Long-Term Tillage Effects on Extractable Organic Matter Fractions in Surface Soil on an Agricultural Midwestern Field Site

This study evaluated whether long-term no-till and chisel tillage differed in total organic carbon (TOC), total nitrogen (TN), alkaline-extractable fractions, and associated indices. A field experiment in Indiana, USA, began in 1983 with moldboard, ridge, chisel, and no-till systems within four crop-sequence plots. In 1998, moldboard was converted to chisel and ridge to no-till; thus, plots had been under their 2017 tillage system for either 34 or 19 years at sampling. Sixteen composite samples were collected from the 0 to 15 cm layer. Because each crop sequence occupied one main plot, crop sequence was treated as field history rather than as an independent factor. A planned contrast compared current no-till with current chisel while accounting for the historical tillage assignments. Compared with chisel, no-till increased TOC by 11%, total extractable carbon (TEC) by 32%, glucose-equivalent-corrected TEC by 32%, total extractable nitrogen by 50%, and total glucose-equivalent carbon by 31%; TN did not differ. Among the indices, only the humification ratio differed between tillage systems. Tillage effects were more consistent for extractable amounts than for relative or optical properties.