Whither the enigma of soil nitrogen balance sheets?
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OPINION PAPER
Whither the enigma of soil nitrogen balance sheets? Phillip M. Chalk
Received: 4 June 2020 / Accepted: 2 September 2020 # Springer Nature Switzerland AG 2020
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.
Keywords Enigma . 15N . N balance . Global budget . Cereals . Partial budget
Introduction One of the most challenging and long-lasting debates in agronomy concerns the ‘enigma of soil nitrogen balance sheets’, the title of a seminal paper published by Allison (1955). In reviewing the many lysimeter, field and glasshouse experiments conducted mainly in the USA and the UK, Allison concluded that N gains via fertilizer addition and wet deposition exceeded N losses by crop harvest, leachates and net changes in total soil N. From such accounting it appeared that N losses were not being comprehensively measured. The development of 15N fertilizer studies, reviewed by Hauck and Bremner (1976), greatly enhanced our understanding of N cycle processes, but it soon became apparent that a major obstacle was our inability to quantitatively account for gaseous N losses. When 15 N-labelled fertilizer recovery in the soil + plant was invariably shown to be less than the N applied, the ‘unaccounted for 15N’ was attributed to nitrate leaching, NH3 volatilization or denitrification, often without any convincing evidence as to the identity of such losses.
The enigma – a question of scale 15
N balance studies
Responsible Editor: Hans Lambers. P. M. Chalk (*) Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Parkville, VIC 3010, Australia e-mail: [email protected]
Laboratory incubation study The goal to achieve a 15N balance in the soil-atmosphere system was initially frustrated by the absence of
Plant Soil
methods to quantitatively measure the 15N abundance of the forms of N in the gas phase. This deficiency was addressed by Craswell et al. (1985) and Strong et al. (1987) who developed an arc method to conver
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