Quantifying changes in soil health over time: Soil organic carbon and nitrogen storage due to long-term tillage system, crop rotation, cover crop, and nitrogen fertilization

Objectives:

• Using the long-term experiment at Ridgetown, which was initiated in 1995 and is applicable to humid-temperate climates, the objectives were to:
  • Assess the differences in soil health (soil organic carbon and total nitrogen) among crop rotations (Corn-Corn (C-C), Corn-Soy (C-S), Soy-Winter Wheat (S-W), Corn-Soy-Winter Wheat (C-S-W), Soy-Winter Wheat/Red Clover (S-Wrc), Corn-Soy-Winter Wheat/Red Clover (C-S-Wrc)), tillage systems (zone-tillage vs. conventional fall plow with spring secondary tillage), and nitrogen (N) fertility treatments (starter vs. starter + 150 lb N/ac).
  • Determine the influence of time on soil organic carbon (SOC) and total nitrogen (TN) accumulation by identifying sequestration rates and the change in storage from the last assessment (2006) to 2019.
  • Quantify the role of red clover cover crop and N fertilizer in crop rotation on SOC, TN storage, and sequestration rates as influenced by tillage.
  • Characterize the relationship of soil health (SOC and TN) with crop yield and variability in yield among years (i.e., resiliency).

Impacts:

• There are very few long-term trials that evaluate so many factors comparing tillage systems, crop rotation, cover crops, and N fertilizer input on soil and crop attributes. This was a powerful opportunity to compare the impact of Ontario relevant production systems on soil health.
• Many farmers know that winter wheat and red clover can improve soil health, lower fertilizer N inputs for corn, and increase crop yield stability (under drought/flooding). Here, we observed that soybean and corn yield stability (standard deviation over 5 yrs) was correlated to SOC stocks, demonstrating the value of investing in soil health practices.  
• As carbon trading (or taxing) policy moves forward, it is imperative that accurate Ontario soil C storage and sequestration rate data are available from as many different and relevant production systems as possible. This study, and others in Ontario, ensures that knowledge is available.

Scientific Summary:

Maintaining crop productivity is critical to Ontario farmers for which healthy, resilient soil plays a key role. This is particularly true in extreme weather years where drought or excessive moisture conditions are exacerbated in poor, degraded soils. A crucial driver of soil health is organic matter and organic carbon (C). While winter wheat and red clover improvements on soil health and system resiliency have been well documented in Ontario, little is known on how quickly changes to soil health can be made and under which management systems. With Federal and Provincial government policies on C trading (or taxing), agriculture may benefit by considering C stored in soil. However, meaningful Ontario data are needed. The quantity of C stored and the rate of C storage under many current different production systems (i.e., different tillage systems, crop rotations, cover crops, and synthetic N fertilizer inputs) may allow for farmers to be compensated for C sequestration. Regardless of policy, having valid Ontario data is critical and the long-term tillage system and crop rotation trial at Ridgetown, ON, is well suited to provide this information.  

At Ridgetown, different tillage systems (no-till vs. fall plow), crop rotations (corn, soybeans, winter wheat with/without red clover), and N fertility treatments to corn and winter wheat were established in 1995. Previous work demonstrated that winter wheat increased soil health after 13 years of production. However, SOC and total nitrogen stocks (two important factors of soil health) had only been measured once (2006), which was prior to when management changes were made (to increase fertilizer N rates, include treatment of frost-seeded red clover into winter wheat, and change from no-till to zone till). Our proposed research was timed to assess the impact of these improvements to the long-term trial and to compare changes from 2006 to 2019 by quantifying SOC and TN stores and sequestration rates from 2006-2019 at Ridgetown. The proposed research identified the combination of Ontario agricultural practices (tillage, crop rotation, cover crops, N fertility) that (i) improved soil health, (ii) had the highest capacity for sequestering SOC and TN over-time, (iii) and provided resiliency to the system by relating soil health with crop yield and variability between years.

Results

Approach Matters: Correctly quantifying the amount of SOC in the lab is critical. However, how SOC concentrations are converted to stocks (i.e., Mg per ha) influences conclusions drawn from the same dataset. For instance, SOC expressed as a concentration (%) showed no difference between tillage systems below 5 cm depth. When taking into consideration the weight of the soil (i.e., equivalent soil mass basis) in each tillage system, then SOC stocks were greater in the no-tillage system to 60 cm depth.

No-till systems can build soil health. Results of this study demonstrated that no-tillage had a greater potential in building soil health (SOC and TN) than the conventional fall plow tillage system in 0-60 cm depth. Soil organic carbon and TN storage was greater in no-tillage than conventional tillage systems at all the tested depth increments. Note: no till was no till before soybean and winter wheat, strip-zone till before corn.

Results indicated that planting red clover did not result in changes to SOC and TN storage. Building SOC depends on good red clover stands, but plant stands depend on the amount of fertilizer N applied. This is a confounding effect. Further explorations are needed to tease apart this interaction. It is worth noting that red clover was only introduced into the experiment in 2008, so perhaps more time is needed to observe changes in C storage.

Reduced applications of fertilizer N did not lower SOC and TN concentrations in all crop rotations and tillage treatments.  We did not detect an influence of in-season N fertilizer (applied to corn and winter wheat) on SOC and TN.

Soil organic carbon did not decline over 13 years. Crop rotations that included winter wheat had the greatest SOC and TN.

External Funding Partners:

Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA) through the Ontario Agri-Food Innovation Alliance funding program.

Mitacs Elevate Program

Project Related Publications:

Chahal, I.; Peng, Y.; Hooker, D., Van Eerd, L. L. Long-term crop rotation diversity, tillage system, and nitrogen fertilizer effects on crop productivity, soil organic carbon and nitrogen concentration in a temperate humid climate. Soil Science Society of America Journal. S-2025-06-0203-OA. Revisions requested. Resubmitted 11 Nov. 2025.

Chahal, I., Peng, Y., Hooker, D., and Van Eerd, L.L. 2025. Long-term tillage and crop rotation effects on soil carbon and nitrogen stocks in southwestern Ontario. Invited submission. Canadian Journal Soil Science. Special Collection on Conservation Tillage. 105:1–10.

Peng, Y., Chahal, I., Hooker, D. and Van Eerd, L.L. 2024.  Comparison of equivalent soil mass approaches to estimate soil organic carbon stocks under long-term tillage. Soil & Tillage Research. 238:106021.