Sustainable nutrient management and agrobiodiversity in Africa

Background/Question/Methods: Increasing soil nitrogen (N) availability to grain crops in Africa is a well-documented way to increase grain yields on a continent where increased food demand far outpaces supply.  Since 2004, the UN Millennium project has recorded increased average maize yields from 1.7 to 4.1 t ha^-1 in villages where improved seed and adequate fertilizer were made available.  In Malawi a fertilizer subsidy introduced in 2006 increased maize production from 1.2 to 2.6 million tons.  However, subsidies are expensive, fertilizers are subject to environmental losses, and widespread distribution is challenging.  Through symbiotic biological N fixation (BNF) with soil bacteria known as rhizobia, legume crops can improve the N economy of an agricultural system and reduce farmer dependence on expensive N fertilizers.  Sustainable nutrient management and future food security in Africa will in-part depend on improving legume crop yield, increasing land area in legume cultivation, and promoting innovative inter-cropping and rotational strategies. 

Results/Conclusions: Research in Malawi has shown that diversified legume cropping strategies in conjunction with reduced fertilizer N inputs produced equivalent grain yields to maize systems supplied with recommended N fertilizer rates.  Diversified systems also had greater soil cover, protein yield and fertilizer efficiency than monoculture systems.  To further understand the potential for legume-diversified systems, research is needed to determine the proportion of legume N derived from BNF in these systems, understand how native communities of soil rhizobia interact with introduced rhizobia and legume species for nodule formation and N-fixation, and explore how farmers can exploit soil N cycling processes to synchronize N availability with crop needs.