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In the north Ethiopian highlands, gully erosion is a significant land degradation process. Although the protective role of vegetation has been demonstrated in many studies, efforts previously made in using woody species for erosion control in the research area are limited, and when applied, survival of the planted seedlings was very low. Lack of experience and fundamental knowledge on species autecology and traits are two important bottlenecks. This study therefore aims at better understanding seedling establishment and growth in a context of tree plantings, with the view to further control gully erosion in a semi-arid environment. To this end survival, growth and development of seedlings of Acacia etbaica Schweinf., Sesbania sesban (L.) Merr. and Dodonaea angustifolia L.f. were monitored during 26 months in a field trial under different site conditions and treatments. The experiment was established at two sites characterized by topographic and edaphic conditions (Vertisol and sandy colluvium) representative for the study area. At each site, seedlings were subjected to different treatments of watering, sheltering and planting position (gully floor, sidewall and shoulder), and a large set of plant growth variables was regularly monitored. Height and biomass growth was fastest for Sesbania, even if affected by different pests. Regardless of the species, plants growing in the (nutrient-rich) Vertisol site had a higher survival rate and an overall better development. Shelter protection significantly increased survival and resulted in taller seedlings. Notwithstanding these general trends, treatment effects were often varying over time and strongly influenced by specific edaphic conditions. Generally, Acacia performed best on gully shoulder and wall, Sesbania on the gully floor and Dodonaea on shoulder positions. Recommendations for planting practice and follow-up research are discussed. [ABSTRACT FROM AUTHOR]

Three woody legumes were planted as two-year `improved fallows' to evaluate their residual nitrogen (N) effects on two subsequent maize crops under minimum and conventional tillage management. Maize monoculture and cowpea-maize-maize sequence treatments were included as controls. N2-fixation was estimated using the 15N natural abundance method to allow the N contribution from the fallows to be partitioned into N2-fixation and soil N capture. Acacia angustissima accumulated the largest amount of both below-ground and above-ground biomass during the 2-year growth period. Using Hyparrhenia grass as the reference plant, the proportion of N2-fixed in litter was 56, 55, 84 and 58% for Acacia, Sesbania sesban, Cajanus cajan, and cowpea, respectively, resulting in inputs of biologically fixed N of 122, 84, 96 and 28 kg N ha-1. Maize growth following the legumes for two subsequent cropping seasons was in most cases not directly related to the N inputs. The first year maize crop was severely infested with cutworms (Agrotis sp.) in the Sesbania and Acacia plots, while the second season maize was affected by drought particularly in the Acacia plots where there was high moisture demand by the re-growing trees. During the second season, N uptake by maize following Sesbania was significantly higher than the rest of the treatments. Conventional tillage resulted in better yields than minimum tillage across all treatments and in both seasons. It can be concluded that (1) improved legume fallow system results in net positive soil N balances, (2) despite large inputs of litter and continued recycling of N during the cropping phase, legume species that re-grow after cutting can result in depressed yields in drought seasons, and (3) tillage is beneficial even after improved legume fallows. [ABSTRACT FROM AUTHOR]

Examines the effects of planted fallows of Sesbania sesban and Calliandra calothyrsus on soil inorganic nitrogen dynamics and subsequent maize crops in Kenya. Determination of ground biomass and nitrogen content; Soil organic matter; Effect of tree fallows on subsequent maize crops.