9. Nitrogen Absorption and Immobilization Patterns as Cataysed by the Roots of Acacia Plants

Large tracts of soil in sub-Saharan Africa are nutrients deficient while inorganic fertilizers are unaffordable for most subsistence farmers. Rotations and/or intercropping of nitrogen fixing trees like Acacia species with crops may alleviate the nitrogen deficiency through biological nitrogen (N₂) fixation and redistribution of subsoil nitrogen to the surface. The study was conducted on an N-deficient, sandy loam (Alfisol) under bimodal rainfall conditions at Kendu Bay, Nyanza Province in Kenya to compare the effectiveness of A. nilotica, A. senegal and A. xanthophloea. This was carried out in order to improve degraded lands by fixing nitrogen from the air, extracting water and nutrients from the soil and investigating the resulting effects on maize yields. Optimal spacing for the Acacia trees for the nitrogen fixation was also evaluated in two seasons to enhance maize yields and nutrients extractions from the soil. Assessment of the relationship between the amount of phosphorus in the soil and nitrogen fixation/availability by the Acacias were also carried out simultaneously. There was poor response in the first season but there was significant increase (P < 0.05) in maize yield due to planting A. nilotica, A. senegal and A. xanthophloea in the second season. The planting of A. nilotica at different spacing in the second season also contributed to the significant increase in the maize yield. In the first season, the Acacias intercropping reduced maize yields. The yield reduction in the first season might be attributed to competition with maize plants when the trees had not grown long enough roots for effective nodulation, and nitrogen fixation while in the second season the Acacia trees had grown long roots that were effective in the nitrogen fixation. Again in the first season close planting spacing of the Acacia reduced yields of the maize. In the second season there was significant (P < 0.05) maize yield increase under closely spaced Acacia trees. These results demonstrate that the benefits of Acacia intercropping in increasing maize yields are more effective after the Acacias have grown and the close spacing of Acacia may be more beneficial in effective nitrogen fixation and supply in the long run. There was increase in phosphorus concentration in maize plants in season two whereby A. nilotica had an increase in phosphorus from 0.183% in season one to 0.363% in season two. The close spacing of the trees was important in phosphorus build up and nitrogen fixation as shown by higher amounts of nitrogen in the soil and maize plants in season two. Soil pH did not vary and remained almost neutral with soil moisture content increasing due to added organic matter from the trees. The findings of this study can be used to intensify agroforestry at the farm level to increase food security in the Lake Victoria Basin and the work should involve all the stakeholders (farmers, extension officers, researchers, non-governmental organizations, leaders, and community based organizations) in decision making and implementation.