HighlightsProductivity of different intercropping patterns was tested in Guinea savanna of northern Ghana.Land Equivalent Ratios in intercropping systems are greater under low soil fertility conditions.Competitive balance between intercrops in poor fields leads to greater Land Equivalent Ratios.Within-row maize-legume intercropping is more productive than distinct row systems.Radiation use efficiency is higher in intercrops than in sole crops.

HighlightsCropping system and soil fertility effects on N2-fixation were tested in northern Ghana.More N2 is fixed in sole cropping than intercropping despite comparable %Ndfa.Poorly fertile fields give limited grain legume benefits despite enhanced %Ndfa.Partial N balances are unreliable indicators of cropping system sustainability.Different grain legumes should be targeted to different sites in the Guinea savanna.

SUMMARYSoil nutrient constraints coupled with erratic rainfall have led to poor crop yields and occasionally to crop failure in sole cropping in the Guinea savanna of West Africa. We explored different maize-grain legume diversification and intensification options that can contribute to mitigating risks of crop failure, increase crop productivity under different soil fertility levels, while improving soil fertility due to biological N 2 -fixation by the legume. There were four relay patterns with cowpea sown first and maize sown at least 2 weeks after sowing (WAS) cowpea; two relay patterns with maize sown first and cowpea sown at least 3 WAS maize in different spatial arrangements. These were compared with groundnut-maize, soybeanmaize, fallow-maize and continuous maize rotations in fields high, medium and poor in fertility at a site each in the southern (SGS) and northern (NGS) Guinea savanna of northern Ghana. Legumes grown in the poorly fertile fields relied more on N 2 -fixation for growth leading to generally larger net N inputs to the soil. Crop yields declined with decreasing soil fertility and were larger in the SGS than in the NGS due to more favourable rainfall and soil fertility. Spatial arrangements of relay intercrops did not have any significant impact on maize and legume grain yields. Sowing maize first followed by a cowpea relay resulted in 0.18-0.26 t ha −1 reduction in cowpea grain yield relative to cowpea sown from the onset. Relaying maize into cowpea led to a 0.29-0.64 t ha −1 reduction in maize grain yield relative to maize sown from the onset in the SGS. In the NGS, a decline of 0.66 and 0.82 t ha −1 in maize grain yield relative to maize sown from the onset was observed due to less rainfall received by the relay maize. Groundnut and soybean induced 0.38-1.01 t ha −1 more grain yield of a subsequent maize relative to continuous maize, and 1.17-1.71 t ha −1 more yield relative to relay maize across both sites. Accumulated crop yields over both years suggest that sowing maize first followed by cowpea relay is a promising ecological intensification option besides the more common legume-maize rotation in the Guinea savanna, as it was comparable with soybean-maize rotation and more productive than the other treatments.

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at 4 p.m. in the Aula. iv Michael Kermah (2020) Sustainable intensification and diversification options with grain legumes for smallholder farming systems in the Guinea savanna of Ghana. 157 pages. PhD Thesis, Wageningen University, Wageningen, NL (2020). With references and summary in English ISBN: 978-94-6395-228-6 DOI: https://doi.org/10.18174/507853v To my late beloved big sister, Janet Ebah Kermah, your toil to ensure the family succeeds will never be in vain, you will forever live in my heart, our hearts. vi vii AbstractFood security is a critical issue in the Guinea savanna of Ghana where about 60% of the rural population, mostly smallholder farmers are food insecure. Food insecurity results from poor crop yields due to low soil fertility compounded by erratic unimodal rainfall and the inability of households to purchase required supplemental food. Rapid population growth means that the numbers of food insecure people are likely to increase, necessitating sustainable intensification and diversification to increase crop production per unit area of land. This thesis focused on testing spatial and temporal intensification and diversification options suitable for the variable biophysical and socio-economic conditions of smallholder farming systems in the Guinea savanna to increase productivity, mitigate the risk of crop failure, and thus to increase food selfsufficiency. One site in the southern Guinea savanna and one in the northern Guinea savanna were selected which differed in biophysical and socio-economic resources. In each site, field experiments were conducted on three fields differing in soil fertility (fertile, medium fertile, poorly fertile) to quantify: N2-fixation and N contribution to soil fertility by grain legumes in sole and intercropping; impact of replacement intercropping on increasing resource use efficiency and crop productivity; and productivity of relay (additive) intercropping and rotation of grain legumes with maize. Scenario analysis was performed with data from the N2AfricaGhana project supplemented with data from the on-farm experiments and literature to test the impacts of intensification and diversification options on household food self-sufficiency. Sole legumes fixed larger amounts of N2 than under intercropping. The soil N balance was generally positive and similar between intercrops and sole crops suggesting that both systems could be sustainable intensification and diversification options. Poor fields stimulated grain legumes to rely on atmospheric N2 for growth leading to more positive soil N balances than in fertile fields. Consequently, legumes in poor fields were more competitive with maize and led to greater intercrop yield advantage than in fertile fields. Across all fields and sites, intercropping enhanced the efficiency in resource use resulting in a 26% to 46% yield advantage over sole cropping. Intercrops were more efficient and productive in the drier northern Guinea savanna than in the wetter southern Guinea savanna. Yet the absolute larger grain yields achieved i...