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Subsurface Phosphorus Risk Assessment Adapted from Information by the University of Delaware When it comes to nutrient management planning for croplands across the United States, it is important to evaluate the phosphorus index to ensure an accurate phosphorus loss risk assessment. Most of these phosphorus index assessments have focused on the risks of phosphorus losses in surface runoff while poorly taking into account the critical role of subsurface phosphorus losses. Subsurface phosphorus is particularly important in areas such as theAtlantic Coastal Plain, where subsurface flow is the pre- dominant pathway of phosphorus transport from croplands using artificial drainage to lower water tables. The cropland must be drained this way to avoid having water within the root zone of plants or standing water, which would disrupt farmers’ ability to use equip- ment and plant successful crops. Farmers are dealing with what is known as “legacy phosphorus”, which is phosphorus left over from past manure applications and con- tinues to contribute to water quality issues. A phosphorus index gives farmers and land managers a tool to identify areas in the landscape where phosphorus sources overlap with the ways water moves phos- phorus through the soils. By using available water quality and soil datasets, researchers from the University of Delaware and other contributing institutions explored methods for evaluating the risk of losing subsurface phosphorus. The researchers calculated the risk by using soil cores collected at different depths from all over the Delmarva Peninsula. They looked at the phosphorus index scores without taking into account any manure application, and instead focused only on contributions of the legacy phosphorus. Amy Shober, lead author of the research said, “For our index, we eliminated the things that we weren’t interested in looking at, so we ultimately got a score that we consider was just for this subsurface risk. We wanted to say, ‘Ok, what is the inherent risk of subsurface losses of phosphorus that was in the soil?’” Once researchers calculated the phosphorus index score, they took a small amount soil and a little bit of de-ionized water, shook it for an hour, and measured how much phosphorus came out of the soil (water-extractable phosphorus), then they correlated the data to see the relationship. “If the phosphorus index subsur- face score was low and the water- extractable phosphorus in the soil at the depth of the water table was low, we would expect a low risk of subsurface phosphorus losses. So, ultimately, we wanted to see scores increasing either linearly or exponentially as soil water extractable phosphorus increased - the higher the risk score, the higher the water-extractable phosphorus level should be,” said Shober. These calculations could serve as a valuable metric, offering insight into the directionality of subsurface phosphorus risk scores when water quality data are inaccessible. Shober said, “We’re hoping that this is something that people can do to move forward with our understanding of subsurface phosphorus loss. In the end, we ended up making some small tweaks to both the Maryland phosphorus man- agement tool (PMT) and the North Carolina phosphorus loss assessment tool (PLAT) that made them score more appropriately against our soils dataset.” Soil cores from Queen Anne’s County, Maryland, courtesy of Amy Shober. 33 WorldWide Drilling Resource ® AUGUST 2018 ENV