Submitted to: African Journal of Plant Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/19/2010
Publication Date: 2/1/2011
Publication URL: hdl.handle.net/10113/48365
Citation: Lupwayi, N.Z., Kennedy, A.C., Chirwa, R.M. 2011. Grain legume impacts on soil biological processes in Sub-Saharan Africa. African Journal of Plant Science. 5:1-7. Interpretive Summary: Cropping systems in the Sub-Saharan Africa often rely on legume-based systems. The use of grain legumes such as dry bean, peas, chickpeas soybeans and lentils, has impacted soil organisms and the processes that they perform. The impacts of grain legumes on soil biota begin from the time seed is planted and continue after crop harvest. We investigated the soil biology changes occurring with legumes in rotation. The microbial and faunal communities found on legume seeds and roots are different from those found on non-legumes. The altered soil biology has mostly beneficial effects to agriculture by adding biologically fixed nitrogen, enhancing nutrient uptake and recycling, reducing greenhouse gas emissions by reducing N fertilizer use, improving soil structure and enhancing biological pest control. Residue and carbon quality characteristics of the legume also alter the soil microbial community. This information can assist growers, field personnel, government agencies and scientists in planning cropping rotations that lead to improved yield, and maintain or improve soil quality in Sub-Saharan Africa.
Technical Abstract: Grain legumes occupy about 20 million hectares in Africa. The major crops are cowpea (Vigna unguiculata), which is grown on about 11 million hectares mostly in west Africa, and common bean (Phaseolus vulgaris), grown on about 5 million hectares mostly in eastern and southern Africa. These grain legumes have impacted soil organisms, including nitrogen fixers, mycorrhizae, fauna, and the processes that they perform. The legume-Rhizobium symbiosis results in dinitrogen (N2) fixation that adds plant available nitrogen to the soil system. The amount of N left in the soil is minimal because most of the N in these legumes is in the grain that is removed at harvest. Non-legume crops grown in rotation with legumes may also benefit from endophytic rhizobia. Mycorrhizal associations that improve plant nutrient and water uptake also benefit from the legume in a cropping system. Besides breaking pest cycles, grain legume crops also reduce disease infestation of non-legume crops by enhancing biological pest control through increased microbial diversity and activity. Legumes may contribute to greenhouse gas (nitrous oxide and carbon dioxide) emissions during nitrification and denitrification of fixed nitrogen. However, because less fertilizer N is used in legume-based cropping systems, overall greenhouse gas emissions are usually less than those in fertilized monoculture cereals. Legume crops also improve soil structure by enhancing the formation and maintenance of soil aggregates. Therefore, grain legumes in Sub-Saharan Africa have positive effects on agriculture by adding and recycling biologically fixed nitrogen, enhancing nutrient uptake, breaking non-legume crop pest cycles, reducing greenhouse gas emissions, and improving soil structure.