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Orthophosphorus Dominates Total P in Tile-drained and Permeable Watersheds


Phosphorus loss is a major contributor to stream eutrophication and hypoxia but recognition is growing that P losses can occur through both highly episodic, event-driven transport or more continuous groundwater or tile drainage discharge. The ratio of dissolved phosphorus (also called orthophosphorus [OP]) to total phosphorus (TP) can be used to evaluate these transport pathways. 


In a paper published in the Journal of Environmental Quality, researchers examined the contribution of OP to TP concentrations and loads in 12 Iowa rivers. The rivers and their watersheds represented a range of watershed characteristics and land use conditions found across the U.S. Midwest.


The team found the ratio of OP to TP varied systematically in Iowa, with ratios exceeding 60% tile-drained and shallow fractured bedrock watersheds and less than 30% in southern and western Iowa dominated by steep slopes and fine-textured soils.  They found that higher OP:TP ratios were associated with greater row crop intensity and more baseflow in the river. 


Determining whether croplands in a watershed are dominated by OP or TP export is an important step in identifying appropriate conservation practices to reduce P loads, since strategies to reduce subsurface OP contributions are different than those designed to reduce TP from surface runoff.

Read the full article in JEQ. Free preview March 23 - March 30