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Scottish Environment, Food and Agriculture Research Institutes (SEFARI) fellowship Blog - 3 August 2020

Scottish arable production and climate change.  A blog by Dr Gemma Miller, the Scottish Environment, Food and Agriculture Research Institutes (SEFARI) fellowship with NFU Scotland.

The factsheet that accompanies this blog, entitled: ‘What are impacts of Scottish arable production on climate change?’ is available at: https://www.nfus.org.uk/policy/environment-and-land-use/sefari-fellowship.aspx

Winters in Scotland are predicted to become warmer and wetter due to climate change. For winter fallow fields, these conditions can lead to increased nitrogen (N) loss through leaching and nitrous oxide (N2O) emissions, as well as soil carbon loss due to increased decomposition and erosion. Cover crops can help mitigate these problems, and are becoming more popular in the UK1. So, what are the benefits of cover crops? And are they a practical option for Scottish arable farmers?



The terms ‘cover crop’ and ‘green manure’ are often used interchangeably, and here I will use cover crop to encompass both. However, the main purpose of a cover crop is to prevent the loss of nutrients from arable soils over winter months and green manures are generally grown with the specific purpose of improving soil structure and health.

As well as helping prevent soil erosion2 and improving soil structure3, cover crops can reduce indirect N2O emissions (emissions from leached nitrates or deposited ammonia). Non-legume cover crops can prevent nitrate leaching4 by holding the N in the plant biomass over winter and then releasing it back to the soil when it is incorporated. This also reduces the requirement for inorganic fertiliser in the subsequent cash crop.

Legume cover crops or leys have the benefit of being able to ‘fix’ atmospheric N. Legumes have a symbiotic relationship with soil bacteria (rhizobia) which live in nodules on the plant’s roots. The bacteria can convert N gas from the atmosphere into ammonia, which the plant can absorb, and in return, the plant provides the bacteria with carbohydrates. In this way, the cover crop is adding additional N to soil on incorporation.

However, the uptake of N by over-winter cover crops may be low due to the short daylength. One study found that naturally regenerating vegetation was just as effective5 at capturing soil N as a non-legume cover crop. There may also be a carbon sequestration benefit as biomass is incorporated into the soil. The extent of carbon sequestration will depend on the soil type and management practices. The net effect of cover crops on the greenhouse gas balance of arable land in Scotland is uncertain as there have been few studies either in Scotland or the UK.

Due to the cooler climate and later harvest in Scotland (e.g. winter wheat may not be harvested until early September), there is less time available to establish cover crops before winter and low temperatures in the winter period can kill off more sensitive species. However, it may be possible to under-sow cereal crops with a cover crop before harvest if it is chosen carefully to minimise competition with the main crop for water and nutrients.

Suitable cover crops for Scotland include fodder radish, mustard, ryegrass, and various clover species. The choice of cover crop species will depend on what the purpose of the cover crop is (e.g. to improve soil organic matter, soil structure, nutrient retention or weed control), on the planned following cash crop, and the timing of cover crop establishment.

Most of the research on the greenhouse gas and wider benefits of sowing cover crops have been conducted on organic or highly fertile soils. Studies on a wide variety of soil types in Scotland are needed to improve our understanding of the benefits and suitability of cover crops. However, based on current knowledge, it is likely that cover crops will provide benefits in terms of increased retention of nutrients and improved soil conditions, so more widespread adoption is actively encouraged.

I would like to thank Professor Bob Rees (SRUC) and Dr Cathy Hawes (JHI) for reviewing and making helpful suggestions on this blog and the accompanying factsheet.

As this is the last in this series of blogs, I would also like to thank my SEFARI colleagues Dr Philip Skuce (Moredun), Dr Charles Bestwick (The Rowett Institute) and Dr Michael Macleod (SRUC), as well as Ruth Taylor, Martin Kennedy and Jonathan Hall (NFUS) for their support and advice throughout this SEFARI Fellowship with NFU Scotland.

References

1 Starr et al (2019) A UK survey of the use and management of cover crops. Annals of Applied Biology 174:179-189

2 Abdalla et al (2019) A critical review of the impacts of cover crops on nitrogen leaching, net greenhouse gas balance and crop productivity. Global Change Biology 25:2530-2543

3 Chen et al (2014) Effects of compaction and cover crops on soil least limiting water range and air permeability. Soil and Tillage Research 136:61-69

4 Cooper et al (2017) Assessing the farm-scale impacts of cover crops and non-inversion tillage regimes on nutrient losses from an arable catchment. Agriculture, Ecosystems & Environment 237:181-193

5 Baggs et al (2000) The fate of nitrogen from incorporated cover crops and green manure residues. Nutrient Cycling in Agroecosystems 56:153-163

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Dunnydeer Farms

5 days ago

It would appear that pre- harvest Roundup May lead to higher release of N2O over the winter than having a green regeneration in the stubble.
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