Agriculture Reference
In-Depth Information
11.3.2
Soc
in
S
mAllholder
S
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11.3.2.1 SOC and Soil Fertility
Improved fallows, intercrops, and woodlot rotations increase the labile fractions of
SOM, which supply nutrients to crops following fallows (Barrios et al. 1997; Beedy
et al. 2010; Kimaro et al. 2011), and can also contribute to improving soil structure,
buildup of SOM, and C stocks, thus contributing to C sequestration. Buildup of SOM
is critical to soil productivity and generally corresponds to nutrient exchange capac-
ity. Release of N from SOM may contribute most of the 40 kg N ha
−1
taken up by the
average maize crop of 1 Mg ha
−1
(Sanchez and Palm 1996; Makumba et al. 2006).
SOC increases the cation exchange capacity (CEC) of the surface soil, which is espe-
cially important for nutrient storage in kaolinitic soils and other light-textured soils
with low CEC. Increasing SOM can reduce P fixation in soils with high iron and
aluminum oxide contents, thus making the P available for plant uptake. High SOC
in fallows results in reduced rates of nutrient leaching due to reduced mineralization
rates (Nyamadzawo et al. 2009). Agboola (1994) reported that 80% of CEC, and
available P, K, Mg, and Ca were highly correlated with SOM levels in some West
African Alfisols. Under fallow systems, the microbial biomass has been shown to
be higher (Nyamadzawo et al. 2009), the microbial community more diverse, and
the rate of plant material decomposition much faster than in nonfallowed systems
(Sarmiento and Bottner 2002), thus ensuring nutrient recycling and timely release of
N and other nutrients. Leguminous fertilizer trees increase SOC levels of soils and
thereby indirectly improve soil fertility. Fernandes et al. (1997) suggested that our
greatest opportunity is that SOM is a renewable resource whose level can be replen-
ished by additions of organic inputs.
11.3.2.2 Potential SOC Increases with Agroforestry
Agroforestry land use systems have been reported to sequester more C than other
forms of agriculture. The amounts of biomass and SOC additions vary with tree
species, soil type, rainfall, and environmental conditions. The extent to which
agroforestry practices will build soil C is controlled by the ability of the tree-crop
combination to produce biomass residue to be transformed into SOM. Plant bio-
mass residues may be deposited on the soil surface for decomposition, incorporated
by tillage, or added to the soil profile at varying depths by root exudates and root
decomposition. Carbon sequestration from cropping system residues varies with soil
temperature and moisture, litter quality, and root dynamics (Post and Kwon 2000).
Albrecht and Kandji (2003) also specify fire, decomposition, leaching, and erosion
as four major avenues of SOC loss. Burning of crop residues and tillage oxidize soil
C. Erosion transports soil C offsite, sometimes into rivers and lakes. Leaching trans-
ports soil C downward in the soil profile and sometimes removes it into streams and
rivers via runoff from fields.
The ability of a given soil to retain organic matter varies strongly with soil tex-
ture (Albrecht and Kandji 2003). SOC oxidizes more quickly in sandy soils and
those with weak aggregation. Organic matter that is not adsorbed to clay colloids
or protected within soil aggregates is quickly decomposed by soil microorganisms
(Six et al. 2000). In fine-textured soils, SOC is adsorbed to clay colloids, protecting
