Soil organic carbon trends over 100 years in the morrow plots

Soil organic carbon (SOC) has been monitored for more than 100 yr in The Morrow Plots at the University of Illinois. From 1904 to 1955, SOC in the top 15 cm of soil decreased in unfertilized plots, but decreased less or remained the same in plots with added manure+lime+phosphate (MLP). A chemical fertilizer treatment (LNPK) was added in 1955, and in 1968, the corn (Zea mays L.)-oat (Avena sativa L.) rotation was converted to corn-soybean (Glycine max L. Merr.), and a second LNPK treatment (LhiNPK) was added with a higher N rate. From 1968 through 1998, SOC values declined by 15 to 19% in LhiNPK, LNPK, and MLP in the corn-soybean rotation and by 8% in one LNPK treatment in corn-oat-hay, but not in other treatments. Soil organic carbon values from 2007-2008 deviated by only 0.2 g C kg^-1 from values projected using regressions from the 1968 to 1998 period. While SOC was lost from the surface soil of unfertilized plots with continuous residue removal during the first half of the 20th century, SOC trends since then are inconsistent, and appear to be inversely correlated to beginning SOC levels. Over all rotations and treatments, SOC loss averaged only 1.1 g C kg^-1 from 1955 to 2008. While no conclusion regarding residue removal effects on SOC is possible, it is clear that adequate nutrient levels are important in maintaining SOC levels.