Embodied phosphorus and the global connections of United States agriculture

Agricultural phosphorus (P) use is intricately linked to food security and water quality. Globalization of agricultural systems and changing diets clearly alter these relationships, yet their specific influence on non-renewable P reserves is less certain. We assessed P fertilizer used for production of food crops, livestock and biofuels in the US agricultural system, explicitly comparing the domestic P use required for US food consumption to the P use embodied in the production of US food imports and exports. By far the largest demand for P fertilizer throughout the US agricultural system was for feed and livestock production (56% of total P fertilizer use, including that for traded commodities). As little as 8% of the total mineral P inputs to US domestic agriculture in 2007 (1905 Gg P) was consumed in US diets in the same year, while larger fractions may have been retained in agricultural soils (28%), associated with different post-harvest losses (40%) or with biofuel refining (10%). One quarter of all P fertilizer used in the US was linked to export production, primarily crops, driving a large net P flux out of the country (338 Gg P). However, US meat consumption relied considerably on P fertilizer use in other countries to produce red meat imports. Changes in domestic farm management and consumer waste could together reduce the P fertilizer required for US food consumption by half, which is comparable to the P fertilizer reduction attainable by cutting domestic meat consumption (44%). US export-oriented agriculture, domestic post-harvest P losses and global demand for meat may ultimately have an important influence on the lifespan of US phosphate rock reserves.