Agroecological Potential of Vigna unguiculata for Improving Sugarcane Soils in Smallholder Farming Systems: Evidence from a Greenhouse Pot Experiment

Soil acidity and nutrient deficiencies limit sustainable sugarcane production in KwaZulu-Natal (KZN), South Africa, where small-scale growers (SSGs) often lack access to lime and fertilizers, leading to progressive soil degradation. This study assessed the potential of Vigna unguiculata L. Walp., a versatile African legume, to support restoration of soil health during fallow periods in acidic, nutrient-poor sugarcane soils. A pot experiment was conducted using soils from four sugarcane sites characterized by low pH and nitrogen (N) and phosphorus (P) deficiencies. Vigna unguiculata cultivation was associated with increased soil pH, reduced exchangeable acidity, and improved N and P availability at several sites. Elevated acid phosphatase and nitrate reductase activities indicated enhanced nutrient cycling. 16 S rRNA sequencing revealed diverse symbiotic bacteria ( Bacillu s, Lysinibacillus , Paenibacillus , Pseudomonas ) and post-cultivation enrichment of free-living N fixers ( Burkholderia , Herbaspirillum ), N-cyclers ( Novosphingobium , Paenibacillus ), and P-solubilizers ( Agrobacterium , Bacillus , Sphingomonas ), taxa previously associated with supporting soil fertility functions. Vigna unguiculata demonstrated high N use efficiency and strong reliance on symbiotically fixed N (35–61%). Low C: N ratios suggested significant potential for biomass contributions to soil organic matter. Overall, these findings provide evidence that V. unguiculata cultivation can influence soil biochemical properties, nutrient-cycling enzyme activities, and culturable bacterial communities in acidic sugarcane soils under greenhouse conditions. The observed changes suggest potential mechanisms through which V. unguiculata may contribute to soil fertility improvement during fallow periods. However, further field-based studies are required to determine the persistence, agronomic relevance, and economic feasibility of these effects in smallholder sugarcane production systems.