Increasing soil carbon with no-till - East versus West

Sunday, October 5, 2014

Tests show that, on its own, no-till is less effective at increasing carbon levels under Ontario's climatic conditions than on the Prairies

by KEITH REID

One of the most common recommendations for improving soil health is to increase its organic matter or organic carbon concentration. Increased soil carbon is linked to improved soil structure, reduced crusting, improved moisture-holding capacity and higher crop yields under stress conditions. Why, then, is it so difficult to build levels of organic carbon in our soils?

The simple answer is that organisms in soil are efficient at breaking down added organic material and using it for their own growth and reproduction. This is particularly true for material mixed into the soil by tillage. Leaving residue on the surface, therefore, should slow down decomposition and allow more of the carbon to stay in the soil. This has been shown to be quite effective in the semi-arid environment of the Prairies, but much less so in Eastern Canada. Why are the responses to no-till different in these parts of the country?

Research by Dr. Ed Gregorich of Agriculture and Agri-Food Canada in Ottawa, in collaboration with colleagues Dr. Bobbi Helgason in Saskatoon and Dr. Henry Janzen in Lethbridge, is helping to answer this question. Using carbon-13-labelled barley straw that was either incorporated into the soil or left on the surface, they were able to track how much of the carbon was disappearing, and where the remainder was ending up. Plots were established in Ottawa and Lethbridge, and measurements were taken for two years after residue application.  

Where the barley straw had been incorporated into the top 10 centimetres of the soil, almost 80 per cent of it had disappeared after two years. There was slightly more carbon retained at Lethbridge than Ottawa in the incorporated treatments, but the difference was only about 10 per cent of the initial carbon application.

The real surprise, however, was what happened in the plots where the straw was left on the soil surface. At Ottawa, almost exactly the same amount of carbon had disappeared in the no-till plots as where the carbon was incorporated. In contrast, the plots at Lethbridge had retained almost twice as much carbon in no-till than where the straw was incorporated.  

Dr. Gregorich and his colleagues went on to look at the amount of the labelled carbon that was showing up in various microbial communities in the soil. There were similar amounts of microbial C in the incorporated treatments at both Ottawa and Lethbridge, and in both cases there was less of the applied C in the microbial biomass where the residues were left on the soil surface.

The difference was much greater, however, in the semi-arid conditions at Lethbridge, particularly during the first year of the study. It is evident that the bacteria and fungi could not easily decompose the material sitting on the surface, where the soil is often dry in semi-arid conditions. This difference was also greater for samples taken in the fall (after the dry summer conditions) than in the spring.

As well, there were subtle differences in the balance between bacterial and fungal carbon in the soil, although they were not consistent over time. The shifts probably represent a succession of microbial communities that break down different parts of the residue (e.g. lignin by fungi versus cellulose by bacteria), and then are incorporated into secondary microbial communities as they die and are eaten in turn. This succession appears to be slower under the semi-arid prairie conditions, when the residue is left on the surface, compared to incorporated residues in either location or the surface-applied residues in Ottawa.

The implication of these findings is that no-till, on its own, may not always significantly increase soil organic carbon levels under the climatic conditions in Ontario, although it does appear to be effective in the Prairies. If we are going to build organic carbon levels in our soils, it will take a combination of practices that both increase the amount of organic matter added to the soil and reduce the losses.

Livestock manure, cover crops and crop rotations that include forages will be necessary components of a "high-C" cropping system. No-till, or reduced till, will have a place, but the impact will be from reduced soil erosion so that more of the newly formed organic matter is protected from washing or blowing away. BF

Keith Reid is Soil Scientist, Agriculture and Agri-Food Canada, Guelph.