Dwindling soil fertility remains a major restraint to smallholder crop production, thus impacting food security in Kenya. Soil fertility challenges are further exacerbated by the high cost of mineral fertilizer. Intentional planting of nitrogen-fixing fast-growing legume fallows has potential for improving soil fertility in smallholder farming systems, among other benefits such as fuel wood supply. However, data on biomass productivity of improved fallows and their influence on sorghum grain yields are still limited. The current study evaluated foliage, wood, and total above-ground biomass productivity of selected improved fallows in Siaya County. It also evaluated the effect of selected improved fallows on nitrogen input and sorghum grain yield. A Randomized Complete Block Design (RCBD) was used to set up the experiment. The experiment had 12 treatments comprising mixed species and monoculture fallows, each replicated thrice. The highest foliage biomass of 15.7 t ha-1 was recorded in the Desmodium uncinatum treatment, which was 392% above the natural fallow (control). Further, pure Crotalaria grahamiana stand had the highest total above-ground biomass (AGB) and wood biomass (WB) of 62.3 t ha-1 and 47.1 t ha-1, respectively. The highest N input of 62.1 kg ha-1 was recorded in Crotalaria grahamiana. The mixture of Sesbania sesban and Desmodium uncinatum resulted in the highest grain yield of 1.8 t ha-1. Significant

positive linear relationship  were reported between foliage biomass and N input, and between N input and sorghum grain yield.

Highlights

• The productivity of improved fallow species is dependent not only on the type of species used but also on their planting arrangement.
• Single-species fallows produced higher foliage and woody biomass compared to mixed-species combinations.
• Mixed-species fallows are best suited for both nutrient cycling through biomass production and fuelwood generation.
• A combination of Sesbania sesban and Tephrosia vogelii emerged as the most promising option for both soil fertility enhancement and fuelwood supply.