Comparing the short- and long-term impacts of subsurface drainage installation on soil physical and biological properties

Subsurface drainage is a common practice used to support agricultural production and increase yields in poorly drained soils. Following decades of subsurface drainage installation, agricultural fields often have increased water discharge and nutrient losses. However, few studies have evaluated the changes in soil properties or soil health metrics at different ages of subsurface drainage. In this study, we attempt to quantify changes to soil properties over time. To achieve this, we sampled six fields in northwest Minnesota representing two timescales: three fields were drained more than 15 years prior to sampling (i.e., subsurface drainage installed prior to 2006), and three fields were drained within 5 years of sampling (i.e., subsurface drainage installed after 2016). We evaluated three soil physical properties: saturated hydraulic conductivity (K_fs), bulk density, and aggregate stability, as well as three soil health metrics at 0 to 15 and 15 to 30 cm: water-extractable organic carbon (WEOC) and nitrogen (WEON), and potentially mineralizable carbon (PMC). The fields with older drainage systems had greater K_fs, WEON (all depths), WEOC (15 to 30 cm), and PMC (15 to 30 cm). There were no differences in bulk density, aggregate stability, WEOC (0 to 15 cm), and PMC (0 to 15 cm). We suspect that the increased K_fs is likely the result of further development of preferential flow pathways in fields with older drainage systems. These preferential flow paths could also be areas with increased microbial diversity and activity, indicated by the higher biological indicators in the fields with older drainage systems. Our findings suggest that nutrient losses, soil physical properties, and soil health metrics evolve over time. These metrics should be tracked as a standard practice in drainage research to improve our understanding of how subsurface drainage installation changes long-term soil properties. This knowledge will improve the information provided to growers and help them more effectively manage their soil’s health and reduce nutrient losses into waterways.