Subsurface drainage intensity effects on soil physical characteristics, plant biomass production, and carbon balance

Many poorly drained soils require subsurface drainage to facilitate crop production, but excessive drainage can lead to loss of soil organic carbon (SOC). The effects of subsurface drains installed at 5‐, 10‐, and 20‐m spacings compared to an undrained 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, following 19 years of installation. The more intense drain spacings significantly reduced bulk density and moisture retention and increased aeration porosity compared to the undrained control in the surface 30‐cm soil depth. Among the 5‐, 10‐, and 20‐m spacings, total biomass production and biomass C input to soil were greatest for 5‐m spacing and least for the 20‐m spacing (average: total plant biomass = 10.1; biomass C = 4.3 Mg ha −1 year −1 ), but biomass production and biomass C input were greater for all three spacing treatments than the undrained control (total biomass = 9.4; biomass C = 4.0 Mg ha −1 year −1 ). All three spacing treatments had greater SOC mass to a 1‐m depth (average = 51.8 Mg C ha −1 ) than the undrained control (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 spacing appeared to have the best combination of increased biomass production and decreased SOC loss over the initial 19 years of drainage.