Nutrient cycling and agroforestry in Africa

One of the main tenets of agroforestry is that trees enhance soil fertility, the capacity of the soil to provide essential nutrients for plant growth. There is often confusion between key terms such as nutrient inputs, nutrient outputs, nutrient balances, nutrient cycling and nutrient capital. They all refer to a soil-plant system, usually at the scale of the farmer's field. Nutrient inputs are additions originating from outside the system, such as nitrogen fixed from the air by legumes or the use of chemical fertilizers. Animal manures are inputs if the manure was produced outside the system Nutrient outputs are those that leave the system through crop harvest removals, soil erosion, leaching, gas volatilization and other processes. The nutrient balance is the difference between nutrient inputs and outputs. Nutrient cycling refers to the transfer of nutrients already in the soil plant system from one component to another, for example the release of nitrogen from soil organic matter as ammonium or nitrate and its subsequent uptake by plants.Other processes involved in nutrient cycling are the return of crop residues such as stover back to the soil; manure and urine deposited by cattle in the system; the incorporation of leguminous green manures into the soil and the transfer of nutrients from trees to crops in agroforestry systems through prunings, leaf drop or root decomposition. The fewer the nutrient losses from the system, the fewer the inputs needed from outside the system to balance the budget. The nutrient capital refers to the soil reserves of nutrients that will be released gradually over a time scale of years or decades. This article first examines the role of agroforestry in nutrient cycling in different ecosystems and then focuses on the two main nutrients, nitrogen and phosphorus, in smallholder maize-based systems of Africa.

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