Quantifying the loss mechanisms of nitrogen

ABSTRACT:

Nitrogen (N) is a key factor in maintaining higher yield production and worldwide economic viability of agricultural systems. Since N is one of the most dynamic and mobile elements, its management is difficult, especially in irrigated systems where significant losses can be produced by leaching or denitrification. The major pathways for N loss are ammonia (NH₃) volatilization; emissions of nitrous oxide (N₂O), oxides of N (NO and NO₂), and dinitrogen (N₂) gases; leaching of nitrates (NO₃); and off-site transport due to wind and water erosion of N tied in the organic matter and in the inorganic NO₃ and ammonium (NH₄) compartments. Nitrogen is dynamic and mobile. Its fate and transport in agricultural systems is affected by management and unpredictable events. Its average worldwide N use efficiencies (NUEs) have been reported to be about 50% and even as low as 33% for cereals.

Farmers usually apply a uniform rate of N to agricultural fields assuming that N sources, sinks, and mechanisms for loss are constant across fields. It is well documented that variability of soil properties that affect N sources makes managing N to maximize NUE difficult. Such variable soil properties include soil organic matter content, residual soil NO₃-N, amount of crop residue returned to the surface soil, yield variability (N sink), and changes in soil chemical and physical properties. Fields vary from coarse gravelly areas where N losses are primarily attributed to NO₃-leaching, to clayey areas where water is ponded and N losses may be primarily dominated by denitrification (N₂/N₂O). Management is being established as the predominant factor that can reduce N losses in the environment. If we are to improve N management to increase NUE we will need to do it within the context of the N cycle accounting for N loss mechanisms and how to manage them. This paper will review how we can quantify these N losses.