ABSTRACT
Nets ....................................................................................250 11.2.2 Maize Yield Increases from Improved Soil Fertility ........................ 251
11.2.2.1 Improved Fallows ............................................................... 251 11.2.2.2 Intercrops ........................................................................... 253 11.2.2.3 Rotational Woodlots ..........................................................254 11.2.2.4 Parkland Systems in Eastern and Southern Africa ............ 255 11.2.2.5 Faidherbia albida: A Successful Parkland Species .......... 255
11.3 Accumulation of Carbon in Biomass and Soil ............................................. 257 11.3.1 Biomass Accumulation in Agroforestry in Eastern and Southern
Africa ................................................................................................ 257 11.3.1.1 Intercropping Agroforestry Systems .................................. 257 11.3.1.2 Improved Fallows ............................................................... 258 11.3.1.3 Rotational Woodlots and Tree Plantations ......................... 259
Soil and water resources worldwide are under stress from the accelerated demands of increasing population. Reduction in soil fertility is driven by increased human population that has reduced land availability and caused a breakdown of traditional fallow systems that smallholder farmers relied on for soil fertility replenishment. Agroforestry is among a suite of sustainable agricultural practices that can rebuild soil fertility and soil organic matter (SOM), and break the cycle of poverty. Many agroforestry species are used for biological fixation of atmospheric nitrogen (N) into available N, root uptake, and recycling of nutrients. Nitrogen that accumulates in the leaves of planted tree fallows and intercrops is released when the biomass decomposes after being incorporated into the soil. Because farming practices of African smallholders tend toward multipurpose mosaics rather than uniform field management, the number of useful combinations of crops with agroforestry is constantly increasing. New tools such as “Useful Tree Species for Africa” facilitate the choice of trees within farming systems. Short-term agroforestry species have increased cereal yields from 10% to 200%, while yield differences under long-term parkland species such as Faidherbia albida have ranged from slight decreases to doubling of yields. Parkland systems have long been used by farmers but are now being recognized by the development community. The multiple sources of the yield benefits under parkland management are currently being documented by researchers. While rebuilding soil fertility, agroforestry also increases biomass buildup and carbon (C) sequestration in farming systems. This increase, however, is highly variable throughout eastern and southern Africa, and the residence time of soil organic carbon (SOC) is controversial. All agroforestry systems for which data are available accumulate biomass faster than the natural systems they emulate. The range of C sequestration by smallholder agroforestry in the tropics has been bracketed between 1.5 and 3.5 Mg C ha−1 year−1. Addition of agroforestry species to farming systems has the potential to either enhance or reduce soil C and greenhouse gas (GHG) emissions. Thus, the study of GHG emissions with agroforestry practices is critical in describing the trade-offs between smallholder and ecosystem benefits from agroforestry.
