(30b) Quantifying Virtual Phosphorus Flows in Interstate Food Trade: Implications for Environmental Sustainability

Authors: 
Vora, N., University of Pittsburgh
Yates, E. M., University of Pittsburgh
Khanna, V., University of Pittsburgh
Phosphorus(P) is one of the essential nutrients for agriculture. It is primarily supplied to crops as inorganic phosphorous fertilizer or though manure application. On farm use of phosphorus efficiency is very low, ranging between 10-15%. Rest of the applied P gets bonded in the soil which may subsequently become available for crop uptake. Current agronomic P recommendations for farmers exclude the accumulated P due to unknown timescales of release. Sustainable management of P is an important topic due to the limited availability of inorganic P sources and its downstream potential to cause eutrophication. Additionally, production of phosphorus fertilizers is highly energy intensive with limited potential of waste stream recovery due to techno-economic constraints. Therefore, an accurate understanding of regional P application and efficiency, combined with interregional food trade is necessary to understand the complexity of P cycle. We combine data on current phosphorus application practices with recommended and existing soil P levels to assess sustainability of P management on farm. Additionally, we use P application data with food trade to quantify phosphorus embodied in the inter-regional trade. This work will enable to identify trends and hotspots in US agriculture P application practices for management of food-water nexus.

We develop a food trade and associated virtual phosphorus model for the U.S. by evaluating over 60 crop items covering commodity groups of cereal grains, fruits, nuts, and vegetables. Regional phosphorus mass balances are constructed using datasets from US Department of Agriculture’s Natural Resource Conservation Service, International Plant Nutrition Institute, and Census of Agriculture. The interstate food trade network is formulated from Freight Analysis Framework and translated to virtual phosphorus network by using crop yields and phosphate fertilizer application rates from Census of Agriculture. Preliminary results indicate that phosphorus fertilizer application differ considerably for crop ranging from 2-10 lb P per ton crop for grains to 10-68 lb P per ton of crop for nuts. To estimate whether calculated P application rates are excessive or necessary for specific soil type, we compare it with recommended P levels in each state. In 2012, 28 states had to recommended levels, including states with highly eutrophic waterbodies such as Ohio, Florida, California, Texas, and Maine. With addition of total P fertilizer across crops in the given year, the soil phosphorus for these states increases by a fraction of 2-4 compared to recommended levels. From food trade perspective, we find that 13 billion US tons of food and 5.3 million US tons of embodied P were transferred across the U.S. Specifically, grains dominate food and total embodied P transfers within the U.S., indicating the importance of considering scale of production. The implications of the study findings will be discussed in the context of balancing on-farm conservation policies for simultaneously enriching soil with nutrients and avoiding excess run-offs.