Whither the enigma of soil nitrogen balance sheets?

Abstract

The enigma of soil nitrogen balance sheets originally described by Allison in 1955 has endured for 65 years. Originally the accounting showed a positive balance between N gains and losses, which indicated a failure to measure all losses, widely suspected of being gaseous N emissions. Measurement of organic, inorganic and gaseous N forms derived from 15N-enriched nitrate added to soils under both anaerobic and aerobic conditions in small- to medium-scale short-term experiments in closed vessels without plants, demonstrated that it is possible to achieve a complete 15N balance. A recent partial N balance at the global scale of the three main cereal crops over a 50-year time span showed a negative balance between N gains and losses, indicating a failure to measure all N gains or to accurately measure net changes in total soil N to a sufficient soil depth. Therefore, if the enigma is to be resolved in a realistic agronomic setting, fundamental decisions need to be made regarding the approach to be adopted, the cereal crop to be studied, the nature of the measurements required and their temporal and spatial scales.