Soil organic carbon responses following two years of subsurface tile drainage installation in Northwest Minnesota

Installation of subsurface tile drainage systems is growing in Minnesota as a management technique to mitigate the impacts of changing precipitation patterns and the initial effect of drainage on soil organic carbon cycling is poorly understood. Altering the water table in arable soils may impact biogeochemical cycling and soil processes by changing soil oxygen levels and subsequently, where soil biota can persist. Drainage is likely to expose previously protected soil organic matter (SOM) to microbial decomposition, with unknown consequences for soil carbon cycling and storage. This two-year study tracked changes in SOM down to 90 cm in silty clay loams at the University of Minnesota Northwest Research and Outreach Center in Crookston, MN following fall 2019 drainage installation. The objective of this project is to help us evaluate changes in carbon cycling in recently drained soils, and whether drainage is likely to increase or decrease total C stocks. These measurements have implications for grower participation in carbon markets and government programs incentivizing climate-smart agriculture.

As bulk soil C stocks may change slowly, we measured potentially mineralizable carbon (PMC) and water-extractable organic carbon (WEOC), indices of labile C, three times per year. Particulate (POM) and mineral-associated organic matter (MAOM), representing short and long-term SOM storage, were measured annually and characterized by their quantity, quality, and relative contribution to total SOC. Finally, carbon dioxide (CO₂) and methane (CH₄) emissions were measured across the 2020 and 2021 growing seasons to observe if microbial byproducts are changing. We found that PMC and WEOC were significantly influenced by the season in which sampling occurred, not drainage. The POM characteristics were influenced by drainage treatments with drained plots containing greater quantities (0-30 and 60-90 cm) and a greater relative contribution of POM-OC to total SOC (30-90 cm). The MAOM characteristics changed over time, independent of drainage treatments, with decreased quality (30-90 cm) and an increased relative contribution of MAOM-OC to total SOC (0-90 cm) in the final year. These results indicate POM is more affected by large-scale management decisions, whereas MAOM is affected by shifts over time in local, small-scale processes.  Drainage did not significantly affect CO₂ and methane CH₄ emissions; however, emissions were significantly less in 2021 due to decreased precipitation with the drought. Overall, we found little evidence of subsurface tile drainage depleting SOM pools or increasing greenhouse gas emissions. Future research should be conducted to evaluate how soil C is affected by subsurface drainage in the long run.