A watershed-scale inventory of phosphorus for the conterminous United States: How phosphorus accumulation in agricultural soils offsets progress in decreasing nutrients from urban areas
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Recent reports suggest that efforts designed to reduce non-point and point source phosphorus (P) pollution are not delivering anticipated improvements in water quality in many areas. Attenuating P pollution is challenging because P accumulates in soils over decades and is transported slowly due to tight biogeochemical cycling. This legacy P likely offsets measures designed to attenuate non-point source loading from agricultural regions and both point and non-point source P loading from urban areas. To analyze and interpret the P trends in across the contiguous United States over time (2002-2012), we compiled peer-reviewed, publicly available datasets of agricultural P flux (i.e., inputs and non-hydrological outputs from fertilizer, crop removal, and livestock production), atmospheric P deposition, human P demand/waste, and point source loads as well as estimate accumulation of legacy P in agricultural lands from 1945-2001. P in agricultural soils has continued to accumulate as fertilizer and manure inputs continue to exceed crop removal rates in many regions. This excess P contributes to legacy P pools, which exceed > 100 kg P ha-1 throughout the Upper Midwest and agricultural regions in the east and west coast. In contrast, P from municipal point source loads has increased slightly nationwide but largely declined in the Northeast, as many states have deployed advanced nutrient removal technologies in wastewater treatment plants. P associated with total human waste has also declined due to various sales bans on P-containing detergents as well non-farm fertilizer use in urban areas, despite population growth over the last decade. Atmospheric P deposition increased throughout the Rockies, potentially contributing to recently reported increases in stream and lake TP concentrations. Overall the P legacy effects were maintained in many regions of the country due to continued P accumulation in agricultural lands, thereby likely perpetuating the continued eutrophication of many of the nation’s downstream ecosystems. Overall this inventory will be particularly useful for managers, who can use it as a starting point to understand the temporal and spatial variation of P cycling when considering management and policy designed to reduce the occurrence of harmful algal blooms and other consequences of excess P.