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The debate over nitrous oxide emissions from cover crops

28th Aug 2015

A few weeks ago (back when the sun was shining and there was hope that we would have a nice hot summer!) I was getting ready for presenting at the National Organic Combinable Crops event and sorting out information to accompany the carbon footprint that we did of the host farm. One of the issues that I was trying to get my head around to explain was the GHG emissions associated with cover crops.

Indeed on the host farm leguminous green manures (mainly red clover) contributed 17% of emissions through nitrous oxide released. In organic systems (as the host farm was) legumes are often seen as the highest risk of nitrous oxide emissions. When the legume is incorporated and the soil bugs mineralise the Nitrogen, it is then available. It is this flush of Nitrogen that needs to be properly managed to be fully utilised by the following crop to avoid losing the Nitrogen that has been biologically fixed.

During researching my talk I found that the area of nitrous oxide emissions from using cover crops was something which needed further research and conflicting results had been found. As this month is nitrous oxide month, it was something that I was keen to come back to and explore further.

The rest of this blog is based on information from a research paper that was published at the end of 2014 (and can be accessed here). This paper has performed a meta-analysis on existing research to try and draw some conclusions as to the dynamics of nitrous oxide emissions from cover crops and how we can maximise the positive benefits that arise from using these crops and minimise the potential losses.

The well-known benefits of cover crops

There are many environmental benefits to incorporating cover crops into rotations including their potential to reduce soil erosion, reduce nitrate leaching losses, build soil organic matter levels (which leads to an increase in soil carbon stocks), reduce pest and weed pressures and provide a biologically fixed source of Nitrogen to the following crop.

How do they impact on Nitrous oxide emissions?

The net impact of cover crops on nitrous oxide emissions is not well understood. Nitrous oxide is emitted as a natural part of soil ecosystem function; however the extent to which it is emitted depends on various factors. These include the available mineral Nitrogen content, the soil water content, the availability of carbon and the physical structure of the soil. Cover crops can affect these factors and as such impact on the amount of nitrous oxide that is emitted.

For example when a cover crop is growing, it can suck up soil mineral Nitrogen (thus minimising the risk of ‘excess’ nitrogen in the soil to turn into N2O), as well as decreasing soil water through transpiration. Once the cover crop is cut, the mulching effect of residues on the soil surface can increase soil water (and thus the potential for the anaerobic conditions needed for denitrification and N2O emissions), depending on weather conditions. Also these residues decomposing on the soil surface can temporarily immobilise soil Nitrogen, and then increase carbon (from the residues) and Nitrogen which will affect emissions as well (it’s all very complicated!).

This research looked at three different suggestions as to the dynamics of cover crops and nitrous oxide emissions. These were:

That the type of cover crop would affect the nitrous oxide emissions (whether it was a nitrogen fixing legume or not). Legumes logically should have a greater potential to increase nitrous oxide emissions due to the fact that they are biologically fixing Nitrogen from the atmosphere.

Rainfall and the residues themselves would have an impact – because the denitrification process (which emits N2O) requires anaerobic conditions, if the soil is waterlogged then oxygen will be lacking, and a carbon source (provided by the residues).

Timing of measurements of N2O would be influential on what the different research papers had concluded. As mentioned earlier, minimising N2O emissions from cover crops has a lot to do with achieving synchrony of Nitrogen and managing the ‘flush’ of nitrogen that comes when the cover crop is incorporated – if measurements are taken during this flush then the emissions will be higher than if they are taken earlier in the growing season.

What did the research find?

After analysing lots of results, there were some conclusions that we can use to help us in understanding the dynamics of nitrous oxide emissions.

Denitrification – this process (which produces N2O) requires a Carbon and a Nitrogen source

Types of cover crop – was found to have an impact on nitrous oxide emissions. Because cover crops take up Nitrogen that might otherwise be lost to leaching, or because legumes fix nitrogen, cover crops may increase soil Nitrogen availability during decomposition and may increase the amount of nitrate available for the denitrification process (and thus nitrous oxide emissions from the field).More research is needed in this area.

When you measure it – as suggested earlier, depending on when you measure emissions the amount will change. Interestingly when looking at longer term experiments this study suggested that there is a net neutral effect of a cover crop on Nitrous oxide emissions, even if particular periods of the year see larger impacts. Again more research needed.The analysis revealed that the highest emissions occurred during the cover crop decomposition period, as the additional carbon added from the residues, with the available mineral Nitrogen led to high emissions of N2O. Emissions from the cover crop growth period were found to be the lowest of all the measurement periods. This suggests that the nitrogen uptake by cover crops holding that nitrogen over the risk period creates a larger sink than a source of nitrous oxide emissions or nitrate leaching.

To incorporate or not? The N2O emissions from studies that ploughed in cover crops rather than leaving them on the surface were significantly higher.Incorporation of cover crop residues contributes to an increase in Nitrous oxide emissions.

The effect of rainfall – Cover crops may alter the soil water status and thus the potential for anaerobic conditions (and denitrification) which normally follow intense rainfall events. Studies looked at had different responses to rainfall, however this report concluded that regardless of the cover crop type, above a certain threshold of rainfall a field with a cover crop is more susceptible to nitrous oxide emissions than one without.

Soil Organic carbon – The study found inconclusive evidence that cover crop biomass was an important factor controlling N2O emissions.

The effect of cover crops on global warming potential – Nitrate lost through leaching from fields is subject to denitrification and Nitrous oxide emissions off-site. Therefore given the ability of cover crops to reduce nitrate leaching losses, cover crops may contribute to an overall decrease in net global warming potential.

What did they conclude?

This is one of the most comprehensive analyses of research to date, but in some ways it throws up more questions than it answers. This highlights the fact that this subject needs more research done into the impact of different cover crops within rotations and the effect on emissions from management of these crops. It also shows that there isn't ‘one size fits all’ answer to these questions, due to the diversity of agricultural systems, crop rotations, soil types, weather (and all the other factors) each system is unique.

This study did conclude that from the literature analysed cover crops increased N2O emissions from the soil surface in 60% of published observations while cover crops decreased nitrous oxide emissions from the soil surface in 40% of studies.

Legume cover crops had higher relative N2O emissions at low N rates and lower emissions at high N rates whereas nitrous oxide emissions from non-legume cover crops increased as N rate increased.

Cover crops on average only lead to a small or negligible increase in N2O emissions when measured for time periods of one year or greater.

More research is needed!

Source: Basche et al (2014), Do cover crops increase or decrease nitrous oxide emissions? A meta - analysis, Journal of Soil and Water Conservation