Livestock effluents, sewage sludge, urban organic waste and industrial effluents - qualified as residual-origin fertilizing materials (referred to here as Mafor, for matières fertilisantes d’origine résiduaire) - constitute sources of fertilizing elements and organic materials that can fertilize or improve agricultural or forest soils. The expert report reviewed the agronomic, environmental and socio-economic impacts of spreading these materials on soils.
Livestock effluents are by far the most important Mafor in terms of volume. Spreading them on agricultural land is a traditional practice in order to fertilize the soil, but because production specialization has concentrated livestock farming in certain regions (notably western and northern France), spreading in these areas is also in a majority on land used for both field crops and pasture. But the use of different Mafor affects the whole of France, varying as a function of different regions: the use of urban sewage sludge and urban composts tends to be more concentrated in regions where no livestock are raised, and in periurban areas.
The spreading of livestock effluents and other Mafor supplies the mineral elements (nitrogen (N), phosphorus (P) and potassium (K)) that are necessary for plant nutrition. It also contributes organic matter than can improve soil properties (organic amendment value). The nitrogen fertilizing value of Mafor is comparable to that of mineral fertilizers with respect to phosphorus and potassium. Indeed, they are the only source of renewable phosphorus, and our reserves of mineral phosphorus are limited. However, nitrate inputs differ from those provided by mineral fertilizers. Mafor can thus be ranked according to the type of input they represent: some are more "fertilizing" because their nitrogen fertilizing value is close to that of mineral inputs, while others are more useful as amendments because they enrich the organic matter levels of soils.
An excess of phosphorus and nitrogen contributed via fertilization can lead to the eutrophication of water (green algae, etc.), while nitrogen is also a source of greenhouse gases. In the expert report, emphasis was also placed with respect to nitrogen on the losses that occur during storage and which require further examination, although this point did not form part of the scope of this study.
The spreading of organic fertilizers thus poses questions regarding their dosage, as each source has its own particular composition (differences between pig slurry and cattle manure, for example, or differences as a function of the treatments applied - composting, liming, etc., and storage duration). The appropriate use of these inputs therefore requires a clear understanding of their characteristics, and often their use in a mixture, so as to better meet the needs targeted.
Losses can be reduced when the materials are incorporated following spreading, or through the use of intercropping with a nitrate catch crop. The expert report thus underlined the agronomic value of such spreading, while highlighting the need for greater technical expertise on the part of farmers (compared with the use of mineral fertilizers) and thus a higher level of risk, in order to minimise their environmental impact. Finally the study evidenced the need for more detailed knowledge of the characteristics of different fertilizing residual materials and their dynamics over time.
Focus: different social challenges and objectives
The use of different Mafor may respond to varying demands and not follow the same logical pathways:
- The spreading of livestock effluents is an ancient practice, a "variant" of the direct emission of faeces onto pasture. Elimination of this practice is rarely envisaged, except in areas suffering from a structural excess according to the Nitrates Directive;
- The agricultural spreading of certain Mafor of industrial origin fits perfectly within a logic of closing the loop. For example, by-products from the sugar beet industry can be applied to fields dedicated to sugar beet crops. Under this logic, a degree of standardisation has sometimes developed, and some of these effluents are sold as organic fertilizers (for example, sugar beet pulp);
- The spreading of municipal Mafor (sewage sludge, urban composts) tends to be perceived as a service rendered by agriculture to society, ridding the latter of its waste. However, the development of processes to treat urban waste (notably methanisation), and use of the "new" Mafor which result, have not been studied from a sociological standpoint, even though this issue is the subject of frequent media attention.
The expert report pointed out that the spreading of livestock effluents, and urban sewage sludge containing faecal matter, may participate in the dissemination of antibiotic resistance. Because they carry numerous bacteria and antibiotic residues, these different effluents offer a favourable environment for the selection of resistant bacteria. More generally, Mafor constitute vectors for pathogenic agents; not only bacteria but also viruses and parasites, etc. To date, however, no microbial contamination arising from a fertilizer has been identified as the source of a public health problem. Some of the treatments applicable to Mafor (liming, composting, anaerobic digestion, etc.) are efficient in reducing their content in pathogenic agents.
Mafor also contain numerous chemical contaminants (both organic and mineral) in the form of trace elements with demonstrated toxicity in living beings at certain doses. But the effects of the treatments applied to Mafor with respect to their contaminant content are less well understood. Studies performed under real-life conditions that comply with current regulations have not demonstrated any accumulation of trace organic compounds in either soils or plants. Their transfer to water courses is nevertheless likely to occur. The expert report highlighted the lack of knowledge on the transformation, fate and toxicity of trace organic compounds. As for trace mineral elements, which are very persistent in the environment, any spreading of Mafor that contain them will contribute to their gradual accumulation in soils.
Nine trace mineral elements and ten trace organic compounds are currently covered by the regulations. The Mafor spread present levels that are below the legal thresholds, but we must remain vigilant regarding cumulated effects following spreading for several years. Finally, the expert report highlighted the lack of regulatory harmonisation between different countries, due to a lack of scientific consensus regarding fundamental data.
About : collective Scientific Expert Reports (ESCo)
A collective scientific expert report (ESCo) is an institutional action that is governed by the National Charter on Institutional Scientific Expert Reports to which INRA, CNRS and IRSTEA subscribed in 2011. It is defined as the analysis and assembly of knowledge generated in highly diverse disciplinary fields that will be pertinent to informing public actions. The analysis is carried out by a multidisciplinary community of expert researchers from different institutions. For the ESCo on Mafor, which was led by INRA's Unit for Collective Scientific Expertise, Foresight and Advanced Studies (DEPE), some thirty experts from different organisations in France and other countries were mobilised for their skills in agronomy, chemistry, microbiology, ecotoxicology, economics, sociology and law, etc. The work of these experts was based on a bibliographical corpus of almost 3000 publications, most of them scientific articles but also including statistical data, study reports and monographs. The ESCo resulted in the production of a report which presented the contributions from these different experts and contained a shorter summary report for use by decision-makers.