July, August 2019 State of the Science reports on water quality

Ohio Farm Bureau’s State of the Science Report is published periodically to provide updates on Ohio and national water quality research and emerging issues. The July edition explores two important ecological questions: ‘Why are agricultural systems considered leaky?’ and ‘What is the natural aging process for lakes and ponds?’ The August edition explores soil phosphorus management strategies and provides an overview of three of the commonly used phosphorus loss assessment methods.

July Report (pdf)

August Report (pdf)

Excerpts from the reports are below.

Why are agricultural systems considered leaky?

Take away messages:

  • All ecosystems evolve and go through a natural aging process. If left undisturbed, the communities of plants and animals will reach a steady state known as a climax community.
  • Resources (nutrients, carbon, etc.) are efficiently used and recycled within a climax community.
  • Non-climax communities are “leaky;” resources are lost from the system and must be replaced.
  • Agricultural systems are not climax communities and thus are classified as leaky systems.
  • Leaky systems are not bad; they just require the addition of resources to replace those that have been removed.
  • Because agricultural systems are leaky systems, additional nutrients must be added to replace those that have been removed by crops and/or transported downstream via erosion and runoff.

What is the natural aging process for lakes and ponds?

Take away messages:

  • All ecosystems evolve and go through a natural aging process. In lakes, this is known as eutrophication.
  • As lakes age, their overall productivity increases due to the input of sediment, nutrients and organic matter from the watershed and the atmosphere.
  • The quantity of biologically useful nutrients like phosphorus and nitrogen dissolved in the waters of a water body determine the trophic (nutritional) state of the water body.
  • Productivity or the nutrient richness of lakes serves as the basis for the trophic status concept of lake classification.
  • This classification system places lakes along a continuum from nutrient poor(oligotrophic) on one end to nutrient rich (eutrophic) on the other. The lakes in themiddle containing moderate levels of nutrients are classified mesotrophic.
  • Ohio’s lakes, especially Lake Erie, are complex systems undergoing continuous changes due to changing weather patterns and introduced exotic species. Ohio’s rich productive landscape naturally leads to nutrient-rich, productive eutrophic lakes.

Soil Phosphorous Management Strategy

An overview of the three commonly used phosphorus loss assessment approaches are provided:

Agronomic-based soil tests can play a role in the environmental management of soil phosphorus but are only the beginning of a more comprehensive approach. Several studies have found that soil test phosphorus levels are related to the release of phosphorus into the solution. Plots of soil test phosphorus against soil solution phosphorus have shown the existence of a two-part relationship and change point (threshold). Soils with soil test phosphorus values above the threshold release phosphorus into the soil solution at a higher rate than do soils with soil test values below the threshold. For several soils from the United States, the threshold occurred in 120-190 mg/kg Mehlich-3 P range (84-133 mg/kg Bray-1 P). These results point to a potential environmental threshold for phosphorus soil test values.

Environmental soil phosphorus thresholds are similar to agronomic limits in that they evaluate the potential for phosphorus loss in runoff based on a single parameter, soil test phosphorus. The assumption is that there is a soil test phosphorus threshold, different from the agronomic limit for crop response, where the soil test phosphorus level leads to a greater potential for phosphorus loss in runoff.

Of the three approaches, the phosphorus index has been the most widely adopted to target phosphorus management. A phosphorus index is an assessment tool used to identify agricultural fields most vulnerable to phosphorus loss by accounting for major source and transport factors controlling phosphorus movement.

Current nutrient management planning seeks to identify critical source areas of phosphorus loss. Critical areas can be single fields within a watershed or portions of a field that are disproportionately responsible for phosphorus export. The phosphorus index developed as a tool to rank the vulnerability of fields as sources of phosphorus loss in overland flow.

On-Field Ohio!

Ohio State University is in the final stages of a phosphorous index field-level evaluation tool to manage phosphorus runoff loss while maintaining agricultural production and protecting water quality. The tool called On-Field Ohio! will be an online, interactive program that provides long-term average for erosion and phosphorus runoff.