Nitrogen budgets and flows in African smallholder farming systems

Nitrogen is often the most limiting nutrient crop production in smallholder farms in Africa. Nitrogen flows and budgets in most farming systems play a crucial role in agricultural production and may lead to either N depletion or accumulation. This review presents trends of N balances and flows in Africa's smallholder farming systems as influenced by the methods used in deriving them, the differences in farming systems and, inherent variability in soil fertility and management strategies. At the farm/household level, wide variations have ranged from negative of as low as -76 kg N ha(-1) to positive N balance of up to +20 kg N ha(-1) within short distances which are caused by the biased allocation of N fertilizer based on the type and value of the crop planted. The review highlights the potential application of N balance approaches as an indicator of soil nutrient mining in smallholder farming systems of sub-Saharan Africa. There is enhanced awareness of nitrogen depletion in Africa from the intensive research carried out in the field. Nevertheless, most of them are snapshot, static assessments, which give little information on the dynamics and spatial variation of nitrogen flows.