Soil Nitrogen Budgets

Authors: Meisinger, John; Calderon, Francisco; JENKINSON, DAVID - ROTHAMSTED,HARPENDEN,UK

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 12/1/2007
Publication Date: 6/20/2008
Citation: Meisinger, J.J., Calderon, F.J., Jenkinson, D.S. 2008. Soil Nitrogen Budgets. American Society of Agronomy Monograph Series. 13:505-562.

Interpretive Summary: Crop nitrogen recoveries are commonly 45% to 70% of the nitrogen inputs for modern field-crop systems. Nitrogen budgets are a valuable tool for improving nitrogen efficiency because they assess the size of various nitrogen pools, nitrogen gains from the atmosphere, nitrogen losses to the environment, and the interactions among nitrogen-cycle processes. Nitrogen budgets are based on the conservation of mass, that simple states that nitrogen inputs minus outputs, equal the change of nitrogen within the system. The consistent application of the same management practices over many years will cause an ecosystem to gain or lose nitrogen at a diminishing rate, until a quasi steady-state nitrogen level is reached. Management practices can lead to large changes in steady-state nitrogen levels, such as tillage of grassland or the initiation/cessation of manuring, or to small changes. Nitrogen budgets can be based on the total-nitrogen or the labeled-nitrogen entering and leaving a system, but these two budgeting approaches are not equivalent. The total-nitrogen budget focuses on the total inputs and losses of the entire system, while the labeled budgets focus on the fate of the labeled nitrogen atoms including its interaction within the soil nitrogen cycle. The choice of a total-nitrogen budget or a labeled budget will depend on the objectives and resources of the project, but careful consideration should be given to the strengths and weaknesses of each approach. Nitrogen budgets represent the product of many transformations carried out by physical, chemical, and biological agents interacting with each other and the environment over time. Common values for the major field-crop nitrogen budget processes are crop uptake 45-65%, leaching or the accumulation of residual nitrogen 10-35%, and gaseous losses through denitrification and ammonia volatilization 10-25%. An important nitrogen budget principle is that nitrogen losses increase rapidly once nitrogen inputs exceed crop assimilation capacity, with lost nitrogen usually accounted for as leaching, denitrification, or an accumulation of residual nitrate. Large-scale nitrogen budgets for farms or watersheds have been valuable for identifying the major nitrogen pathways and the spatial pattern of nitrogen surpluses. Whole-farm budgets on livestock farms have also proven valuable for evaluating management scenarios for improving nitrogen recoveries. Soil nitrogen budgets will continue to challenge agricultural scientists by slowly revealing fundamental principles that are contained in a background of contrasting results arising from the variability of biological systems. By understanding these principles and the factors influencing them, scientists and nutrient managers will have a stronger foundation for improving nitrogen use efficiency and concurrently reducing nitrogen losses to the environment.

Technical Abstract: Nitrogen (N) recoveries are commonly 45% to 70% for modern field-crop systems. Nitrogen budgets are a valuable tool for improving N efficiency because they assess the size of various N pools, N gains from the atmosphere, N losses to the environment, and the interactions among soil-N-cycle processes. Nitrogen budgets are based on the conservation of mass, with the deceptively simple statement that N inputs minus N outputs, equal the change of organic-N within the system. The consistent application of the same management practices over many years will cause an ecosystem to gain or lose N at a diminishing rate, until a quasi steady-state N level is reached. Management practices can lead to large changes in steady-state N levels, such as tillage of grassland or the initiation/cessation of manuring, or to small changes in organic-N. Nitrogen budgets can be based on the total-N or the labeled-N entering and leaving a system, but these two N budgeting approaches are not equivalent. The total-N budget focuses on the total-N inputs and losses of the entire system, while the labeled budgets focus on the fate of the labeled N including the 15N interaction within the soil N cycle. The choice of a total-N budget or a 15N budget will depend on the objectives and resources of the project, but careful consideration should be given to the fundamental strengths and weaknesses of each approach. Nitrogen budgets represent the product of many transformations carried out by physical, chemical, and biological agents interacting with each other and the environment over time. The major field-crop N budget processes with common values are: crop uptake amounting to 45-65%, leaching or the accumulation of residual N accounting for 10-35%, and gaseous losses through denitrification and ammonia volatilization of 10-25%. An important N-budget principle is that N losses increase rapidly once N inputs exceed crop assimilation capacity, with lost N usually accounted for as leaching, denitrification, or an accumulation of residual nitrate. Large-scale N budgets for farms or watersheds have been valuable for identifying the major N pathways and the spatial pattern of N surpluses. Whole-farm budgets on livestock farms have also proven valuable for evaluating management scenarios for improving N recoveries. Soil N budgets will continue to challenge agricultural scientists by slowly revealing fundamental principles that are woven within a matrix of contrasting results and the inevitable variability of biological systems. By understanding these principles and the factors influencing them, basic and applied scientists will have a stronger foundation for improving N use efficiency and concurrently reducing N losses to the environment.