For more than 2000 years, legumes have been used in crop rotations to enhance the growth and yield of the subsequent cereal crops. While legumes can leave residual N in the soil for the subsequent crop, this cannot account for all the rotation benefit (Bolton et al 1976; Hesterman et al. 1986; Fyson, Oaks 1990). Consequently, a variety of mechanisms have been proposed, including legume effects on altering soil structure or nutrient balance, breaking the disease cycle or opening channels in soil for deeper rooting.
The data described in this report indicated that soil H2 fertilization may play a significant role in contributing to the benefit of legume crops in rotation with cereals. Although the mechanism by which H2 fertilization of soils enhances plant growth has yet to be proven, the most likely explanation involves the enhanced growth of H2-oxidizing microorganisms in the soil. These organisms may improve the nutrient status of soil or act as plant growth promoting rhizobacteria (PGPR), enhancing the plants disease resistance or growth regulator balance (Hart et al. 1986; Williams, Sparling 1988; Insam et al 1991; Srivastava, Singh 1991; Bankole, Adebanjo 1996; Omar, AbdAlla 1998). These bacteria population changes in the soils adjacent to H2 releasing nodules may be associated with the enhanced growth response of plants rotated with legumes. If so, H2 fertilization of soils could achieve some of the beneficial effects of crop rotation, without the need to implement actual crop rotation.
This study would also help to account for the evolutionary questions surrounding why HUP" symbioses have thrived when there are genes (in many cases within the same genus and species) for the more energetically efficient HUP+ symbioses. Perhaps the plant growth advantages of the HUP" symbioses offset the greater energy efficiency of the HUP+ symbioses.
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