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Phosphorus in the Chesapeake:An Overview on Nutrient Pollution in the Chesapeake Bay and Efforts to Limit Phosphorus Runoff in the Watershed

Kathleen DaleyNeil Saunders

1 May 2015

Copyright 2015 G2 Associates, Inc., Great Falls, Virginia, All Rights Reserved.

G2 Associates hereby authorizes you to copy this document for non-commercial uses within your organization only. In consideration of this authorization, you agree that any copy of these documents that you make shall retain all copyright and other proprietary notices contained herein. II. 2III. Phosphorus1. Introduction Phosphorus is an essential nutrient for plant and animal life, both terrestrial and aquatic. However, phosphorus causes negative environmental impacts to underwater ecosystems when this nutrient enters waterways at levels higher than aquatic organisms require. Increased nutrient and sediment loads to waterways like the Chesapeake Bay watershed result in algal blooms, which take up oxygen needed by fish and shellfish in the Bay. Excess nutrients and sediment can also block sunlight to underwater grasses, and cause die-offs of underwater vegetation. Phosphorus enters waterways naturally from erosion, runoff, and soil leaching. Human activity, especially agricultural practices, increases phosphorus runoff to the Bay at levels that are harmful to aquatic life. This problem is prevalent in the Chesapeake Bay watershed, where chicken production takes place on a massive scale; watershed states produce hundreds of millions of chickens each year. In the Chesapeake region, chicken manure, high in phosphorus, is loaded on soils as a fertilizer at levels higher than the crops need. Many of these soils already have high phosphorus levels before manure is added to the surface. The excess phosphorus leaches and runs off into the Bay.

0. The Phosphorus CyclePhosphorus is a natural part of the environment; it is found in our bodies, in soils, in water, and in bedrock. Phosphorus is an essential nutrient for all living organisms, and is used in energy transfer and in the passage of genetic information in the DNA of cells. The nutrient increases freshwater productivity, and is essential to crop production. The phosphorus cycle refers to the circulation of phosphorus within the natural world. Phosphorus as a pure element is rare; the nutrient is generally found in the environment as part of a phosphate molecule.[endnoteRef:-1] Most phosphorus reserves are found in rock, and released through mining, weathering and leaching. The nutrient circulates through freshwater and terrestrial ecosystems when taken up by plants, grazers, predators, and parasites. Phosphorus returns to the ecosystem when these organisms die and decompose. Most phosphorus, however, reaches the sea, and is lost to the deep ocean. Some phosphorus returns to the terrestrial ecosystem when fish, which take up phosphorus from algae, are harvested, or through the collection of seabird guano for fertilizer. Unlike nitrogen, a nutrient that can be recycled and returned to terrestrial ecosystems through atmospheric deposition, most phosphorus is eventually lost to the ocean.[endnoteRef:0] [-1: EPA, 5.6 Phosphorus, Accessed 2015, http://water.epa.gov/type/rsl/monitoring/vms56.cfm. ] [0: 2 Encyclopedia Brittanica, Phosphorus Cycle, Accessed 2015, http://www.britannica.com/EBchecked/topic/457621/phosphorus-cycle. ]

Figure 1: The Phosphorus Cycle. Image source: http://britannica.com/EBchecked/topic/457621/phosphorus-cycle.

The Phosphorus Cycle in the Aquatic Environment

Figure 2: The phosphorus cycle in the aquatic environment. Image source: EPA, http://water.epa.gov/type/rsl/monitoring/vms56.cfm.Phosphorus occurs in two forms in aquatic ecosystems, as organic and inorganic phosphate. Plant and animal tissues of aquatic organisms contain organic phosphate, while underwater plants take up inorganic phosphate from non-living sources, and convert it to organic phosphate. Other aquatic organisms take up organic phosphate by consuming plants, other animals, or decomposing organic matter. Animal waste and decomposing organic matter contain organic phosphate that is converted to inorganic phosphate by bacterial decomposition on the floor of a body of water. When the floor is stirred up, inorganic phosphate cycles back through the aquatic environment.[endnoteRef:1] [1: EPA, 5.6 Phosphorus. ]

Phosphorus Reserves and TransportPhosphorus is a valuable finite resource, found in select locations across the globe. A majority of global phosphate rock reserves are in countries such as Morocco, which holds 38% of global phosphate reserves, as well as China (27%), South Africa (10%), the United States (10%) and Jordan (8%).[endnoteRef:2] While many regions around the globe lack sufficient levels of phosphorus in their soils, and rely on the import of expensive fertilizers to crop producers, the United States contains regions where soils are overloaded with the nutrient, including the Chesapeake Bay region. [2: Dr. Andrew Sharpley, The Role of Phosphorus Management in the Green Pastures and Blue Waters Paradox, (Presented at The State of the Science of Phosphorus, Wye Mills, Maryland, January 30, 2015). ]

Figure 3: Global phosphate reserves. Image source: http://www.greatquest.com/s/PhosphateWhy.asp.

Certain U.S. lands, including agricultural fields in the Chesapeake Bay watershed, are over-loaded with phosphorus due to the current system of mining, transport, and application of the nutrient. Prior to the mid-twentieth century, nutrients were cycled within and on a farm. With the adoption of large-scale industrial operations for the growth of crops and animals, this cycle was broken. Now, the country participates in an unsustainable system of transport and application of phosphorus. This finite resource is mined and shipped cross-country at a rate that cannot be maintained. Phosphate is mined in Florida, shipped to the Midwest to be used as fertilizer for crops, which are then shipped to the Mid-Atlantic states for concentrated animal feeding operations, and finally deposited on the land in animal waste, used as fertilizer.[endnoteRef:3] [3: Dr. Douglas R. Smith, Balance and Legacy Phosphorus, (Presented at The State of the Science on Phosphorus, Wye Mills, Maryland, January 30, 2015). ]

In the Chesapeake Bay region, animal feed high in phosphorus is shipped to the Delmarva Peninsula, a region known for its concentrated feeding operations for chicken. Chicken litter from these operations is directly applied to local croplands to meet plant needs, primarily for nitrogen. While chicken litter is a source of readily available fertilizer for crops, the litter is often applied to land that already has high concentrations of phosphorus, increasing phosphorus levels in the soil beyond plant needs. Runoff and leaching from these phosphorus-laden soils causes a buildup of the nutrient in waterways leading to the Chesapeake Bay. While phosphorus is a good thing in watersheds, serious problems occur when phosphorus loads reach high levels.[endnoteRef:4] [4: Smith, Balance and Legacy Phosphorus. ]

Figure 4: Phosphorus transport within the United States. Image source: https://passel.unl.edu/pages/printinformationmodule.php?idinformationmodule=1118084123.

Phosphorus in Soil: Loss to the Aquatic Environment, Manure Application and Legacy PhosphorusAgriculture is one of the biggest contributors to phosphorus pollution in the Chesapeake Bay. Phosphorus pollution from agriculture occurs through stormwater or irrigation runoff (surface flow) and leaching from soils (subsurface flow), as well as sediment loss from wind and erosion.[endnoteRef:5] Phosphorus loss to the aquatic environment occurs through four main transport methods. These include phosphorus loss in waterborne sediment (erosion), soluble phosphorus dissolved in surface water (surface water runoff from rainfall and irrigation), phosphorus dissolved in leachate (leaching), and loss from windborne sediment.[endnoteRef:6] Phosphorus that does not remain in the soil, or is not taken up by crops, reaches the waterway through these transport methods. [5: Sharpley, The Role of Phosphorus Management in the Green Pastures and Blue Waters Paradox.] [6: Natural Resources Conservation Service, Model Simulation of Soil Loss, Nutrient Loss, and Change in Soil Organic Carbon Associated with Crop Production, 2006.]

Most phosphorus runoff from agricultural areas comes from a critical source area a portion of the land where high source meets high transport. This is also known as the 80/20 rule - 80% of phosphorus comes from 20% of the land area. A majority of phosphorus that enters watersheds as agricultural runoff comes from large storm events, from this small area of land.[endnoteRef:7] Erosion, surface runoff, and leaching are the most common methods of phosphorus loss on agricultural fields. Croplands lose pounds of phosphorus per acre every year (see Table 1 below), from irrigation and storm events, especially when soils are over-saturated with the nutrient. [7: Sharpley, The Role of Phosphorus Management in the Green Pastures and Blue Waters Paradox.]

Total Phosphorus Loss from Agricultural Fields to the Aquatic Environment

Pathway% LossLoss in Lbs/Acre/Year

Waterborne sediment63%1.5 lbs/acre/year

Soluble phosphorus dissolved in surface water20%0.5 lbs/acre/year

Phosphorus loss with windborne sediment15%0.4 lbs/acre/ year

Phosphorus dissolved in leachate