Les cycles biologiques des engrais azotés et phosphorés

• As the principal constituent of proteins, essential components of living matter, nitrogen plays a vital role in plant metabolism. A growth factor, it also serves to improve product quality. For example, a minimum protein content in cereals is required for bread production. • In soil, nitrogen exists in organic form (humus) or mineral form (ammonium NH4+, nitrate NO3-). Plants, with the exception of legumes (alfalfa, clover, peas…), cannot absorb nitrogen in its gaseous form. Nitrogen must therefore be supplied through fertilizers. Bacteria present in the soil transform it into nitrates for absorption by plants; this is the mineralization process. Most of plants' nitrogen nutrition is ensured by nitrates.

• Nitrogen is a significant environmental risk factor when present in excess in the soil. Indeed, nitrogen in the form of nitrate ions is highly soluble, poorly retained by soils, and non-degradable. When supplied in excessive quantities to crops, the surplus fraction can be leached out of the root zone into soil depths, causing pollution of surface and groundwater. • Nitrogen should therefore be supplied, as much as possible, in appropriate quantities and just before plant absorption, to prevent this leaching. A crop covering the soil during winter helps limit this leaching, which is promoted by precipitation infiltration. • Furthermore, some nitrogen, particularly from livestock effluent, volatilizes into the air during spreading, in the form of: • ammonia (NH3), which contributes to environmental acidification; • or nitrous oxide (N2O), a powerful greenhouse gas.

• The main levers for controlling and improving the environmental impact of nitrogen fertilization are based on: • crop rotation selection; • rotating crops by introducing legume plants capable of fixing atmospheric nitrogen; • implementing "Catch Crops for Nitrates (CIPAN)" to avoid bare soil and prevent leaching, or Energy Crops (CIVE); • best fertilization practices of "right rate and right time" according to plant needs and natural inputs (soil, atmospheric deposition); • developing alternative agriculture such as organic farming, which excludes all synthetic fertilizer inputs in its practices.

An essential constituent for plant growth and energy storage or transfer, phosphorus is indispensable for plants, even though they use it in smaller quantities than nitrogen or potassium.

Its cycle is relatively complex. Since atmospheric deposition is negligible, inputs come essentially from mineral and organic inputs. Plants absorb phosphate ions primarily in the form of H2PO4- and HPO42-. Mainly present in these forms, it is less soluble than nitrogen. Relatively immobile in soil, it can nevertheless be partially transported by sediments into surface waters and is frequently found in rivers and streams.

• Excessive phosphorus concentration in surface waters can cause eutrophication of slow-flowing rivers, lakes, reservoirs, and coastal zones. This phenomenon is characterized by blue-green algae proliferation, reduced light infiltration, oxygen depletion in surface waters, disappearance of benthic invertebrates, and production of toxins harmful to fish, livestock, and humans. • Soils are also exposed to eutrophication risk when excessive nutrient quantities lead to oxygen depletion and prevent natural microorganisms from functioning correctly.

Only intensive pig and poultry farming zones experience total phosphorus inputs (mineral and organic) sometimes excessive relative to crop needs. Indeed, organic effluent spreading there is still often based on nitrogen quantities supplied, without accounting for phosphorus.

Within plants, phosphorus, drawn from the environment, accumulates primarily as phytate. Specific enzymes, phytases, hydrolyze phytate to release phosphorus usable for organism growth.

Unlike plants, monogastric livestock such as pigs and poultry cannot use phytate present in plant-based diets as a phosphorus source. Indeed, unlike ruminants, they lack phytases in their digestive tract. Phosphate is then supplied to them as inorganic form (mineral salt) or by adding phytase to their feed. The presence of excessive phosphorus quantities (mineral salt and phytate) in animal feed causes partial rejection to the environment if not used by the animal.

The main levers for controlling and improving the environmental impact of excess phosphorus are based on limiting soil erosion.