Diversification and intensification are synergistic: field trials evidence from nitrogen-managed maize–cowpea intercropping in Malawi

Omondi, J.O.; Simwaka, P.; Kamwana, F.; Siyeni, D.; Sika, G.; Kadwala, P.; Msukwa, W.; Kyei-Boahen, S.; Akinwale, G.; Alene, A.

Diversification and intensification are synergistic: field trials evidence from nitrogen-managed maize–cowpea intercropping in Malawi

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Context Sustaining maize productivity on nitrogen (N)-depleted soils while enhancing system efficiency and profitability remains a central challenge in sub-Saharan Africa. Maize–legume intercropping can improve soil health and land-use efficiency, yet its performance under intensified mineral N management and rhizobial inoculation remains insufficiently quantified. Objective This study evaluated the productivity, physiological responses, land-use efficiency, farmer preference and profitability of maize–cowpea intercropping under contrasting fertilizer-applied N rates and cowpea inoculation across agro-ecological zones in Malawi. Methods Maize-based trials were conducted at two research stations during 2020/2021 and 2021/2022 seasons, followed by farmer-managed validation across seven districts during 2022/2023 and 2023/2024 seasons. Treatments combined maize N rates (0, 100 and 200 kg N ha⁻¹) with cowpea N fertilization or rhizobia inoculation. Grain yield, chlorophyll content (SPAD), nodulation traits, and leaf nutrient status were measured. Land equivalent ratio (LER), mixed-model analysis, relative-importance analysis and structural equation modelling were used to quantify treatment effects and yield pathways; farmer participatory evaluation and partial budgeting assessed acceptability and profitability. Results Maize yield increased strongly with fertilizer-applied N and improved cowpea management. On-station, the zero-N maize control averaged 4.4 t ha−1 at Chitala and 0.45 t ha−1 at Makoka, whereas the best integrated treatment (200 kg N ha−1 maize + inoculated cowpea) yielded 12.0 and 13.7 t ha−1, respectively. On-farm, the same treatment reached 4.7–7.6 t ha−1 in high-performing districts during 2023/2024, compared with < 2 t ha−1 for zero-N maize in several sites. Cowpea yield and nodulation were more strongly associated with inoculation and leaf N status than with direct mineral N addition, although high maize N reduced nodule dry weight. LER exceeded 1.0 in 80% of intercropping treatments and 1.2 in 70% of cases. Integrated high-N + inoculation treatments generated the highest farmer preference and increased net returns by 2–5 folds relative to low-inputs systems. Conclusions and Implications Diversification and intensification function synergistically when mineral N management is integrated with functional legumes. Moderate-to-high N rates combined with inoculation offer strong performance, but trade-offs related to nitrogen-use efficiency and risk must be considered. System-based N recommendations that incorporate rhizobial inoculation within maize–cowpea intercropping can enhance productivity, nitrogen-use efficiency, land-use efficiency, and profitability. This approach provides a scalable resilience pathway for sustainable intensification in sub-Saharan Africa.