Optimizing Peri-URban Ecosystems (PURE) to re-couple urban-rural symbiosis

Highlights

• Rapid urbanization, along with economic development, is dramatically changing the balance of biogeochemical cycles and impacting ecosystem services;

• Nutrient stocks are being enriched, through human bio-concentration, but are not being returned to support primary production in rural/peri-urban environments;

• We propose the concept of the Peri-URban ecosystem (PURE) to address the issue of sustainable urbanization;

• The framework of Critical Zone science is integrated into the management of PURE.

Abstract

Globally, rapid urbanization, along with economic development, is dramatically changing the balance of biogeochemical cycles, impacting upon ecosystem services and impinging on United Nation global sustainability goals (inter alia: sustainable cities and communities; responsible consumption and production; good health and well-being; clean water and sanitation, and; to protect and conserve life on land and below water). A key feature of the urban ecosystems is that nutrient stocks, carbon (C), nitrogen (N) and phosphorus (P), are being enriched. Furthermore, urban ecosystems are highly engineered, biogeochemical cycling of nutrients within urban ecosystems is spatially segregated, and nutrients exported (e.g. in food) from rural/peri-urban areas are not being returned to support primary production in these environments. To redress these imbalances we propose the concept of the Peri-URban Ecosystem (PURE). Through the merging of conceptual approaches that relate to Critical Zone science and the dynamics of successional climax PURE serves at the symbiotic interface between rural/natural and urban ecosystems and allow re-coupling of resource flows. PURE provides a framework for tackling the most pressing of societal challenges and supporting global sustainability goals.

Introduction

Rapid urbanization, in many parts of the world, is driven by the desire for economic improvement coupled with the diminished employment opportunities in rural regions. As a consequence of unprecedented urbanization, globally more than 50% of the world population now live in cities (Grimm et al., 2008). The trade-off of urbanization is that less people now produce our food with an associated intensification of production, and agricultural land around metropolitan boundaries is being sealed over for buildings and transport infrastructure. Now, more than ever, the understanding and management of urban ecosystems have become an essential component of sustainable development.

A key feature of urban ecosystems is that nutrient stocks, carbon (C), nitrogen (N) and phosphorus (P), are being imported into urban ecosystem (through both natural and anthropogenic pathways). Significantly, these nutrients are not being returned to support primary production in rural/peri-urban environments from where they originated. For example, the food production required to sustain a city's population typically takes place in rural environments, but nutrient-rich wastes (resulting from food consumption) are emitted and processed in urban settings. Thus, reuse of urban nutrient wastes in rural and peri-urban environments is precluded from sustaining further production because: these wastes, and their associated nutrients, are often lost due to their discharge to water courses or incorporation in landfills; inadequate sewage collecting infrastructure and wastewater treatment approaches (that might realize suitable products to support soil improvement), and; a lack of mechanisms to return nutrients recovered from the urban environment to their point of origin. Overall, this cycle perpetuates a net gain of nutrients in the urban environment and a commensurate loss of nutrients from rural/peri-urban environments.

In order to redress nutrient losses in rural/peri-urban environments, and to sustain food-supply, chemical fertilizers are required to replenish this nutrient deficit. While this ‘fixes’ the soil nutrient problem the current use patterns of chemical fertilizers are unsustainable. Firstly, these practices result in the increased likelihood of nutrients being leached form soil into watercourses and causing damage to aquatic environments and additionally contributing to the rural to urban efflux of nutrients. Secondly, fertilizer production is heavily reliant upon with fossil fuels and as consequence production of inorganic fertilizers has a large carbon-footprint.