Methodologies for determining N losses

• Institute of Soil Science, China
• Institute of Soil Science, China
• Malaysian Agricultural Research and Development Institute, Malaysia

Lack of nitrogen is a major factor limiting production of Asian crops, especially rice. Increased use of nitrogen fertiliser has resulted, and most Asian countries now rely largely on urea. However, efficiency of fertiliser use has approved very poor, with rice crops recovering as little as 10% of the applied nitrogen and rarely more than 50%. Recent research has suggested that ammonia volatilisation may contribution heavily to the losses.
First, the project will develop and test simple methods of measuring this volatilization and other forms of nitrogen loss from flooded rice-fields. Using these methods, the team will seek to identify the major causes of inefficiency in a range of representative cropping systems, fertiliser practices and soil and climatic environments. It will then assist in rapid diagnosis, identification and prediction of which of those conditions are conducive to gaseous nitrogen losses, and promote the design and development of more cost-efficient fertiliser forms and practices.
Laboratory experiments can lead to misinterpretation of the importance of gaseous losses because any enclosures or manipulations of the ecosystems tend to cause distortions. Techniques developed in CSIRO Canberra over the last 10 years enable accurate measurement of NH3 and N2O losses in a variety of undisturbed field environments, but as used so far, they require a large number of measurements of air movement, NH3 concentrations and flow rates into traps at a range of heights and times. The project will test simplified methods-first in Australia and then in Southeast Asia. Preliminary results on a prototype isokinetic sampler suggest that it can give satisfactory results, at least for a specified range of wind speeds and crop canopies.
Measurement of NH3 loss in a particular experiment provides only one data point, covering a particular combination of fertiliser application, soil type, microbiological population, weather conditions etc. Building up an analysable bank of information involves many data points. In a alternative approach during the field testing the scientists will collect basic information on the various parameters known to affect ammonia volatilisation. From this, they will develop a model of the processes and factors, which relates losses to conditions in the surface soil or floodwater and in the atmosphere. The model will enable them to predict the potential for NH3 loss in a range of situations.
Losses of nitrogen other than NH3 can occur as N2 (and to minor extents as N2O and NO), or in runoff and leachate. In addition, some applied nitrogen becomes immobilised as insoluble forms in the soil, either transiently or for long periods. The scientists will monitor such processes, conducting balance measurements with 15N-labelled urea at all field testing sites in Australia and Asia.
Collaborative field testing will begin in Malaysia and China, but is expected to extend to other Asian countries, in part through the International Network for Soil Fertility and Fertiliser Efficiency for Rice.