Sustainable grain legume-cereal production systems through management of nitrogen fixation

• National Agricultural Research Council, Nepal
• North West Frontier Province Agricultural University, Peshawar, Pakistan
• University for Arid Agriculture, Rawalpindi, Pakistan
• National Agricultural Research Council, Pakistan
• Agricultural Research Institute, Peshawar, Pakistan
• CSIRO Plant Industry, Australia

Insufficient nitrogen in soils severely restricts crop growth. Many soils in the world are naturally low in nitrogen and while this is acceptable for the native plants in those areas, modern crop varieties suffer. The only solutions are to apply nitrogenous fertiliser or to grow legumes (which have roots that contain bacteria capable of 'fixing' atmospheric nitrogen in a form available for plant growth).
In Pakistan and Nepal, crop production is hard-pressed to keep pace with the needs of their rapidly growing populations. In both countries there is a shortage of foreign currency reserves to buy nitrogenous fertilisers and little opportunity to expand cropped areas. Use of legumes would therefore be beneficial, but the practice of including them in the cropping cycle is not widespread.
The northern cereal belt of Australia also faces major soil nitrogen deficiencies. There are several similarities between this region and Pakistan and Nepal, in terms of the crops grown, the dominance of winter cereals and the restricted use of legumes.
In 1994 ACIAR funded project LWR2/1992/010, with the broad objectives of developing and promoting the use of legumes to provide soil nitrogen. The intention was to enhance the production of cereals in rain-fed areas of the three countries, by intercropping them with legumes or planting legume/cereal rotations. However, the system needed studying in order to maximise productivity.
This project was designed to build on the work of the earlier project and extend the scope to include modelling of the cropping systems. LWR2/1992/010 had surveyed more than 300 fields to measure levels of nitrogen fixation and to determine the factors that affected fixation, and researchers had undertaken experiments to evaluate the short-term effects of rotating legumes with cereal crops.

The project aimed to increase the productivity and sustainability of cereal cropping systems in Pakistan, Nepal and Australia through increased use of legumes and more effective application of their abilities of to enrich soil nitrogen (N2).

The researchers continued the existing rotation experiments in all three countries. They refined the current method of measuring nitrogen fixation in the soil, and used it to study the fixation rates of crop legumes such as mung bean, common bean and ground nut species. At the same time they monitored the effects of the legumes on other soil nitrogen processes.
They also recorded the practices used by farmers in all locations to identify those that correlated most closely with improved nitrogen fixation rates. They conducted field surveys of soil nitrogen when it was necessary to set benchmark levels.
As well, the team developed computer models, based on the APSIM modelling package, which had the ability to predict the degree of nitrogen fixation of selected legume species in particular places. Researchers also produced and disseminated information packages about legume use, nitrogen fixation and nitrogen management in cereal-production systems, tailored to suit the conditions of the respective countries. This was an important feature because farmers have a vital role to play in boosting productivity by actively managing soil nitrogen.

Legume N2 fixation was substantial in most of the experiments. Values exceeding 300 kg N/ha were recorded for irrigated pigeonpea and mungbean in the North West Frontier Province (NWFP), Pakistan, and for soybean in the Hills of Nepal. But more commonly, values were in the range 50-150 kg N/ha. The values for the Australian experiments were similar to those for Pakistan and Nepal.
Legumes increased yields in the cereal crops that followed. The majority of responses, relative to yields in a cereal monoculture, were in the range of 20 to 100%. Cereals also responded to inputs of fertiliser N, particularly in the low-N soils of Pakistan. Responses were commonly 100-200%.
Economic analysis, using gross margin, dominance and marginal analysis of a number of the rotation experiments, revealed large benefits of legumes - particularly chickpea and faba bean in Australia and soybean and black gram in Pakistan. The legume economic benefit reflected both its high grain price and impact on cereal yields.
The high productivity rotations incorporating legumes and fertiliser N increased soil organic fertility. For example, after 4 years of the chickpea-wheat rotation experiments in Pakistan, soil organic matter had increased between 28 and 56% through inclusion of chickpea and between 35 and 72% through inputs of fertiliser N in the wheat monoculture. Values for increases in soil total N were 22-56% (chickpea) and 66-100% (fertiliser N).
Both growth and N2 fixation models for faba bean and chickpea developed in APSIM are being refined as a result of the project's field experiments at Gatton in southeast Queensland and through verification using other data sets. A key outcome of the experimental program was that faba bean showed a greater capacity for N2 fixation than chickpea and, when soil nitrate supply far exceeded demand, spared soil nitrate. This finding has implications for APSIM's N2 fixation model.
APSIM-generated simulations of some NWFP Agricultural University data sets showed good agreement with observed data for wheat (-N) and mungbean, and reasonable predictions for responses of the wheat to both legumes and fertiliser N. APSIM proved an excellent tool for scenario analysis (e.g. fertiliser N rates, residue management) in such systems.
The xylem solute method for measuring legume N2 fixation was calibrated for cowpea, mungbean and black gram, adding to the list of calibrated species. Sampling procedures for the method have been simplified - previously 7-8 samplings were recommended, now it is 1-2.
Surveys of 107 farmer crops of soybean, lentil, chickpea, mash bean, pigeonpea, grasspea, faba bean and ground nut in Nepal showed that percentage nitrogen derived from N2 fixation values were almost always >50%, with generally higher values for the winter species than for the summer legumes. Total N fixed was primarily influenced by crop growth. Researchers estimated that about 30,000 tonnes N is fixed annually in Nepal by legumes, valued at A$30 million.
Quality assurance was introduced into the laboratory operations of the three Pakistani institutes. A report of the NWFP group, which may well represent all groups, noted that the ACIAR project had played a major role in the capacity building of its institutions. The research team was able to establish appropriate lab and field facilities for research, and to conduct training courses/workshops in Peshawar for researchers and students.