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.
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.