How can increased use of biological N2 fixation in agriculture benefit the environment?

Summary

Biological N2 fixation (BNF), either asymbiotic, associative or symbiotic is considered a free and renewable resource, which should constitute an integral part of sustainable agro-ecosystems globally. Since the 1950ties and the invention of industrial synthesis of N-fertilizer there was a rapid increase in the use of fertilizer N worldwide and a parallel decline in the use of leguminous plants and BNF, especially in the developed world. Fertilizers have boosted crop yields, but the intensive agriculture often associated with heavy fertilizer use has resulted in increasing negative side effects on the atmospheric and aquatic environments. BNF, either alone or in combination with fertilizers and animal manures, may prove to be a better solution to nitrogen supply of cropping systems in the future. This review focuses on the potential benefit of BNF on the environment. The effect of the BNF process per se on soil acidification, rhizosphere processes and plant CO2 fixation is discussed. Since fertilizer N has substituted BNF in agriculture the re-substitution is considered. What is the consequence of fertilizer N production for the energy use in the society? The effect of fertilizer production on the release of the greenhouse gas CO2 is estimated, and found to be less than 1% of the global anthropogenic emission of CO2. The role of BNF on nitrogen cycling processed in cropping systems are discussed, with center of attention on loss process such as NH3 volatilization, N2O emission and NO3 leaching. It is concluded that BNF is less likely than fertilizer to cause losses during pre-cropping and cropping, but post-harvest the risk of N losses may sometimes be greater, due to effects of the biochemical qualities of legume residues on N cycle processes. However, management options are available to minimize these risks. Legumes are an integral part of approaches to soil fertility building in any system and N2 fixing may have other “ecological services” in the agroecosystem, e.g. improved soils structure, break-crop effect, deep rooting, erosion protection and contributing to greater biological diversity. Many of these effects are beneficial in terms of improving the environmental quality and legumes or other ways of including BNF should be an integral part of future cropping systems.