Agriculture Reference
In-Depth Information
11.3.2 SOC in Smallholder Settings ............................................................ 260
11.3.2.1 SOC and Soil Fertility ....................................................... 260
11.3.2.2 Potential SOC Increases with Agroforestry ....................... 260
11.3.2.3 Intercrops, Improved Fallows, and Woodlots .................... 261
11.3.2.4 Connection with Carbon Markets ...................................... 263
11.3.3 GHG Emissions ................................................................................264
11.3.3.1 Soil Carbon Dioxide, Methane, and Nitrous Oxide
Emissions ...........................................................................264
11.3.3.2 Emissions of GHGs in Agroforestry in Eastern and
Southern Africa .................................................................. 265
11.3.3.3 Suggested Future Studies ................................................... 266
11.4 Conclusions, Challenges, and Future Needs ................................................. 266
References .............................................................................................................. 267
11.1 INTRODUCTION
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 decom-
poses after being incorporated into the soil. Because farming practices of African
smallholders tend toward multipurpose mosaics rather than uniform field manage-
ment, 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 rec-
ognized 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 through-
out 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 poten-
tial 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.
