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Tillage and emissions, ploughing through the science

5th Sep 2016

Tillage has received considerable attention from researchers and policy makers concerned with climate change mitigation and reducing greenhouse gas emissions.  As farmers and growers we have choices as to how we prepare the soil; reduce weed growth, incorporate fertiliser, manures and organic matter, and ultimately the growing system we use to produce crops.  The effect of reducing tillage on the emissions of greenhouse gases has been studied by various research organisations over the last few years.  One of the reasons for this may be that if it is possible to find a method of tillage that sequesters carbon and reduces greenhouse gas emissions while at the same time growing profitable crops, then it would be a fierce weapon in agriculture’s armoury against climate change.

Emissions from tillage

The emissions that are concerned with tillage are mainly carbon dioxide and then to a lesser extent nitrous oxide and methane.  When soil is cultivated, carbon dioxide is released into the atmosphere. This is principally as a result of the oxidation of the soil organic matter (SOM) by microbial activity that is stimulated by available oxygen following a mechanical cultivation.  As well as the release of carbon dioxide from the soil, there are also the associated emissions with the machinery and fuel use (which we will look at in more depth next week).

Over the last few years there have been a growing number of farmers and growers who are adopting reduced or zero tillage systems, (see East Hendred Farm case study as an example).   Reduced (sometimes called conservation) tillage has been suggested as a management strategy that offers many benefits to farmers in terms of sustainability credentials.  These benefits include increased soil organic matter levels, increased carbon sequestration, mitigation of greenhouse gas emissions, greater aggregate stability and biological activity, and prevention of soil erosion and runoff.  One study has concluded that across Europe, if 50% of farmers adopted a no till approach, 0.4% of all anthropogenic CO₂ emissions could be offset.

However with the diverse nature of farming, it is not as simple as it first seems.  With variations in soil type and structure across the UK, reduced tillage is not suitable for all farming systems.

Reduced tillage: the science so far

Under appropriate conditions reduced tillage systems may improve yield performance as well as energy and resource efficiency and may mitigate CO₂ emissions and increase the level of Carbon that is sequestered in the soil.  However the benefits of reduced tillage are soil specific.  Research has shown that particularly on soils with a high loam or clay content and where the climate is cool and wet, then the soil may end up with increased dry bulk density and reduced aeration of the soil, which will stimulate the release of nitrous oxide which will then offset the benefits in terms of CO₂ reduction.

With the associated benefits of reduced tillage in terms of fuel and time savings, less wear and tear on machinery as well as the benefits to the soil structure in terms of productivity and reduced emissions, then you would think that there would be a lot more farmers doing it.  However ploughing still remains widespread.  The major reason as to why this is, it that, reduced tillage has been shown in some studies to have a negative impact on yield.  Further causes for negative effects on yields in no-till systems compared to reduced or conventional tillage systems can be aggravated disease and pest development resulting from large quantities of crop and root residues close to the soil surface.

Management of Nitrogen in different tillage systems

Nitrous oxide emissions from soils after the use of N fertilisers can be higher in zero tilled than conventional tillage systems.  Indeed some studies are now showing that while reduced tillage systems may result in the accumulation of more carbon in the soil compared to conventional tillage, there is an increase in nitrous oxide and methane emissions.  As such if reducing tillage also results in increased nitrous oxide and methane emissions, the benefits of increased soil carbon sequestration in relation to greenhouse gas emissions will be offset.

However the results are not consistent.  In some studies, conversion from conventional tillage to no till, can significantly reduce methane and nitrous oxide emissions, and in some research when high levels of N fertiliser are applied the emissions from nitrous oxide are much higher than in a conventionally ploughed system.  As such looking at the Nitrogen fertiliser policy, and planning applications to only apply what the crop needs, when the crop can take it up will help to reduce these losses.

Moving forward

The research does all seem to conclude that in the right soil conditions, reducing tillage will result in reduced CO₂ emissions and an increased level of soil carbon sequestration.  However what is still not clear is what the effects are of other management practices on these fields, for example applying N fertiliser on the emissions.  The two aspects of soil management that seem to have the largest effect on emissions are tillage and fertiliser management.  As such moving forward, we need more research to allow us as farmers to get the best result from reducing fuel and input costs, and reducing emissions as well as growing profitable crops.