Nitrogen is an essential nutrient in agriculture. Its reactive forms are the focus of concerns because they are responsible for a multitude of impacts on the environment and health. This was highlighted by the 2011 European Nitrogen Assessment, which drew up a critical inquiry on nitrogen imbalances due to livestock farming systems. The ambivalent status of nitrogen, both a resource for agriculture and a pollutant for environment, requires legal systems to find the equilibrium between the fertilising potential of the animal wastes and its possible negative effects on the environment. The European policies on nitrate and gaseous pollutants are the subject of much litigation with the European Commission. In this context, the French government asked for a synthesis of scientific knowledge on flows and fate of nitrogen related to livestock farms. Articles following in this review draw (i) the current situation, (ii) explain social and economic causes of the territorial variability of nitrogen pressure, (iii) quantify the flow on farms, (iv) look at indicators, (v) regulation instruments and finally (vi) identify options for reducing nitrogen pressure caused by livestock farming. In terms of materials and methods, a particular importance was given to peer recognition, and plurality inside the panel of experts and in the literature selection.

This article describes the nitrogen flows in the environment and points to the specificities of the livestock production. Till the beginning of the 20th century, the symbiotic fixation and the recycling of animal excreta supplied the nitrogen necessary for the fertility of soil. In 1913, the Haber-Bosch process allowed the industrial synthesis of ammonia and made possible the fertilisation without association of crop production with the livestock farming. The efficiency of the nitrogen in livestock farming is low with nearly half or more of the inputs losses to the environment. These losses have diverse impacts that intervene at various spatial scales owing to the nitrogen cascade. Quantitative assessment of nitrogen flows at the scale of regions started in the early 1980s in Western Europe and North America. These studies provided estimates of the spatial variability of nitrogen discharge within a region. They confirmed the differences between areas with a high animal density such as Brittany (western region, France) and other regions. It was also found that the same nitrogenous losses could lead to different levels of environmental impacts according to the sensibility of a given environment and its capacity to cope with nitrogen excess. Climate, soils characteristics, animal density, and proportions of agricultural land under annual and perennial crops are drivers of this sensibility.

Scientific literature in social sciences that deals with nitrate embraces two centuries, whereas very little socio-economic work has addressed other forms of reactive nitrogen. Nitrogen has always had an ambivalent role as both a raw material indispensable for the development of agricultural and a source of negative impacts. This ambivalence has accompanied the social history of livestock production and can explain the conflicting nature of the subject and the moderate environmental efficiency of environmental policies. The legal system is particularly complex. The main cause of territorial pollutions is linked to the industrialisation and spatial concentration of livestock production in France, as in numerous countries in Europe and North America. This conglomeration movement is translated by a concentration of animal manure, which drives to nitrogen balance surplus and associated serious environmental consequences.

The nitrogen efficiency is the ratio between the output of nitrogen in the animal products and the input required for the livestock production. This ratio is a driver of the economic profitability and can be calculated at various levels of the production system: animal, field or farm. Calculated at the scale of the animal, it is generally low with less than half-ingested nitrogen remaining in the milk, the eggs or the meat in the form of proteins; the major part of the nitrogen being rejected in the environment. Significant gains were achieved in the past via the genetic improvement and the adjustment of feed supply. At the farm level, the efficiency increases to 45% to 50%, thanks to the recycling of animal excreta as fertilisers. From excretion to land application of manure, the losses of nitrogen are very variable depending on the animal species and the manure management system. Considering the risks of pollution swapping, all management and handling steps need to be considered. Collective initiatives or local rules on agricultural practices allow new opportunities to restore nitrogen balances on local territory.

This article describes several indicators and methods for estimating nitrogen flows and balance sheets. It reviews their strengths and weaknesses according to user types and their objectives. The nitrogen balance sheets, based on the difference between inputs and outputs are by far the most-often used estimator of nitrogen management on livestock farms. Among those, the soil surface balance helps in guiding the fertilisation and the farmgate balance is logically higher. The indicators of practices are easy to use but are poor predictors of nitrogen losses, whereas the indicators of emissions allow for estimating nitrogen losses using either direct measurements or models thereby providing a dynamic assessment of these flows. The coupling of balance sheets and indicators of emissions offers a certain potential for improvement of diagnosis and support of decision making. Finally, the indicators of impacts, including the life cycle analyses are currently the most popular for qualifying the impacts of animal production.

This article examines how the concerns of economic efficiency and of redistribution can be integrated into the design of the policies that aim to control the nitrogen flows. Generally speaking, the economists recommend to determine the degree of pollution and to arbitrate between the gains and the transaction costs associated to the implementation of the policy. Regarding instruments, the reduction of the diffuse nitrogen pollution needs a quantitative or economic instrument owing to the difficulty of valuing the environmental efforts of the producers on markets. Unlike for pesticides, the quota is here preferred to the tax, in particular in the case of fertilisers. The polluter-payer principle should be applied to the risks of accidental pollution on livestock farms. Results of a long process under the European Commission pressure, the policies of EU members in Northern Europe are more in line with the economic prescriptions than those of France. Recent evolutions go nevertheless forward, even if penalties for enforcing the practices standards are always missing.