Comparing N2O emissions from organic and mineral fertilisers

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A recent study compares the effects of organic, 'natural' fertilisers, such as compost, with mineral, synthetic fertilisers, such as urea, on N2O emissions from Mediterranean soil. It suggests that there is little difference between the fertilisers, but that pig slurry offers the best overall balance in terms of emissions and crop yield.

Nitrous oxide (N2O) is a powerful greenhouse gas (GHG). Although N2O exists in smaller quantities than CO2 in the atmosphere, it is a more powerful GHG. One of its main anthropogenic sources is agricultural soil, the result of applying organic and mineral fertilisers. These fertilisers also produce nitric oxide (NO) which damages the ozone layer.

Previous studies on the effects of applying natural organic and mineral fertilisers on N2O and NO emissions have produced contradictory findings. Emissions are influenced by conditions such as soil type, crop, temperature and water levels in the soil. Research, conducted under the EU NitroEurope project1, investigated the effect of both types of fertiliser on a non-irrigated crop under Mediterranean conditions. It also investigated the effect of the first rainfall events in autumn on changing levels of N2O and NO.

Eighteen 30 m2 plots were selected at a location near Madrid where no crop had been sown and no fertiliser applied in the previous 5 years. Individual plots were treated with four natural organic fertilisers (composts and liquid manures), a mineral fertiliser (urea) and a control treatment without any nitrogen fertiliser. Barley was sown on the same day in each plot. Changes in N2O and NO were measured during the whole experimental period from January to October.

There were no significant differences between the fertilisers and very low emissions were measured for all treatments and throughout the study period. However, to compare, composted, organic municipal solid waste (MSW) produced fewest N2O emissions and composted crop residue with sludge (CCR+S), also a natural organic treatment, produced the least NO. Three of the four natural organic fertilisers (MSW, CCR+S and untreated pig slurry) produced smaller NO emissions than urea, and a NO sink was observed at several points, particularly when soil temperatures were low. The authors suggest that further studies are needed to investigate this process. The research observed brief rises of N2O emissions in autumn when the first rainfall events occur and the dry soil becomes wet.

An emission factor was calculated which related N2O emissions to the amount of barley produced. This indicated that digested pig slurry is the most efficient fertiliser for this type of Mediterranean agrosystem if the aim is to reduce N2O emissions without reducing crop yield. In comparison, composted residues mixed with sewage sludge and urea both had high emissions factors, i.e. it would not be possible to reduce N2O emissions without also reducing crop yield.

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