Soil Organic Carbon Trends Over 100 Years in the Morrow Plots.

The Morrow Plots on the campus of the University of Illinois were established in 1876, in a Flanagan silt loam (fine, montmorillonitic, mesic Aquic Arguidoll) soil developed in 100-150 cm of loess over calcareous till, with an A horizon about 46 cm thick. Plot 3 has had continuous corn since the inception of the experiment. In 1904 soil fertility treatments were implemented as manure+lime+P (MLP) treatments, and, since 1955, as lime+N+P+K (LNPK) treatments on the north half of the main plots. Crop residue was removed from all plots before 1955. From 1904 to 1955, SOC in the top 15 cm of soil decreased, from 21.1 to 14.0 g kg^-1 (a loss of 13.8 Mg C ha^-1) in the unfertilized plots, but was unchanged (21.5 g kg^-1 to 21.7 g kg^-1) for the MLP treatment. From 1955 to 1967 corn residue was removed from all but the LNPK treatment. During this period SOC values changed from 14.0 g kg^-1 to 14.7 g kg^-1 (an increase of 1.17 Mg C ha^-1) for the unfertilized treatment, and from 21.7 g kg^-1 to 21.5 g kg^-1 (unchanged) for the MLP treatment. The LNPK treatment, with 19.6 Mg C ha^-1 of residue returned over this period, had SOC values increase from 14.0 g kg^-1 to 15.7 g kg^-1 (an increase of 3.3 Mg C ha^-1). After 1967 corn residue was returned to all treatments.  SOC changes for 1967-2007 were 14.7 g kg^-1 to 14.4 g kg^-1 (unchanged) for unfertilized treatment, from 21.5 g kg^-1 to 19.4 g kg^-1 (decreased by 1.2 Mg C ha^-1) for the MLP treatment and from 15.7 g kg^-1 to 16.5 g kg^-1 (increased by 1.6 Mg C ha^-1) for LNPK. Thus it appears that, while SOC was lost at similar rates from the surface soil of unfertilized and modestly-fertilized plots under continuous residue removal during the first half of the twentieth century, loss rates have stabilized, and in the case of well-fertilized continuous corn with residue returned, may be increasing slightly over the past fifty years.