Tillage??

This post is going to look at another crop management strategy, aiming to increase soil carbon content, tillage.  Tillage is the preparation of soil before crops are planted, it can be done by hand or machine and involves processes such as digging and overturning.  Altering tillage methods can now be used as a mitigation strategy for climate change across the world due to advances in farm machinery and farming methods.  

Tillage causes disturbance of the soil which stimulates losses of carbon through enhanced decomposition and erosion.  Therefore it has been put forward that changing conventional tillage methods to reduced/conservational tillage or no-till can result in carbon gain in the soil and a reduction in carbon dioxide emissions as a result of a lessened use of farm equipment.  The picture below describes the variation between the different types of tillage.  

A study done by West and Marland (2002) in the USA suggest that reduced tillage practices could contribute to making US agriculture ‘carbon neutral’ over the next forty years.  In this study, they compared conventional tillage methods with no-till.  They estimated the energy use and carbon emissions from all aspects of crop growing, such as including fuels, fertilisers and farm machinery.  They estimated no-till emitted less carbon dioxide from agricultural operations than conventional tillage and that no-till increased carbon sequestration due to less soil disturbance.  The enhanced carbon sequestration is limited over time as once it reaches a peak the soil will be unable to absorb anymore carbon, however they argue that if no-till practices are continued then the soil carbon content will be maintained, and the reduction in carbon dioxide emitted to the atmosphere as a result of lessened fossil fuel use will continue indefinitely.  Cerri et al (2004) undertook a similar study in Brazil and found that converting to no-till methods is increase carbon sequestration in the soil, currently accumulating 9 Mt C per year.  

Changing to reduced tillage or no-till seems a successful way of lowering carbon emissions, but there is some uncertainty whether converting to these methods is beneficial in all locations.  Hermle et al (2007) studied tillage in North East Switzerland.  They analysed different tillage methods in the region, from no-till to ploughing and found there was little difference in the amount of carbon sequestrated.  The reason given for this is the climatic conditions, being moist cold-temperate solid.  

Another problem that has arisen as a result of suggested tillage mitigation strategies is the impact on nitrous oxide emissions.  This is discussed by Li et al (2005), it is thought that switching from conventional to no-till can increase nitrous oxide emissions by 2.5±0.5 kg N ha^-1 yr.^-1 for humid environments and 0.8±1.0 kg N ha^-1 yr.^-1 for dry environments which offsets some of the carbon sequestration gains as nitrous oxide is also important greenhouse gas.  Some have found these nitrous oxide emissions to subside after a few years after the conversion but these results are variable.  Li et al do state that despite the nitrous oxide emissions, no-till cropping is beneficial as it reduces fossil fuel use, it reduces soil erosion, enhances soil fertility and also water holding capacity.  

Using reduced tillage or no-till does seem on the whole to be a successful mitigation strategy to improve carbon sequestration.  In many studies across the world it has shown to be effective in increase soil carbon content.  Using reduced tillage or no-till is also beneficial as it reduces carbon dioxide emissions from the practice of farming, such as machinery use.  These methods also improve the quality and fertility of the soil as less erosion occurs and less nutrients are lost, therefore despite uncertainty about nitrous oxide emissions, if the soil holds its nutrients better then perhaps fewer fertilisers can be used on the soil reducing nitrous oxide emissions in the long run.