Conservation Drainage Network

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How Long-Term Subsurface Drainage Changes Soil Properties

Summary prepared by: Sushant Mehan, Department of Agricultural & Biosystems Engineering, South Dakota State University

Frankl, A. L., Sherbine, K. T., Strock, J. S., Fernández, F. G., Cates, A. M., & Pease, L. A. (2023). Comparing the short- and long-term impacts of subsurface drainage installation on soil physical and biological properties. Journal of Soil and Water Conservation, 78(6), 457–465. https://doi.org/10.2489/jswc.2023.00147

Major Findings:

Researchers evaluated seven agricultural fields in northwest Minnesota with different drainage histories:

  • >15 years since drainage installation
  • <5 years since drainage installation
  • One undrained reference field

Soils were sampled at 0–15 cm and 15–30 cm depths during the 2021 growing season. Measurements included:

  • Saturated hydraulic conductivity (Kfs)
  • Bulk density
  • Aggregate stability
  • Soil biological indicators:
    • Potentially mineralizable carbon (PMC)
    • Water-extractable organic carbon (WEOC)

Results showed:

  • Fields drained for more than 15 years had significantly higher saturated hydraulic conductivity, indicating faster water movement through the soil profile (Figure 1). This increase is likely due to the long-term development of preferential flow paths, such as macropores and cracks, that connect the soil surface to subsurface drains.
  • At depths below 15 cm, fields with older drainage systems showed higher biologically active carbon and nitrogen pools, including greater potentially mineralizable carbon, water-extractable organic carbon, and water-extractable organic nitrogen (Figure 2). These indicators suggest enhanced microbial activity in deeper soil layers over time.
  • Despite expectations, bulk density and aggregate stability showed little difference between newer and older drainage systems. Tillage practices appeared to have a stronger influence on these properties than drainage age alone.

Figure 1. Fields drained for more than 15 years showed faster water movement through soil compared to recently drained fields, indicating long-term development of preferential flow pathways. (Taken from Frankl et al. (2023)).

Contributions to the conservation drainage community

Drainage impacts are dynamic, not one-time changes. Older drainage systems may: (i) Move water and nutrients more efficiently; (ii) Increase connectivity between surface soils and drains; and (iii) Influence long-term nutrient loss pathways. Soil health indicators can improve below the surface, even if surface properties appear unchanged. Considering drainage age alongside management practices may improve decisions related to nutrient management, drainage water recycling, controlled drainage, and edge-of-field practices.

Subsurface drainage systems continue to reshape soil physical and biological processes for decades after installation. Tracking soil health and hydrologic indicators over time provides a more complete picture of drainage impacts and supports better long-term water quality management.

Figure 1. Soils in fields with long-term subsurface drainage showed greater biologically active carbon and nitrogen pools, particularly below 15 cm, indicating increased microbial activity over time. (Taken from Frankl et al. (2023)).