All of the nitrogen that is fixed out of the atmosphere goes into plants and then possibly into animals and then as those animals and plants die, they start to be decomposed, and that then enters into soil organic matter. So soil is a suite of different types of particles – sands, silts and clays – and some of the clays in particular are able to protect that organic matter from further decay.
So you get large build-ups of organic matter in soil, and that contains mainly nitrogen and carbon. There is more carbon in soils than in all the plants above the soil, and if you also add in the amount of carbon that’s in the atmosphere.
So there’s a tremendous amount of carbon in every hectare of land. For example, in New Zealand, it wouldn’t be unusual for a pasture to have something like 150 tonnes of carbon per hectare down to about a metre’s depth. Now along with that carbon is a lot of nitrogen, usually in a ratio about 10:1, so for every 10 tonnes of carbon, you’ll have about a tonne of nitrogen stored in the organic matter.
Professor Louis Schipper, University of Waikato
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