As we work to reduce agriculture’s impact on the environment, a crucial focus is on soils, which play a vital role in storing and cycling essential nutrients like carbon, nitrogen, and phosphorus. Biogeochemists, including Andrew Margenot from the Agroecosystem Sustainability Center, are aiming to uncover important insights. However, a significant challenge lies in accurately assessing where phosphorus has accumulated over the past century, particularly since human activity has increased its presence in the biosphere.
Margenot, who serves as an Associate Professor of Soil Science in the Department of Crop Sciences at the University of Illinois Urbana-Champaign, along with other global phosphorus experts, recently published a position paper in Global Change Biology. This document aims to create a framework for understanding phosphorus cycling during the Anthropocene, the current geological epoch defined by significant human impact.
The team also includes notable researchers such as Leo Condron, a Biochemistry Professor at Lincoln University in New Zealand; Genevieve Metson, an Associate Professor in Geography and Environment at the University of Western Ontario; Philip Haygarth from the Lancaster University Environment Centre in the UK; and Jordan Wade, a Soil Health Assessment Lead at Syngenta Group in Basel, Switzerland, along with Ph.D. student Prince Agyeman from the Czech Republic and research scientist Shengnan Zhou and postdoctoral researcher Suwei Xu from Margenot’s research team.
“Our objective is to explore all viable methods to measure legacy phosphorus. We aim for a comprehensive overview of where it is practical to measure and where it may not be necessary. This allows us to establish priorities and non-priorities, providing a cohesive strategy for future actions,” stated Margenot.
The paper emphasizes phosphorus within the “terrestrial aquatic continuum,” highlighting the interaction between water and soil across various spatial and temporal scales. “A significant aspect of our analysis is addressing uncertainty,” said Margenot. “This poses challenges for policymakers who feel pressured to develop solutions for urgent issues.”
It could take up to a century for the legacy phosphorus that has accumulated in stream channels and soil to disperse into waterways. Consequently, identifying the amount and locations of this residual phosphorus is of utmost importance.
“When addressing legacy phosphorus that will influence water quality for the next century or more, we lack fundamental data on where to begin. However, we must find a way to navigate this uncertainty: we shouldn’t be overzealous, but we also can’t afford to wait decades to decide how to proceed,” he added.
To propose future phosphorus management strategies, researchers can assess how much phosphorus was added to the soil and how much was lost through activities like harvest or erosion. By calculating this balance—what was introduced minus what was removed—it becomes possible to estimate the amount of residual phosphorus remaining.
The position paper also showcases several case studies of legacy phosphorus using two of the world’s oldest continuous agriculture test sites: the Rothamsted Experiment in Harpenden, England, established in 1843, and the Morrow Plots at the University of Illinois, founded in 1876. A key finding was that accumulated phosphorus is typically found within the top 12 inches of soil, and its form often differs from what was originally applied. This discovery is crucial to understanding phosphorus dynamics.
“Phosphorus is usually added in inorganic forms as phosphate, which is highly soluble in water and susceptible to loss,” Margenot noted. “Our findings indicate that even though significant phosphorus is present as phosphate, its accumulation does not remain in this soluble form in the soil. Instead, it transitions into forms bound to organic matter, iron, and calcium. Thus, we cannot assume that the residual phosphorus applied is readily available for crop uptake or poses a risk of being washed away.”
Beyond evaluating the issue thoroughly, the consortium proposed several recommendations.
1) Researchers need to improve their estimation validation processes. “Oftentimes, we overlook minor phosphorus input and output amounts. These measurement gaps can amplify uncertainty over time,” Margenot explained.
2) Scientists often lack access to sufficient data for accurate estimations. The group suggests initiatives to involve the private sector, such as soil testing labs collaborating with researchers to tap into existing datasets.
3) A standardized methodology for measuring data is essential. “In many instances, fundamental measurements remain uncollected,” Margenot points out. “We need to combine various complementary methods that may individually be inadequate, but collectively provide robust insights. While mapping phosphorus accumulation over the past 70 years may seem daunting, it is vital to pinpoint where it matters in terms of agricultural use and water quality challenges.”
4) There should be efforts to identify hot spots of legacy phosphorus to prioritize resources for mitigating negative water quality impacts. Margenot’s team is already engaging in this work in Illinois. “We don’t need to map the entire state,” he said. “We have identified priority watersheds based on USGS measurements.”
The paper highlights the need to persuade researchers and funding agencies to allocate resources efficiently to generate impactful data. “Acquiring the last 5 percent of the necessary data can often require half the investment needed to obtain the initial 95 percent,” Margenot remarked.
For policymakers, Margenot believes it will be crucial to communicate that estimates regarding legacy phosphorus are currently provisional and should be treated as evolving information. “Policies must be flexible,” he emphasized.
“The global phosphorus cycle has been more significantly altered by human activities than nitrogen,” Margenot stated. “While we’ve roughly doubled the nitrogen in circulation within the biosphere, we’ve quadrupled the phosphorus.”
