Soil Nitrogen Budgets.

Nitrogen budgets are valuable for improving N efficiency because they assess N inputs and N outputs, plus N interactions, on a “systems basis”. Nitrogen budgets are based on the deceptively simple conservation of mass, which states that N inputs minus outputs equals the change of N. 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 leading to large changes in steady-state N are tillage of grassland and the initiation/cessation of manuring. Nitrogen budgets can be based on the total-N or the labeled-N entering and leaving a system, but these two approaches are not equivalent. The total-N budget focuses on the total inputs and losses, while the labeled budgets focus on the fate of the labeled N atoms including its interaction in the soil-N cycle. 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 agronomic N budgets are: crop uptake 45-65%, leaching or nitrate accumulation 0-35%, and denitrification plus ammonia volatilization 10-25%. An important N budget principle is that N losses increase rapidly once N inputs exceed crop assimilation capacity, with N commonly accounted for as leaching, denitrification, or as residual nitrate. Whole-farm N budgets on livestock farms are valuable for evaluating improved N management scenarios. Soil N budgets will continue to challenge scientists by slowly revealing fundamental principles that are embedded in a background of contrasting results arising from the variability of biological systems. By understanding these principles and the factors influencing them, scientists will have a stronger foundation for improving N use efficiency and reduce N losses to the environment.