The effects can vary greatly from one organic product to another. The potential reduction of emission depends on the type of organic products, on the storage or waste source used or energy source for the production of the given organic fertiliser. Some type of products or situation could even provide increase emissions. In addition, consideration must be given to when and how fertilisers are added to avoid pollution and improve effectiveness. The effect will be different if this is a partial or total substitution of mineral fertilisers with organic fertilisers.

1- Upstream: The key difference between the emissions from the two types of fertilizers is based on the energy consumption and GHG emissions during the production of the synthetic fertilizer that is replaced. Depending the process of the organic fertiliser, it will receive a more or less heavy part of the uptstream impact with its “waste” status in the LCA allocation.

2- In the field, the effect on nitrous oxide (N₂O) emissions depends on climate and fertilizer type, with lower N₂O emission from organic N fertilizer compared to synthetic N fertilizer in wet climates, similar otherwise. Effects on N₂O also depend on environmental factors (such as soil organic C content, soil texture, drainage, soil pH, temperature and freeze-thaw cycle) and management practices (such as N application rate; fertilizer type, liquid or solid form of organic fertilizer; irrigation and type of crop). Synthetic fertilizer provides a more immediate and concentrated source of nitrogen which may lead to higher N₂O emissions if not managed properly. Organic fertilizers release nitrogen more slowly as they decompose, which can result in lower N₂O emissions, and favor soil health and microbial diversity, potentially reducing N₂O emissions over time. The choice of the recipient crop is crucial to get avoid leaching conditions and to target the right timing to matching the crop demands. The priority is to target a good synchronisation between the mineralisation of organic fertilisers and the period of high nitrogen uptake by the crop : easier with spring or summer crops such as maize than wintern crop such as wheat, except for the case of rapeseed a winter crop very efficient to use nitrogen avalable in the soil in autumn.

Therefore the scope for improvement lies mainly in making better use of avalable organic fertilisers by improving spreading conditions to reduce nitrogen losses and taking greater account of their fertilising value, bearing in mind that there is considerable variability (depending on the conditions of the year).

3- The effect on SOC depends on the type of organic fertilizer: organic fertilizer with large amount of easily decomposable C stimulate microbial growth which stabilize SOC. Other factors include (i) fertilizer quality: organic fertilizers with low C:N ratio are decomposed faster; (ii) climate and soil texture: smaller SOC increase in warm and dry conditions and higher SOC increase in fine-textured soils.

The effect depends on the quantities applied, the nature of the product and the characteristics of the soil. The part of the mineral nitrogen (TAN) content of the organic fertilisers will define the level of NH3 emissions right at the application (leading to nitrogen redeposition elsewhere) and the total N content will determine the level of N2O emissions from the soils linked to the soil microbiological activity.