Abstract
Cities are a key system in anthropogenic phosphorus (P) cycling because they concentrate both P demand and waste production. Urban agriculture (UA) has been proposed as a means to improve P management by recycling cities’ P-rich waste back into local food production. However, we have a limited understanding of the role UA currently plays in the P cycle of cities or its potential to recycle local P waste. Using existing data combined with surveys of local UA practitioners, we quantified the role of UA in the P cycle of Montreal, Canada to explore the potential for UA to recycle local P waste. We also used existing data to complete a substance flow analysis of P flows in the overall food system of Montreal. In 2012, Montreal imported 3.5 Gg of P in food, of which 2.63 Gg ultimately accumulated in landfills, 0.36 Gg were discharged to local waters, and only 0.09 Gg were recycled through composting. We found that UA is only a small sub-system in the overall P cycle of the city, contributing just 0.44% of the P consumed as food in the city. However, within the UA system, the rate of recycling is high: 73% of inputs applied to soil were from recycled sources. While a Quebec mandate to recycle 100% of all organic waste by 2020 might increase the role of UA in P recycling, the area of land in UA is too small to accommodate all P waste produced on the island. UA may, however, be a valuable pathway to improve urban P sustainability by acting as an activity that changes residents’ relationship to, and understanding of, the food system and increases their acceptance of composting.
Highlights
People have significantly altered the P biogeochemical cycle, changing P flows between ecosystems [1], modifying the geographic distribution of P stocks around the world [2], and greatly accelerating the global P cycle [3]
To calculate P flows through the Montreal food system we quantified P in food imports to the island (1), food consumed on the island (2), human urine and feces produced on the island (3), sewage waste going to the wastewater treatment plant (4), sewage treatment plant losses to the Saint-Lawrence river (5), biosolids sent to landfill (6), septic storage (7), food and green waste produced on island (8), food and green waste produced recycled through compost (9), and food and green waste produced sent to landfill (10, numbers refer to Fig. 2 and Table 1)
P cycling in the Montreal Urban agriculture (UA) system
Summary
People have significantly altered the P biogeochemical cycle, changing P flows between ecosystems [1], modifying the geographic distribution of P stocks around the world [2], and greatly accelerating the global P cycle [3]. Global P cycling naturally happens on geological time scales, where P is eroded from rocks, tightly recycled through ecosystems, eventually ending up in the ocean where it is reincorporated into sediments [4]. People have accelerated the extraction process through mining to produce P fertilizer for agricultural systems [5], roughly tripling the mobilization of P at the global scale [1]. Phosphorus Cycling in Montreal bellandpostgrad-belletsuperieures_eng.asp) to GSM and National Science and Engineering Research Council Discovery grant The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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