RESEARCH AND ANALYSIS Efficiency Through Proximity Changes in Phosphorus Cycling at the Urban–Agricultural Interface of a Rapidly Urbanizing Desert Region Genevi` eve Metson, Rimjhim Aggarwal, and Daniel L. Childers Summary In tightly coupled socioecological systems, such as cities, the interactions between socio- economic and biophysical characteristics of an area strongly influence ecosystem function. Very often the effects of socioeconomic activities on ecosystem function are unintended, but can impact the sustainability of a city and can have irreversible effects. The food system in its entirety, from production to treatment of human waste, is one of the most important contributors to the way phosphorus (P) cycles through cities. In this article we examined the changes in P dynamics at the urban–agricultural interface of the Phoenix, Arizona, USA, metropolitan area between 1978 and 2008. We found that the contribution of cotton to harvested P decreased while the contribution of alfalfa, which is used as feed for local dairy cows, increased over the study period. This change in cropping pattern was accompanied by growth in the dairy industry and increased internal recycling of P due to dairy cow manure application to alfalfa fields and the local recycling of biosolids and treated wastewater. The proximity of urban populations with dairies and feed production and low runoff in this arid climate have facilitated this serendipitous recycling. Currently P is not strongly regulated or intentionally managed in this system, but farmers’ behaviors, shaped largely by market forces and policies related to water recycling, unintentionally affect P cycling. This underscores the need to move from unintentional to deliberate and holistic management of P dynamics through collaborations between practitioners and researchers in order to enhance urban sustainability. Keywords: biosolids industrial ecology manure Phoenix sustainability urban ecology Supporting information is available on the JIE Web site Introduction Phosphorus (P) is an essential nutrient for all living or- ganisms and is often the limiting nutrient in agricultural soils (meaning that the growth of plants is often limited by the amount of P available to them) (Chapin et al. 2002; Drech- sel et al. 2001; Pierrou 1979; Runge-Metzger 1995). Both P and nitrogen (N), another nutrient, are building blocks of all organisms and cycle through ecosystems in both their organic and inorganic forms (via organisms, water, air, etc.). However, while the majority of N is found as N 2 gas, which is fixed by plants and subsequently incorporated into other organic and in- Address correspondence to: Genevieve Metson, Department of Natural Resource Science, McGill University, 111 Lackeshore Road, Ste. Anne de Bellevue, QC H9X 3V9, Canada. Email: genevieve.metson@mail.mcgill.ca © 2012 by Yale University DOI: 10.1111/j.1530-9290.2012.00554.x Volume 16, Number 6 organic complexes, P follows a mineral, sedimentary cycle. That is, the majority of P is found in mineral form, which weathers so that P can be incorporated into organic materials, and even- tually returned to sediments. On shorter time scales, P is also recycled within ecosystems as animal and plant waste to soils where it is taken up again. Modern agricultural systems depend on mined P for fertilizer to maintain high crop yields, and the production of fertilizer is the most important cause of human alteration of the global P cycle (Smil 2000; Turner 1990). Of the 160 million tons of phosphate rock currently extracted per year, 90% is for fertilizer production, making agriculture, and the food chain that follows, 914 Journal of Industrial Ecology www.wileyonlinelibrary.com/journal/jie