Phosphorus Management and Legacy Phosphorus
About 35 scientists and others affiliated with the Southern Extension and Research Activity (SERA 17) group met in person (with some remote attendees) in San Antonio on November 14 to hear presentations of recent research and updates about the group’s activities. Here we summarize for our readers a subset of the research presentations, which addressed the use of critical source areas in managing phosphorus, the terminology we use to describe “legacy phosphorus”, and legacy phosphorus drawdown experiments.
Critical Source Areas
Dr. Rich McDowell (Lincoln University, NZ) began the day’s talks by discussing the concept of critical source areas to improve water quality. A critical source area is a landscape feature that contributes disproportionate levels of some pollutant, in our case, phosphorus, to waterways. (The size of these areas can change with saturation, leading to the concept of “variable source areas”.) One policy approach to nutrient loss reduction is to identify these areas and focus actions to reduce nutrient losses, thereby making actions more cost-effective than an untargeted approach. This approach is embodied in the P-indices used across many US states with mixed results for water quality, at least in part because there is considerable variability in how P-indices are defined, implemented and enforced.
McDowell described how alternative approaches don’t always work. Simply setting soil test phosphorus thresholds for farms doesn’t always work for reducing phosphorus losses because the landscape interacts with the hydrology of the system, i.e. variations in how water flows across different areas of a landscape and connects to downstream waters can lead to differences in nutrient loads from soils that have the same soil test phosphorus levels. Likewise, simply requiring phosphorus balances of farms (balancing inputs and outputs) doesn’t always achieve the desired reductions because phosphorus losses vary greatly according to hydrological processes not captured by balances, such as streambank erosion and large amounts of runoff from phosphorus deficient soils that sum to large loads.
In McDowell’s New Zealand context, farmers are required to develop freshwater farm plans underpinned by the identification of critical source areas where cost-effective measures can then be targeted. Implementation of best management practices that target these CSAs are then audited every few years. Through long-term catchment-scale studies, McDowell and collaborators were able to demonstrate that implementing practices in this way did, in fact, reduce nutrient loads.
However, a more typical use-case is not at the catchment scale, but at the field/farm scale, and this was evidenced by a survey of 55 internationally published studies of P losses from CSAs that his team identified in the literature. At larger scales, other transport processes can dominate the flows coming from critical source areas. Similarly, in a flat, featureless landscape, average losses across the landscape tend to dominate those from a critical source (and such areas are harder to identify than in topographically varied areas). Moreover, McDowell noted that 50% of global P loss comes from the top 10% of discharge events. Such events can flood entire landscapes, and the runoff from CSAs is overwhelmed by cross-catchment runoff. This issue will become more salient as climate change leads to more such events.
Ultimately, CSAs are an appropriate policy instrument in certain circumstances but not all. If the approach is taken, McDowell noted that it needs to be communicated to the public with simple and common-sense explanations. Policymakers and scientists also need to give people realistic expectations of the lags between intervention implementation and likely water quality improvements.
More information can be found in a recent publication cited here: McDowell, R., Kleinman, P.J.A., Haygarth, P., McGrath, J.M., Smith, D., Heathwaite, L., Iho, A., Schoumans, O., Nash, D., 2024. A review of the development and implementation of the critical source area concept: A reflection of Andrew Sharpley’s role in improving water quality. J of Env Quality jeq2.20551. https://doi.org/10.1002/jeq2.20551. You can also hear more about Rich McDowell’s research on diffuse nitrogen and phosphorus loads by watching the video abstract we produced of his Geosciences Data Journal paper on Phosphorus Science Now!
Defining Legacy Phosphorus
As fields of inquiry evolve, it’s not unusual for specialists to begin struggling to reach consensus about defining more precise terminology to describe their work. Blood has nearly been spilled debating the taxonomies of biological species, for example. While these discussions can seem abstruse and overly academic, it’s important to standardize language to avoid confusion. Otherwise, data collected on related but distinct phenomenon can be difficult to parse and analyze. At the conference, researchers who study so-called “legacy phosphorus” discussed how to best name different pools of stored phosphorus to reduce ambiguities.
Dr. Andrew Margenot (University of Illinois, Urbana-Champaign) kicked off the discussion about by describing historical uses of the terms “legacy phosphorus” and “residual phosphorus”. Through an extensive literature survey, Margenot traced the latter term to a 1925 article that used it to describe the frontloading of phosphorus application to build a soil stock as an alternative to annual application to meet agronomic needs. He traced the former term to a paper in 1996 that describe phosphorus entrained in the watershed in the Florida Everglades. The terms have been conflated with each other and many others (old/new phosphorus, anthropogenic/geogenic, applied/native, etc.), and it’s not always clear exactly which pool of phosphorus is being describe when a particular term is used.
Margenot described the various attributes or dimensions that terms used to describe these pools try to capture, including the magnitude of the stock (agnostic to flux or source); its form (species of interest, labile v. recalcitrant); its origin (anthropogenic or geogenic, regardless of speciation); its age (e.g., recently applied or applied long ago); and its impact (e.g. agronomic benefit or water column detriment). He noted that we often think in terms of binaries when adopting terminology (e.g. old v. new), but that these attributes really occur across spectra of values (e.g. oldest to newest). As one proposal, he suggested we could adopt multi-dimensional definitions, such as “labile new anthropogenic P” and “mixed lability, old, geogenic P”.
His talk was followed by one from Dr. Amy Shober (University of Delaware) who was part of a SERA-17 working group that met over three years to develop a shared legacy phosphorus lexicon. Shober described how we often discuss legacy P in either an agronomic context, where it represents a resource opportunity (soil pools farmers can tap), or an environmental context, where it represents a pollution problem. At the end of the day, we measure phosphorus concentrations, and some possible functional definitions were described.
Researchers in the room were encouraged to consistently report total P in their work as a way to track the larger store of P in soils and sediments, encompassing legacy P.
Dr. Pete Kleinman finished the discussion by describing an effort he is leading to develop a unique perspective piece on legacy phosphorus to be published in the Journal of Environmental Quality. The approach will be to solicit and assemble a range of disciplinary perspectives from 30-50 authors on how the term legacy P is used in fields such as agronomy, hydrology, oceanography. Demographic information about the authors will be collected, as will their opinions about such topics as:
- How does legacy P intersect with your work?
- What context/scale do you work in?
- How do you define legacy P?
- Is it consistent with others?
- What gaps in legacy P science and management should be addressed?
By looking across these perspectives and correlating them to the knowledge domains from which they derive, the hope is that a clearer, shared lexicon can be derived. A manuscript is scheduled to be submitted in September 2025.
Legacy Phosphorus Drawdown
Dr. Luke Gatiboni (North Carolina State University and STEPS Center) discussed the rate of drawdown of legacy phosphorus achieved by ceasing phosphates application. Two published studies had demonstrated a 3-5% reduction per year in soil test phosphorus during a period of drawdown, and Gatiboni’s team was interested in whether these rates were more broadly applicable and what factors influenced drawdown rate.
To study this, they conducted a meta-analysis that ultimately selected 14 different published studies of a total of 56 drawdown sites/treatments from across the US and Canada. Drawdown rates were statistically correlated to a set of characteristics that included the soil test method used, whether they contained low or high P soils, the type of crop rotation used, and the soil type present. For many of the studies, yield data was missing, as were P export data.
Their analysis suggested that the initial P availability was far more predictive of drawdown rate than other factors. The soil test method used had some predictive power, but, perhaps surprisingly, the crop rotation used and the soil texture were not very predictive. In the first 15 years of drawdown, the rates of drawdown ranged from 0.9 to 7.8 mg P/kg soil/yr, and it generally took 8-15 years to achieve 50% reductions in STP.