Quantifying the upstream flux of phosphorus to Minnesota’s Twin Cities urban food-shed

Background/Question/Methods

Phosphorus (P) is a non-renewable resource, essential for agriculture and human food production, which is being depleted rapidly within the U.S., and more slowly globally.  Inefficient use of urban P contributes to eutrophication of receiving waters. Hence, moving toward a closed “circular economy” for P may become essential. Most mined P enters human systems via application of fertilizer to agricultural land, which becomes recycled via manure, exported as crops or animals, and then enters urban food systems after processing.  The goal of this study was to track P from fertilizer to the urban plate, linking urban food production in the Twin Cities Metropolitan Area (TCMA) to its agricultural source, produced primarily on Minnesota farmland. At the farm level, we used state-level agricultural intensive farm interviews, literature, and expert information to model P flows through crop and animal production systems, analyzed P losses during processing, and modeled P inputs to the urban plate from national dietary studies, mapped into the TCMA based on the distribution of age, gender, and ethnicity.

Results/Conclusions

Human food accounted for 41% of the P inputs and 46% of the P outputs from the TCMA; only about 1% of food waste P is recycled. Food corresponding to at least 80% of the dietary P-input for TCMA households could be produced entirely within Minnesota’s agricultural system, hence the “food-shed” is more-or-less directly connected to urban consumers. Top food products which contribute the largest input of dietary P are milk (22% of P), cheese (12%), wheat flour (11%), beef (7%), chicken (6%), and pork (5%).  Mapping out an agricultural footprint which can support this urban ecosystem enables P use to be conceptualized through a circular economy model, with Minnesota as the food-shed. P use efficiency was calculated for individual systems including: corn (1.15), hog (0.47), dairy (0.36), and beef (0.20).  Using our model to altering TCMA consumption patterns to reflect 1950 meat intakes resulted in 21% less land area and 17% less P fertilizer required.  Only 9% of the P in a cow becomes retail cuts. To account for the other 91% of P, we are analyzing flows through slaughterhouses and rendering plants. The potential for increasing P use efficiency and identifying barriers or opportunities for improvement between agricultural and urban systems will be discussed.