Alley cropping in the moist savanna of West-Africa: II. Impact on soil productivity in a North-to-South transect in Togo

The moist savanna of West-Africa is characterized by a wide range of climat es and soil types. The impact of the biophysical environment on hedgerow N uptake, wood production and maize grain yield was assessed for three years in three alley cropping trials with a selected number of hedgerow species i n Glidji (Southern Togo), Amoutchou (Central Togo), and Sarakawa (Northern Togo), Senna siamea hedgerows accumulated significantly more N in the first pruning in Glidji (129-138 kg N ha(-1)) and Sarakawa (102-185 kg N ha(-1)) than in Amoutchou (17-26 kg N ha(-1)). This difference in N uptake was att ributed to the infertile subsoil in Amoutchou, which was sandy up to 1 m an d had a shallow groundwater-table. The amount of N accumulated in the Gliri cidia sepium biomass varied between 38 kg N ha(-1) in Glidji and 142 kg N h a(-1) in Amoutchou. Averaged over all species and sites, 9 to 29% and 9 to 39% of the annual N accumulation in the hedgerow biomass is incorporated in the second, respectively third pruning. The Gliricidia trees produced betw een 12 and 26 ton fresh matter ha(-1) of wood and the Senna trees between 4 and 38 ton fresh matter ha(-1). Maize grain yield in Glidji was not affect ed by treatments (3196 kg ha(-1), on average). In Amoutchou, the highest gr ain production was observed in the Gliricidia treatment (2774 kg ha(-1) vs 1007 kg ha(-1) in the control), while in Sarakawa, the Gliricidia (3786 kg ha(-1)) and Senna (3842 kg ha(-1)) plots produced a greater grain yield tha n the control plots (2123 kg ha(-1)). Maize yield increase in the alley cro pping systems relative to the control plots was related to the soil total N content. Top and sub-soil characteristics were shown to be an important mo difier of the functioning of alley cropping systems and should be taken int o account when deciding on whether to use alley cropping and when selecting the hedgerow species.