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Dissolving uncertainty: A comprehensive evaluation of dissolved P in tile drainage

As major phosphorus-related water impairments continue to generate headlines and stir regulatory interest, there is an increasing need to better understand factors impacting phosphorus (P) transported from areas where agriculture is heavily underpinned by tile drainage. Substantial opportunities to reduce Illinois’ point-source P loads exist, but the agricultural sector will nevertheless need to play a role in meeting P reduction targets.

This work, funded starting in 2016 by the Illinois Nutrient Research and Education Council, builds on the momentum of the previously established and publicly available “Measured Annual Nutrient loads from AGricultural Environments” (MANAGE) database to answer questions about how and why P is transported in subsurface drainage. Original support from the 4Rs Research Fund in 2014 led to the development of a “Drain Load” component in the database. An initial review of over 400 drainage studies (Figure 1) led to inclusion of 91 individual journal publications and 1279 site-years of drainage nutrient load data that facilitated a quantitative review of the water quality and crop yield impacts of artificially drained agronomic systems across North America.

400 studies reviewed_map2

    Figure 1. An initial review of over 400 drainage studies from around the globe was performed in 2014 (Map: M. Schwartz/The Conservation Fund).

Major findings of this previously funded work included that wetter years resulted in significantly greater drainage discharge and nutrient loads than dry years (Figure 2), and there was a notable seasonal impact within years (that is, non-growing season nutrient transport poses a particular concern). The importance of drainage hydrology was a strong theme that emerged throughout the analyses which carries a special significance in the face of a highly variable climate.

Another major finding was that, historically, dissolved N loads in subsurface drainage have received much greater attention than P loads, and this now presents a large gap in knowledge. For example, look at the number of studies (“n = …”) represented by each bar in Figure 2.

MANAGE_Drain Load_basic nutrient results_1

Figure 2. Wetter site-years in the MANAGE Drain Load database had significantly greater dissolved nitrogen, dissolved phosphorus, and total phosphorus loads in agricultural drainage than drier site-years.

To provide a deeper analysis across scientific literature regarding drainage P transport, both new literature and literature previously reviewed for the MANAGE “Drain Load” table will be reviewed for drainage P and N concentration data to create a new MANAGE “Drain Concentration” table. This new data compilation will then be statistically analyzed to determine the impacts of controllable factors (e.g., crop selection, tillage, 4R practices) and uncontrollable factors (e.g., precipitation, soil drainage class) on dissolved P in drainage waters. The MANAGE Drain Concentration table will inform new insights about (1) the extent of dissolved reactive P in drainage, (2) controllable and uncontrollable factors impacting its presence, and (3) yield consequences of practices impacting its presence. Ultimately, this work aims to improve targeting and increase implementation of appropriate P loss reduction practices to improve in water quality in Illinois and across the Mississippi River Basin.

Dr. R. Daren Harmel, USDA ARS, is a collaborator on this work.  Mr. Allan Hertzberger is the graduate student project lead.

Peer-reviewed outputs from these projects include:

  1. Christianson, L., and R. D. Harmel. 2015. The MANAGE Drain Load database: Review and compilation of more than fifty years of North American drainage nutrient studies. Ag. Water Manage. 159:277-289.
  2. Christianson, L., and R. D. Harmel. 2015. 4R water quality impacts: An assessment and synthesis of forty years of drainage nitrogen losses. J. Environ. Qual. 44(6):1852-1860.
  3. Christianson, L., R. D. Harmel, D. Smith, M. Williams, and K. King. 2016. Assessment and synthesis of 50 years of published drainage phosphorus losses. J. Environ. Qual. doi:10.2134/jeq2015.12.0593.
  4. Harmel, R.D., L.E. Christianson, M. McBroom, D.R. Smith, and K.D. Higgs. 2016 (Accepted). Expansion of the MANAGE database with forest and drainage studies. J. Am. Water Res.  Association.