Abstract: Subsurface drainage (i.e., tile drainage) lowers high water tables in poorly drained agricultural soils. Grass seed farmers in the Willamette Valley, OR, rely on tile drainage to reduce saturated conditions during the months-long rainy season. Improvement in soil moisture allows growers to have timely access to fields, extend the growing season, expand cropping options, and achieve higher yields. Willamette Valley soils are high in soil carbon (C), and grass seed production lends itself to adhering to many soil health principles that aim to sequester and maintain C within the soil. However, there is virtually no research that investigates the impact of tile drainage systems on soil health and the fate of soil C. Our research objective was to determine whether new-tiled (< 5 years post installation), old-tile (5 > years post installation), and untiled (no history of subsurface drainage) fields increase, decrease, or have no change on soil C stocks. To accomplish our objectives, we conducted a field study in the southern Willamette Valley that identified an age gradient of tile drainage systems (n=5 of each field treatment) within grass seed cropping systems. We collected cores from the Dayton series, characterized by a distinct textural change from silt loam to heavy clay in the Bt horizon. We sampled to a depth of 1 m with a UTV-mounted hydraulic probe and sectioned cores based on horizons falling above (Ap/E), within (Bt), and below (BC) the argillic horizon. To understand the impact of drainage on C storage dynamics, we measured total C, microbial biomass and respiration, and potentially oxidizable C and other soil health indicators. We used an equivalent soil mass approach to determine the soil C stocks based on horizon depths. Preliminary results show cumulative soil C did not differ between drainage treatments, and soil C decreased with depth. Additionally, there were no differences in soil C between the drainage treatments in the Ap/E and Bt horizons. However, there was more soil C in the old-tile fields within the deepest (BC) horizon (p < 0.001). These findings indicate that tile drainage may promote the accumulation of deep soil C in the long term. The results of this study will provide growers with information that they can use to inform the management practices utilized on their farms.
