When micro-organisms decompose SOM, N is ‘mineralised’. How much N is released and when this release occurs determines whether it will become available for plant growth and/or represent a risk of loss to the environment in humid regions. These questions have stimulated development of laboratory methods for PMN (Walley and Yates 2002; Wang et al. 2003).
Two laboratory-based methods for PMN are included. The first (7D1), which has several optional analytical finishes, is a chemical procedure that utilises hot-KCl extraction of air-dry samples. Extraction time and temperature are 95°C for 16 h, as most NH4-N is expected to be released under these conditions (Wang et al. 1996).
The second PMN method (7D2) utilises anaerobic incubation in the laboratory for seven days, based broadly on the methods of Waring and Bremner (1964) and Keeney and Bremner (1966). The method requires fresh, field-moist soil as a starting point (air-dried soils are unsuitable).