Focus on continuous living cover, not adding more crops to rotation, to see soil carbon gains

Alison King, Katelyn Congreves, Bill Deen, Kari Dunfield, Myna Simpson, Paul Voroney, and Claudia Wagner-Riddle. 2020. Crop rotations differ in soil carbon stabilization efficiency, but the response to quality of structural plant inputs is ambiguous. Plant and Soil. doi:10.1007/s11104-020-04728-5
Study Summary by Alison King

key messages

  • Using the long-term rotation experiment in Elora, researchers studied the impact of crop rotations on soil carbon levels
  • The amount of continuous living crop cover was the best predictor of soil carbon levels, rather than the number of crops in the rotation
  • Crop rotations with perennials or cover crops are more likely to sequester carbon than those without

Diverse crop rotations  – those with a greater variety of crops over time – have been recently promoted as a means to conserve or increase of soil organic carbon, a key indicator of soil health. But taking a critical view of this claim, is it supported by science? And if not diversity, what should be the priority in planning crop rotations, if the goal is to increase soil organic carbon?

To answer this question, University of Guelph researchers studied six crop rotations with increasing rotational diversity – from continuous corn to a corn-soybean-wheat rotation with red clover after wheat – at the Ontario Crops Research Centre–Elora’s long-term rotation trial. The researchers also investigated other characteristics of crop rotations that could control soil organic carbon levels, such as carbon added to the soil by crop roots, and whether the crop rotations included a cover crop or perennial forage (alfalfa).

What they found

Simply having more crops in rotation had no obvious effect on soil organic carbon. In fact, the corn-based crop rotation with the highest soil organic carbon levels (corn-corn-alfalfa-alfalfa) had the same amount of rotational diversity as the crop rotation with the lowest soil organic carbon levels (corn-corn-soybean-soybean).

Instead, the amount of carbon provided by roots and the inclusion of alfalfa were the best predictors of the crop rotations that supported the highest soil organic carbon. Including soybean in rotation decreased soil organic carbon, likely due to its low root biomass, or possibly other factors.

Even though cover cropping effectively increases soil organic carbon in other long-term trials, at the Elora long-term rotation trial there wasn’t a detectable difference between the corn-corn-soybean-wheat rotation and the same with a red clover cover crop. Because this rotation only included a cover crop once every four years, the results suggest that cover cropping needs to be practiced more frequently to see soil carbon gains. Continuous living cover – as is provided by frequent cover cropping or perennial crops – makes use of windows of time in the shoulder seasons for plants to be pumping carbon from the atmosphere into soil where it has a chance to accumulate over the long-term.

Why it matters

For anyone interested in managing crop rotations to improve soil organic carbon (and soil health), this study highlights that managing for diversity won’t necessarily target soil organic carbon. While there are other important reasons for rotating crops, rotation alone won’t guarantee soil carbon gains. To get a carbon benefit from crop rotation choices, focus on keeping living roots in the ground: frequently planting cover crops and rotating in perennial forages.

How they did it

From the long-term rotation trial in Elora, the six crop rotations included in the study were: continuous corn, continuous alfalfa, corn-corn-alfalfa-alfalfa, corn-corn-soybean-soybean, corn-corn-soybean-winter wheat, and corn-corn-soybean-winter wheat(red clover). All crop rotations received conventional tillage, which consisted of moldboard ploughing after grain crops followed by spring cultivation, with a few exceptions. Continuous alfalfa was moldboard ploughed once every 4 years. Soil samples were collected from the topsoil (0 – 8 inches) and carbon contributions from roots were estimated based on yield data over the course of the trial (37 years). 

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