Timely agricultural production activities in poorly drained soils require that drain tiles be installed to maintain a suitable water table level. However, excessive drainage can lead to a loss of soil organic carbon (SOC), an increase in atmospheric CO2 and potential climate change. The effects of drain tiles installed at 5-, 10-, and 20-m spacings compared to a non-drained control (40-m spacing) on soil physical properties, biomass production, and C balance were evaluated for a low organic matter silt loam soil in Indiana, USA, following 20 yrs of installation. Drain spacing significantly reduced bulk density and moisture retention and increased total porosity compared to the non-drained control in the surface 30 cm soil depth. Soil organic C was significantly increased for the 20-m spacing relative to the non-drained control in the surface 5 cm soil depth. Equivalent SOC mass calculated to 1 m depth ranged from 48 Mg C ha-1 for non-drained to 52 Mg C ha-1 in the 20-m drain spacing. Among the 5-, 10-, and 20-m spacings, total biomass production and biomass C input to soil was greatest for 5 m and least for the 20-m spacing (average: total plant biomass = 11.1 and biomass C = 4.7 Mg ha-1 yr-1), but biomass production and biomass C input was greater for all 3 spacing treatments than the non-drained control (total biomass = 10.3 and biomass C = 4.3 Mg ha-1 yr-1). Although biomass C input into soil among the 5-, 10- and 20-m spacings was greatest for 5 m, SOC accumulated to 1 m depth was smallest for the 5-m spacing and greatest for the 20-m spacing, indicating that SOC loss was greatest for the more intensively drained 5-m spacing. However, all three drain spacing treatments had greater SOC (average = 51.8 Mg C ha-1) than the non-drained control soil (48.3 Mg C ha-1). The results showed that for soils low in SOC, long-term subsurface drainage at the appropriate drain spacing could be beneficial to C accumulation. In this soil, the 20-m drain spacing appeared to have the best combination of increased biomass production and decreased SOC loss.