Soil Organic Carbon Trends Over 100 Years

Accumulated soil organic carbon is an important factor in quality and productivity of soil, and maintaining or increasing soil organic carbon is an important goal of current crop and soil management. The current interest in removing crop residue to serve as a fuel source and the ongoing concern about the use of fertilizer, especially nitrogen, has increased the interest in research that addresses the effects of cropping system, residue removal, and nitrogen fertilizer rates on soil organic carbon.

The purpose of this study, conducted by researchers from the Univ. of Illinois, was to reexamine existing soil organic carbon (SOC) data from the Morrow Plots, including the use of regression to assess changes in SOC with rotation, management, and nitrogen (N) fertilizer use over time.

Some corrections with regard to management of the Morrow Plots were made, and limitations of this experiment with regard to drawing conclusions about small changes in SOC are acknowledged and discussed in the paper by the researchers.

The researchers state that soil organic carbon levels in the Morrow Plots declined during the first half of the 20th century, with greater declines in plots that received no soil amendments than in those that received modest additions of nutrients as manure+lime+phosphate  (MLP).

Declines in SOC occurred in unfertilized plots in all three cropping systems, but started at a lower level and declined more in continuous corn than in the corn–oat and corn–oat–hay rotations.

As a result, SOC levels differed by more than twofold by 1955, with the lowest value in unfertilized plots of continuous corn and the highest in MLP treatments in rotated corn.  

The rate of change in SOC among treatments during this period was positively correlated with average corn yield. All crop residues were removed, but the positive correlation between crop (and residue) productivity from 1905 through the mid-1950s and SOC levels at the end of this period indicates that residue removal had less effect on SOC than did factors such as location within the Morrow Plots and crop rotation.

Changes in SOC levels in the Morrow Plots from 1955 to 2008 were relatively small in most cases, and were not very consistently related to rotation and treatment. Losses were largest in the corn–oat (to 1967)/corn–soybean (1968 and after) rotation, lowest in continuous corn, and intermediate in the corn–oat–hay rotation.

Plots with moderate additions of nutrients manure+lime+phosphate  (MLP) consistently lost SOC, unfertilized plots lost small amounts of SOC, and chemical fertilizer additions resulted in some increases and some decreases in SOC.

The negative correlation between beginning SOC level and the change in SOC level during the second half of the 20th century indicates, according to the researchers, that treatment effects may have had less direct influence on SOC than did beginning levels of SOC. This possibility supported by the finding that a 2007 measurement of electrical conductivity, which incorporates SOC level and other inherent soil properties, revealed variability that was largely unrelated to cropping system or soil amendments treatments.

The researchers state that despite the lack of replication, inherent differences in soil properties, and the clear influence of location within the plots on yields, the Morrow Plots continue to produce knowledge regarding the effects of crop agriculture on soils. Long-term trends show that SOC values tend to move lower for some decades after they are first farmed, but that this decrease is less when modest levels of plant nutrients are provided, either by additions or by N-fixing crops in rotation.

In conclusion, these researchers state that 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).

The researchers state that while no conclusion regarding residue removal effects on SOC is possible, it is clear that adequate nutrient levels are important in maintaining SOC levels


Material taken and adapted from:

Soil Organic Carbon Trends Over 100 Years in the Morrow Plots
Emerson D. Nafziger and Robert E. Dunker
Published in Agron. J. 103:234–242 (2011)
Published online 9 Dec. 2010
Abstract | Full Text 

Feature photo of Morrow Plots courtesy of Wikipedia Commons.