Can urban P conservation help to prevent the brown devolution?

Abstract

Achieving better understanding phosphorus (P) flows through urban ecosystems is needed to conserve P, as non-renewable phosphate rock deposits become depleted and the global human population increases. A baseline mass flow analysis (MFA) for P developed for the Twin Cities Watershed (TCW, which includes most of the Minneapolis-St. Paul metropolitan region) showed that most P input was stored in the system (65%) or leaked from it (31%); only 4% was deliberately exported as useful products. In a realistic, comprehensive conservation scenario P input was reduced by 15%; deliberate export of P in the form of sewage sludge, food waste, and landscape waste was 68% of P input. In this scenario, increased deliberate export was accomplished by decreasing leakage (to 9% of input) and storage (to 23% of input). If used as agricultural fertilizer, the deliberately exported P in the conservation scenario would support about half of the food production required by the TCW.

Introduction

Since the 1970s, when evidence of the relationship between phosphorus (P) and lake eutrophication became overwhelming (Schindler, 2006), the main goal of phosphorus (P) management in cities has been to control eutrophication. Eutrophication (nutrient enrichment) increases algae abundance, thereby reducing clarity, shifts the dominance of algae toward blue-greens, and depletes hypolimnetic oxygen, causing release of reduced chemicals (H₂S, Mn²⁺, Fe²⁺). Impacts of eutrophication to human well being include reduced recreational enjoyment, reduced property values of lakeshore homes, increased cost of water treatment, and impaired drinking water quality. Exported P also contributes to eutrophication of estuaries, particularly the creation of anoxic zones, which impairs fisheries, often with considerable economic loss (SAB, 2007).

Policies to reduce P pollution from US cities have included expanded use of advanced (“tertiary”) wastewater treatment systems and bans on P-containing detergents in 28 states during the 1970s and 1980s, eventually leading to a voluntary industry phase-out (Litke, 1999). In recent years, some states and local governments have also restricted the use of lawn P fertilizers and/or banned the use of P-containing automatic dishwasher detergents (Rosen and Horgan, 2005, USA Today. 2010. 16 states ban phosphate-laden dishwasher soap, 2010). Farmers have also become far more efficient it their use of P over the past several decades, reducing their input of P fertilizer while increasing removal of crop P (Bundy, 1998). The agricultural P balance for the Mississippi-Atchafalaya River basin, which was positive most years from 1950–1990 (i.e., more P added to agricultural systems than removed as products) became negative in the 1990s (i.e., the agricultural system was mining P stored in soils) (SAB, 2007). As a result of increased P use efficiency, agricultural P fertilizer use in the US has declined by ∼15–20% since the mid-1970s (ERS, 2010).

Despite these efforts, an analysis of P trends in rivers in the US for the period 1993–2004 (Sprague and Lorenz, 2009) revealed more sites with upward trends (24%) than downward trends (16%), with no change at 40% of the sites. P fluxes to the Gulf of Mexico increased by 12% between 1980–1996 and 2001–2003 (SAB, 2007). Hence, it appears that P leakage from our cities and farms continues.

While the motivation to reduce eutrophication is still pressing, a newer and potentially more serious concern is the exhaustion of phosphate rock (Herring and Fantel, 1993, Vaccari, 2009), the source of most P fertilizer. If we do not adopt P conservation policies over the next few decades, the green revolution made possible by industrial fixation of nitrogen (Haber–Bosch process) in the early 20th century (Smil, 2001) could be followed by a “brown devolution” in the mid-to-late 21st century, as rising population and food wealth (e.g., greater meat consumption) collide with diminishing P resources, with devastating consequences for humanity.

Preventing the brown devolution will require us to use less P and to use it more efficiently. This paper focuses on P conservation in urban systems, which may house two-thirds of the world’s population by 2030 (FAO, 2001). Although direct P inputs to cities (at least in the US) are small relative to agricultural inputs, consumption of food in cities drives agricultural P fertilization. Moreover, in the US at least, there is probably greater future opportunity to conserve P in cities than in agriculture, because P cycling in agricultural systems has already been tightened over the past few decades.

The few studies of urban P cycling that have been conducted indicate that most P entering cities is stored (in landfills or soils) or is exported via wastewater. Tangsubkul et al. (2005) found that 50% of P imported into Sydney, Australia, was stored within the system and that most of the rest was exported as sewage effluent. In a study of 266 counties in the Chesapeake Bay watershed, Russel et al. (2008) found a strong correlation between Net Anthropogenic Phosphorus Index (NAPI, the difference between imported P and net exports of animal and crops) and human population, with NAPI values of 40–80 kg P ha⁻¹ yr⁻¹ in the counties with densest urban populations. Similarly, Schussler et al. (2007) found in a study of 11 Minnesota watersheds that while P inputs (kg ha⁻¹ yr⁻¹) were higher in agricultural watersheds than in urbanized watersheds, net P retention was highest in more urbanized watersheds, because there was little “deliberate export” of P in the form of agricultural products.

This study examines P balances for the Twin Cites Watershed (TCW), which includes most of the Minneapolis-St. Paul (Minnesota, USA) metropolitan region. P balances include a baseline P balance (for 2000), and three conservation scenarios (reduced input; reduced storage; and reduced leakage), followed by a comprehensive conservation scenario that includes all three conservation approaches. The comprehensive conservation scenario reduced P input by 15%, reduced P storage by 71%, and reduced P leakage by 74%, while increasing deliberate P export by 1200%. The P supplied by the comprehensive conservation scenario would be sufficient to grow about half of the food needed by the TCW.