Food legumes have a unique potential to improve both human nutrition and agricultural sustainability; they provide a high-quality source of nutrients, N inputs through biological nitrogen fixation, and are considered a high-value crop for marketing. However, their potential frequently goes unrealized in some of the poorest regions of the world, where the predominance of highly weathered tropical soils constrains legume productivity. Most legumes perform poorly under low fertility conditions, especially where acid soil conditions cause free iron (Fe) and aluminum (Al) oxides to bind soil phosphorus (P) into forms unavailable to plants. Legumes are especially sensitive to low P availability since biological nitrogen fixation has a high P requirement. Application of P fertilizer is only a partial solution since P fertilizers are resource-dependent and expensive, and subject to the same chemical processes that immobilize native soil P. Besides phosphorus, nitrogen could be another important limiting factor for soybean growth due to the lack of effectively native rhizobial strains in most tropical and subtropical areas. Application of N fertilizer not only wastes resources but also causes environmental pollution.
There has been significant progress in genetic improvement of common beans (Phaseolus vulgaris) and soybeans (Glycine max) for better adaptation to low soil P. Research at both Penn State University (PSU) and South China Agricultural University (SCAU) has demonstrated the central importance of root morphology (esp. root hair characteristics) and architecture in conferring P-efficiency in common bean. These root traits have been used at SCAU to screen for P-efficiency among soybean landraces and bred lines, as well as create new genotypes with improved P efficiency and enhanced nitrogen fixing capacity.
The current project represents the second phase of an earlier CCRP project (2002-2005), which targeted the development of P-efficient soybean varieties P-deficient soils of South China. During phase one, collaborators at SCAU and PSU validated that the same traits that are important for P-efficiency in common bean are also important in soybean. The team also developed new P-efficient soybean varieties for the low P soils of South China, some of which have been commercially released to farmers in South China. These new genotypes have shown substantial yield gains in low P soil compared with conventional genotypes, in some trials doubling yield without additional inputs, thereby demonstrating the potential of this approach.
During phase two, collaborators from Mozambique have joined the research team and the project's mandate area have been expanded to include this southern African country where more P-efficient soybean and bean varieties could help to improve the health and well-being of some of the world's most economically disadvantaged rural communities. Evaluating legume performance under Mozambique conditions will also be important for looking more closely at potentially important genotype-by-environment interactions for P-efficient root traits. For example, shallow-rooted genotypes are able to acquire more P from the soil since in most soils P availability is greatest in the topsoil and declines with depth. However, shallow-rooted genotypes are more sensitive to drought, which is also an important constraint in many agroecosystems, especially those in Southern Africa. In such systems, genotypes with root systems that balance both deep and shallow soil exploration may be required. This project are currently evaluating ecological tradeoffs for root architectural and morphological traits in the specific contexts of South China and Mozambique.
Phosphorus-efficient soybeans and beans are likely to be extremely useful in the low fertility soils of South China and Mozambique. In order to realize the potential of this technology, Phase 2 of the project is focusing on the following activities: (1) continue the soybean breeding program in China and begin a bean and/or soybean breeding program in Mozambique with specific focus on P efficiency, (2) develop a better understanding of how traits conferring P efficiency in legumes affect crop responses to other environmental factors, (3) determine how P efficient legumes will affect agroecosystem productivity and sustainability, as well as the economic well being of rural communities, and (4) develop African scientific capacity in plant nutrition so that this effort can be sustained.