Knowledge for better food systems

Soil Association report on soil carbon

The Soil Association has published 'Just say N2O: From manufactured fertiliser to biologically-fixed nitrogen.' This report reviews the extent to which organic systems can meet the double challenge of reducing nitrogen losses and building stores of soil organic nitrogen so as to reduce dependency on manufactured nitrogen.

It argues that scientific evidence shows that the lower nitrogen inputs in organic farming can lead to lower N2O emissions compared to non-organic farms but that, given the imperative to reduce our reliance on manufactured nitrogen and improve the efficiency of nitrogen use more research is needed in a number of key areas. These are as follows:

  • The Government should investigate the likelihood that legume-based organic systems and those using manufactured nitrogen behave differently in terms of nitrogen cycling, retention and loss. There is an urgent need to understand the consequence of this for long-term soil fertility, reducing GHG emissions, storing carbon in soil and reducing our dependence on manufactured fertilisers.
  • The Government should fund research that looks in detail at N2O emissions from organic systems to bring scientific understanding to the same level as will be provided by the Government’s ‘InvenN2Ory’ project for non-organic farming practices. This would contribute to the ‘futureproofing’ of the UK GHG Inventory and ensure that emissions from organic systems are represented using accurate emissions factors.
  • There is an urgent need for further research into best practice for organic farms and other agro-ecological farming systems on how N2O emissions and other nitrogen losses can be minimised. Research is also needed into innovative methods already being trialled on organic farms. For example, alternatives to ploughing in legumes such as crops direct drilled into legumes, the use of perennially based cropping systems and agroforestry.
  • The European Nitrogen Assessment called for a lowering of the human consumption of animal protein as a way of also tackling nitrogen excesses. Research into the impact of nitrogen use and pollution as a consequence of a shift in diets in the UK to lower consumption of meat and dairy products, especially from animals fed on grain rather than grass, should be commissioned to accompany existing evidence of the climate change and health benefits.
  • Using clover on grassland to fix new nitrogen, rather than manufactured nitrogen is a practice that can be readily adopted by non-organic farmers. The use of winter cover crops by those growing spring sown crops should also be encouraged. The Government should provide financial incentives to help farmers implement such measures, and these could be included as part of the ‘greening’ of Pillar 1 of the Common Agriculture Policy.

You can download the report here.

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FCRN Admin's picture
Submitted by FCRN Admin on

There is consensus that all agricultural sectors should strive towards efficient use of nitrogen. The Soil Association report presents a case against using fertilizer N, focused around nitrous oxide emissions from denitrification. Nitrous oxide emissions are highest when there are large concentrations of nitrate in the soil, with plenty of carbon to stimulate microbial activity and warm temperatures. So avoiding nitrous oxide emissions depends on avoiding large additions of N to the soil, both as organic manures or fertilizer N, when crop demand and uptake is weak.

Managing the N cycle is one of the most difficult issues when balancing agricultural and environmental concerns. Biological nitrogen fixation by legumes in agriculture has been the main focus of my research for many years. If legume green manures, or large amounts of green crop residues are returned to the soil the perfect conditions for nitrous oxide emissions are created. One of the greatest problems of nitrous oxide or leaching losses in both conventional or organic agriculture is managing the huge pulse of mineralized N that is released when clover/grass pastures or leys are ploughed under. An advantage of fertilizer N is that is can be applied as top-dressing, when crops need it most. A perfect N supply in synchrony with crop demand is hard to achieve in all agricultural systems, and particularly difficult in organic agriculture.

I live the Netherlands where the whole country is classified a ‘nitrate sensitive zone’ and agriculture operates under strict regulations. The largest problem for the N cycle is created due to the importation of feed for livestock that creates a huge surplus manure. Organic farms use only 20% manure from organic animal production, and therefore both help to solve the problem of the manure mountain, but also rely on it for crop production. I support increasing use of clover/grass pastures in conventional farms to substitute imported feeds, but this will not automatically reduce nitrous oxide emissions.

Since the early 1980s research led by Rothamsted in the UK played a leading role in showing up inefficiencies of N fertilizer use, and both rates of fertilizer and the total amount used has declined strongly in Europe since. Further reductions would mean a fall in yield, which farmers and policy would include in their calculations if considering conversion to organic production. Any moves to reduce nitrous oxide emissions must avoid “pollution swapping”, as measures to reduce ammonia loss or denitrification may result in larger nitrate leaching.

You can respond to Ken by email here, and I’d be grateful if you’d copy me in.  Let me know you want me to circulate your comments more widely.

For more about Ken’s work on nitrogen fixation through the N2Africa project, see here.

N2AFRICA is a large scale, science research project focused on putting nitrogen fixation to work for smallholder farmers growing legume crops in Africa.

Its goals, by the end of its four year period will be to have:

  • identified niches for targeting nitrogen fixing legumes
  • tested multi-purpose legumes to provide food, animal feed, and improved soil fertility
  • promoted the adoption of improved legume varieties
  • supported the development of inoculum production capacity through collaboration with private sector partners
  • developed and strengthened capacity for legumes research and technology dissemination
  • delivered improved varieties of legumes and inoculant technologies to more than 225,000 smallholder farmers in eight countries of sub-Saharan Africa.