The fate of nitrogen from crop residues of broccoli, leek and sugar beet

Environmental concern has lead to legislation on fertilization to reduce nutrient losses to the environment. Reducing N input may be inadequate for crops that have a high N content in their residues. Reducing N input will negatively affect yield, but the residues remain. Management of crop residues may then be a more effective strategy to reduce N losses, notably nitrate leaching. Two experiments were carried out in 2006-07 and 2007-08 to study all emission routes of N from crop residues of broccoli, leek and sugar beet, and to determine the contribution of N from the residues to nitrate in groundwater. Crop residues were surface applied or rototilled, and compared with a blank and application of fertilizer N. About 11 to 16% of the N-content was lost to the air. Total N-losses to the air as ammonia and through denitrification were little affected by tillage, but partitioning was. When residues were left on the soil surface, most N was lost as ammonia, when the residues were rototilled, most N was lost through denitrification. Year strongly affected N leaching from crop residues, and the fraction of N from crop residues that was leached was twice as high in 2006-07 than in 2007-08. Tillage increased N leaching from broccoli, but had little effect with leek or sugar beet. Between 20 and 60% of the N content of residues of leek and broccoli was leached. Removal of residues after harvest will therefore contribute to reduce N leaching. However, options for treatment and re-use of the residues need to be studied as well as the effects on nitrate leaching and gaseous losses to assess the effects of the system as a whole