Long‐Term Soil Organic Carbon as Affected by Tillage and Cropping Systems

The use of conservation tillage systems for corn ( Zea mays L.) and soybean [ Glycine max (L.) Merr.] production has increased in recent years because of several factors including their potential to reduce losses or sequester soil organic C (SOC). This study evaluated the effects on SOC of three cropping systems (continuous corn [CC], continuous soybean [CSB], and soybean–corn [SB‐C]) in six primary tillage systems (chisel, disk, plow, no‐till, ridge‐till, and subtill) under rainfed conditions in southeastern Nebraska. Soil samples were collected in depth increments of 0 to 7.5, 7.5 to 15, and 15 to 30 cm in the fall of 1989 and 2004 after harvest and analyzed for SOC. No significant differences in SOC concentrations were obtained among tillage treatments in any depth in a partial sampling of a study that was done in 1989. Tillage treatment and cropping system both significantly affected SOC concentrations and reserves at all depths in 2004, but only bulk density at a few depths. No‐till SOC reserves ranged from 4.8 to 11.6 Mg ha −1 greater than SOC reserves in the other tillage treatments in the 0‐ to 30‐cm depth. Similarly, SOC concentrations and reserves were greatest in CC and least in CSB, with intermediate values for SB‐C in all tillage systems. Soil organic C levels were maintained or even increased in all tillage systems; however, the greatest increases were obtained in systems with the least amount of soil disturbance, which strongly supports the adoption and use of conservation tillage systems for soil sustainability.