Few researchers have assessed the possibly interacting effects of long-term tillage and rotation practices on organic carbon (OC) sequestration in soil to depths well beyond the maximum depth of tillage operations while also studying carbon dioxide (CO2) emissions from the soil surface of those same experiments. This study was conducted from 2003 to 2005 on tillage and rotation experiments initiated 30 yrs ago in West-Central Indiana on a dark prairie soil with silty clay loam texture.. Our objectives were to determine how tillage systems such as moldboard plow (MP), chisel (CP), and no-till affected OC retention and surface soil CO2 emissions. These tillage systems were investigated in continuous corn and corn-soybean rotations. Soil OC distribution was determined from soil cores in multiple increments to a 1.0 m depth in late 2003 and early 2004. Gas fluxes from the soil surface were measured at weekly or biweekly intervals for up to 14 weeks in the corn growing seasons of 2004 and 2005. The increase in soil OC with no-till relative to moldboard plow averaged just 8 t/ha (or 5% on an equivalent mass basis) in both rotations. Rotation systems had little impact on OC; continuous corn was not superior to the soybean-corn rotation in either no-till or moldboard plow systems. While no-till clearly resulted in more OC and N accumulation in the surface 15 cm than moldboard plow, the relative no-till advantage declined sharply with depth. Indeed, moldboard plowing resulted in substantially more OC, relative to no-till, in the 30-50 cm depth interval despite moldboard plowing consistently to less than a 25 cm depth. Growing season CO2 emissions were significantly affected by rotation but not by tillage treatments. . CO2 emission was higher under continuous corn than with corn following soybean. Our results suggest that conclusions about soil OC gains under long-term no-till are highly dependent on sampling depth and, therefore, tillage comparisons should be based on samples taken much deeper than the deepest depth of direct soil disturbance by tillage implements. After 3 decades of consistent tillage and crop rotation management, tillage system impacts on overall soil OC retention and seasonal CO2 emissions were less than expected. Continuous corn did not store more soil OC than rotation corn, perhaps because continuous corn emitted more CO2 from the soil surface than corn- soybean rotation systems.
