningen University (KB-14-007) funded by the Ministry of Economic Affairs of the Kingdom of the Netherlands. We also thank the local government of St. Eustatius and local farmers for their support in data collection.
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