Native non-N₂-fixing and exotic N₂-fixing tree species provide higher-quality green manure than exotic fast-growing species in sub-Saharan Africa

Poor soil fertility is a major constraint on agricultural productivity in sub-Saharan Africa. The use of green manure from agroforestry trees offers strong potential to improve soil fertility, yet litter decomposition and nutrient release patterns remain poorly understood across different tree species and functional groups. Here, we investigated litter decomposition and nutrient release patterns of six common agroforestry tree species across two contrasting districts in Rwanda (Musanze and Kayonza) under field conditions during the dry and wet seasons using a litterbag experiment over 120 days. At each site, three representative species were selected, each representing one functional group: exotic N2-fixing species (Alnus acuminata and Calliandra calothyrsus), exotic non-N2-fixing species (Eucalyptus globulus and Grevillea robusta), and native non-N2-fixing species (Markhamia lutea and Croton megalocarpus). Mixed-effects models were used to quantify litter mass loss and the release of nutrients and carbon compounds (C, N, P, K, lignin, and tannins). Decomposition rates and the release of nutrient and carbon compounds differed significantly among species and were strongly associated with litter quality. Exotic non-N2-fixing species decomposed more slowly and released fewer nutrients, which was associated with higher litter C:N ratios and lignin content. In contrast, exotic N2-fixing and native non-N2-fixing species, characterized by more favourable litter quality, exhibited faster decomposition and higher nutrient release. Seasonal effects were site dependent. In Kayonza (semi-arid), decomposition was slower during the wet than the dry season, whereas in Musanze (humid highlands), no significant seasonal effects were observed. Our findings highlight that native tree species, often underutilized in agroforestry, can provide high-quality green manure comparable to commonly promoted exotic N2-fixing species. These results provide practical guidance for tree species selection in agroforestry and tree-based restoration initiatives on farmlands and other productive landscapes in Sub-Saharan Africa, with implications for improving soil fertility, agricultural productivity and biodiversity conservation.