Agriculture Reference
In-Depth Information
Several other studies have also reported N fixation by legume trees, with reported
values ranging from 100 to 200 kg ha
−1
(Table 11.2). The benefits of the N contribu-
tion of fertilizer trees on a regional scale (Zambia, Malawi, Zimbabwe, Tanzania,
and Mozambique) as of 2003 were estimated to be from $6.27 to $7.13 million per
year in savings on the purchase of mineral fertilizer (Ajayi et al. 2005).
Trees may supply crop nutrients through leaf drop, root exudates, and senescence,
as well as supply timber and fuelwood. In a study in Tanzania,
Acacia polyacantha
and
Gliricidia
leaves had higher nutrient concentrations and lower C-to-N ratios,
reflecting higher leaf quality compared with those of
Acacia crassicarpa
and
Acacia
mangium
(Kimaro et al. 2008). Nitrogen uptake of maize follows a similar pattern
(Table 11.2), with
A. polyacantha
and
Gliricidia
supplying >100 kg ha
−1
of N to the
maize and the exotic acacias supplying less N for subsequent crops.
11.2.1.2 Other Macro- and Micronutrients
The ability of agroforestry trees to fix N for cereal crops is often emphasized.
However, a closer analysis shows that the benefits of agroforestry go beyond N fixa-
tion.
The use of agroforestry trees has proven to also enhance P availability to sub-
sequent crops (Chikowo 2004; Ayuk and Mafongoya 2002; Ajayi et al. 2005; Jose et
al. 2000) Many soils in southern Africa are P deficient, and expensive inorganic P
supplements are needed for crop production. Fertilizer trees can economically close
this gap as they can improve P availability through the secretion of organic acids and
increased mycorrhizal fungi populations in the soil. The fungi that are associated
with increased P availability in agricultural soils are arbuscular mycorrhizal (AM)
fungi (phylum Glomeromycota)
(Harrier and Watson 2003). Bagayoko et al. (2000)
reported that N-fixing legumes resulted in better colonization of cereal roots and an
increase in AM fungal populations in the soil. Fertilizer trees may act as reservoirs
for AM fungi populations in crop root zones (Ingleby et al. 2007). The mixing of
crops and trees that occurs in agroforestry systems may also result in a more equi-
table occurrence of mycorrhizae throughout the root zone because deep-rooted trees
will distribute the AM fungi to deeper layers and increase the volume of soil from
which nutrients are extracted (Cardoso et al. 2003). Management practices such as
agroforestry that allow a buildup of AM fungi in soils would alleviate P deficiency
while enhancing N fixation (Houngnandan et al. 2000). AM fungi have been pro-
posed as a partial solution to nutrient deficiencies in tropical soils (Cardoso and
Kuyper 2006), and the combination of AM fungi and agroforestry may have a role in
the alleviation of micronutrient deficiencies in depleted smallholder soils.
11.2.1.3 Role of Fertilizer Trees as Nutrient Pumps and Safety Nets
In cropping systems located in high-rainfall areas, where net N mineralization
exceeds N uptake by crops, infiltrating water carries nutrients to the subsoil, result-
ing in a buildup of subsoil N that ranges from 70 to 315 kg ha
−1
(Hartemink et al.
1996). This is supported by Chikowo (2004) who reported that up to 45 kg N ha
−1
is lost through leaching during the cropping phase of maize fallow rotations. Under
conditions where the soils have substantial anion exchange capacity, e.g., Oxisols and
oxic Alfisols, leached N and other nutrients are retained in the subsoil beyond the
reach of most crops and can only be accessed by tree roots (Mekonnen et al. 1997).
