A nitrogen fix for farmers

Demand for food is likely to double within the next fifty years but to double yield will require doubling fertilizer availability and, in the case of nitrogen, unless biological nitrogen fixation (BNF) can be improved, a tripling of mineral fertilizer will be required. To triple present day production would require burning of 500 million tonnes of coal, or its equivalent, annually. Technically this should be possible but environmental and political pressures are working in the opposite direction. Even if the inevitable increases in CO2 emissions were accepted, application of such large amounts of nitrogen would undoubtely result in increasing leaching into groundwater. Biological nitrogen fixation (BNF), the process by which soil bacteria fix atmospheric nitrogen, has been the subject of research for the past 100 years. The big-chemical processes are well understood and nuclear technology using the 15N isotope means that researchers have an accurate method for quantifying BNF. There is a strong and continuing commitment by the scientific community to the basic research but what are the options currently available to farmers in the developing world for enhancing BNF within their own farming systems? There are four principal means by which biological nitrogen fixation can benefit agriculture: symbiotic nitrogen fixation by the crop itself; symbiotic nitrogen fixation by the Azolla/Anahaena azollae complex, which is used as a green manure; non-symbiotic nitrogen fixation at crop level by, for example, blue-green algae, azospirillum and other rhizospheric bacteria, and the use of nitrogen fixing trees or shrubs to improve the nitrogen content of the soil. Symbiotic nitrogen fixation by leguminous crops has been the subject of much research. It has long been known that different legumes have different nitrogen fixing capacity and furthermore, that within each species, cultivars differ in efficiency. BNF can be greatly increased by matching the compatibility of the host plant and the strain of companion rhizobial bacteria. Where the crop legume is a new introduction to the farming system, as is often the case with soybean, naturally occurring rhizobia compatible with the new host are unlikely to be available in the soil and inoculation, either of the seed or the soil, is needed. Trials under field conditions are essential since work by, among others, Dr Gudni Hardarson at FAD-IAEA Joint Division laboratories at Siebersdorf, Austria has shown that failure of legumes to achieve economically attractive yields is often the result of inhospitable environmental conditions. Rhizobia efficiency and efficacy The ability of rhizobia to survive in the soil depends, among other factors, on soil texture, organic matter content and pH value. Improving land management for legumes should lead to tangible results, some of which will be due to increased nitrogen fixation. Even the best legume will not perform well when it is cultivated in the wrong place or at the wrong time. Inoculation has been shown to improve yields of soybean, for example, by 50% or more. But, says Dr Michael Obaton of INRA (currently with ICARDA), work is still required to make the inoculant easier for farmers to use and to improve its keeping quality, especially in temperatures above 30° C. Furthermore, inoculation stimulates nodule growth at the root crown only. Subsequent nodulation on lateral and deeper roots is from indigenous bacteria already within the soil. While there is plenty of evidence of the successful application of inoculation technology at the resource-rich, large farm level where innovation carries little risk, the use of inoculum at small farm level is minimal according to Dr David Hubbell of the University of Florida. This is despite the fact that technology has been available commercially, in appropriate form for small farm application, for eighty years. It is the transfer of information to farmers that has become stuck in the neck of the research bottle. Scientists have been filling this bottle with research results for years but policy makers and those responsible for setting agricultural priorities have a firm hold on the cork and the corkscrew. Azolla and tree legumes Azolla, with its symbiotic partner the N2 fixing blue green-alga Anabaena azollae, has been used for centuries as a nitrogen fixing fertilizer in some rice growing countries. However, the realizable potential of Azolla as a green manure is restricted by, among other factors, very labour intensive management. According to Dr Pierre Roger of ORSTOM, given the choice of purchasing cheap, industrially produced, balanced fertilizer or spending three to four months cultivating Azolla, dividing it, and digging to incorporate it, most farmers would choose the factory product and it seems that more are indeed doing so. Non-symbiotic, or free living, N2 fixing organisms include blue-green algae and rhizospheric bacteria such as azospirillum and Bacillus polymyxa. Blue-green algae, which make a significant contribution to N fixation in paddy rice culture, are limited by low pH, P deficiency and the use of mineral N fertiliser. Furthermore, non-indigenous strains rarely establish themselves. These findings suggest that more attention should be paid to cultural practices to support indigenous strains, such as liming of acidic soils, split application of P and deep placement of mineral N which avoids some of its inhibitory effect (Roger). According to Dr Thierry Heulin from CNRA, bacteria associated with the root system (the rhizospheric bacteria) of cereals and forage grasses are able to fix nitrogen and promote plant growth, but these research findings are not yet implemented at the farmer level. The impact of leguminous trees and shrubs on soil fertility has also been the subject of research for many years and the technologies for intercropping annual crops with leguminous trees have been made widely available to farmers most recently by the International Centre for Research in Agroforestry (ICRAF) and the International Institute for Tropical Agriculture (IITA). Increased use of inorganic fertiliser is inescapable, but a parallel return to greater exploitation of BNF would seem to make good sense. There are three challenges to be met. The first is to make sure that the research serves the farmer rather than the researcher. The second is to convey existing and, as it becomes available, future knowledge to farmers. And thirdly farmers must be placed in a position to implement the knowledge, if they wish to do so.

Demand for food is likely to double within the next fifty years but to double yield will require doubling fertilizer availability and, in the case of nitrogen, unless biological nitrogen fixation (BNF) can be improved, a tripling of mineral...