Background to recent estimates of nitrogen fixation
Nitrogen-fixing legumes such as peas and beans, clovers and vetches, were the main source of nitrogen in croplands until the early 1900s. They were extensively tested and grown in the Improvements era after 1700, and became an essential part of crop rotations, even though the biological mechanism of fixation was not then known. By fixing N gas from the air, they could provide their own nitrogen for growth, and the residues they left in the soil, in the form of roots and root nodules, provided much of the N for the next crop also.
Legumes receded in the 1900s, displaced by mineral N fertiliser, and then by imports of N-rich stock-feed from the Americas. Agriculture in north west Europe soon became dependent on manufactures and imports. However, the rising cost of N-pollution, the monetary cost of the fertiliser and the lost benefits, now increasingly realised, of legumes to the wider food web and soil structure, has brought on a reappraisal of legumes in mainstream crop production.
So it is incongruous that there are very few measurements of N fixation in northern, temperate-zone, agricultural fields. Individual measures of fixation in crops and pastures are not in themselves considered to be high-profile science, compared to, say, the insights to the mechanism of fixation at biochemical and genomic levels. Yet without knowing kilograms of nitrogen per hectare (kg/ha) fixed in real crops and pastures, it is difficult to design and justify new crop and forage systems in which manufactures and imports are replaced by local N.
The recent paper by Pete Iannetta and others in Frontiers in Plant Science fills in some of the gaps. The contribution of N fixation was estimated in a range of grain and forage legumes, sometimes grown alone and sometimes with other crops in mixtures. The analysis was based on existing data from field experiments managed by collaborating groups in different parts of Europe. The study added value to these data by estimating components of the N-balance, including fixation as the mass of N entering the system that could not be accounted for by any other means.