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Today’s’ blog post will be short and sweet. Its objective is to bring you a visual representation of terrestrial nitrogen transformations and hopefully help illustrate why legumes have such an important role in efficient and sustainable agriculture….
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Lets start with a basic model with the main components of the Nitrogen Cycle in a typical agricultural system.
Crops need nitrogen to create proteins, cellulose, lignin and chlorophyll. Their growth is therefore limited by how much nitrogen is available in the soil. To maintain yield, farmers often need to add nitrogen in the form of animal manure, crop residues or synthetic fertiliser.
So lets add some synthetic fertiliser to the soil and see what happens….
Notice how the levels of gaseous nitrogen from the soil increases. This is because often fertiliser is applied in a haphazard way, often in greater quantities that what is required. Not only is this wasteful but it can be detrimental to the environment.
The increase in nitrous oxide (N2O) is contributing to climate change in a major way ….
…and fertiliser is typically used in conjunction with ploughing. This can compact the porous soil structure over time, eroding nutrients and disrupting the soil’s water holding capacity. This causes further losses of nitrogen and other nutrients from the soil which impedes crop growth over time.
To maintain crop yield, farmers may have to add more fertiliser but this will only serve to keep them in the vicious cycle. This phenomenon is know as the ‘Law of Diminishing Returns’ and without careful management can lock agriculture into a downward spiral of decreasing inefficiency and increasing costs.
Lets try and different approach and use Nitrogen Fixing Legumes instead…..
Now, instead of major losses of N2O, most N remains in the soil to be taken up by the crops.
No ploughing is needed to push the fertiliser down into the soil; it is naturally pulled down by the legume’s root system. This means the soil structure is maintained and can hold more water and nutrients.
With healthier soil, gradually the soil microbes and invertebrates begin to return…
These microbes help out in the other nutrient cycles in the soil. Many are photosynthetic and absorb CO2 from the atmosphere.
Healthy soil is the world’s largest carbon sink…
Using legumes as an alternative to fertiliser can thus help mitigate the role of agriculture in climate change and improve systematic efficiency, output and economic security for farmers.
And that is why legumes matter!
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Lal, Rattan. 2009. ‘Soil degradation as a reason for inadequate human nutrition’, Food Security, 1: 45-57.
Jensen, Erik Steen, Georg Carlsson, and Henrik Hauggaard-Nielsen. 2020. ‘Intercropping of grain legumes and cereals improves the use of soil N resources and reduces the requirement for synthetic fertilizer N: A global-scale analysis’, Agronomy for Sustainable Development, 40: 1-9.
Rockström, Johan, Will Steffen, Kevin Noone, Åsa Persson, F Stuart Chapin III, Eric Lambin, Timothy M Lenton, Marten Scheffer, Carl Folke, and Hans Joachim Schellnhuber. 2009. ‘Planetary boundaries: exploring the safe operating space for humanity’, Ecology and society, 14.
Valentim, Judson Ferreira, and Carlos Mauricio Soares de Andrade. 2020. “Strategies leading to successful wide adoption of mixed grass-legume pastures for sustainable intensification of beef cattle production systems in the Brazilian Amazon.” In Embrapa Acre-Artigo em anais de congresso (ALICE). In: INTERNATIONAL SYMPOSIUM ON AGRICULTURAL TECHNOLOGY ADOPTION, 1., 2019 ….