Reducing inorganic nitrogen inputs for small-scale sugarcane growers in KwaZulu-Natal using legume species (Vigna unguiculata and Cajanus cajan).

Abstract

Declining soil fertility and health pose a significant threat to food security and agricultural sustainability. In South Africa, sugarcane production plays a crucial role in the economy, contributing significantly to gross domestic product (GDP) through exports and providing income for many smallholder growers. However, between 2007 and 2017, sugarcane production in the country declined by 52%. Despite being among the largest sugarcane163 producing provinces, smallholder growers in KwaZulu-Natal have experienced substantial reductions in yield, threatening both their livelihoods and the broader economic stability of the sector. One of the primary causes of declining sugarcane production is the widespread practice of continuous monoculture, which depletes soil nutrients, reduces microbial diversity, and increases soil acidity. Monoculture selects for crop-specific microbes, leading to a decline in beneficial bacterial populations and their associated enzyme activities, which are essential for nutrient cycling. This degradation in soil health underscores the urgent need for sustainable agricultural practices, such as the integration of leguminous cover crops, to restore soil fertility and improve sugarcane productivity. The use of legumes as cover crops has been widely recognised for enhancing soil organic carbon, improving nutrient cycling, and increasing microbial activity. Among leguminous species, Cajanus cajan and Vigna unguiculata are particularly valuable due to their ability to fix atmospheric nitrogen, improve soil nutrient dynamics, and enhance microbial diversity. This study aimed to evaluate the impact of C. cajan and V. unguiculata as sustainable cover crops on soil nutrient dynamics in sugarcane plantations managed by smallholder growers in KwaZulu-Natal. The study was conducted under greenhouse conditions, with temperature ranging from 13–16 °C at night to 28–35 °C during day, with humidity levels between 70% 180 and 80%. Soil samples were collected from sugarcane plantations in Gingindlovu, Mvutshini, Mpembeni, Hibberdene, and Mzinto to serve as the growth substrates. 181 Pre-planting and post182 harvest soil sample were analysed for nutrient concentrations, nutrient cycling bacteria, along with associated extracellular enzyme activities. The results demonstrated that both legume species contributed to improvements in soil health. Post-harvest, soil pH slightly increased across all plantation sites, counteracting the acidity induced by continuous sugarcane monoculture. The abundance of plant growth-promoting bacteria, including Arthrobacter, Agrobacterium, Bacillus, Lysinibacillus, Caulobacter, and Pseudomonas, increased significantly. Plants grown in Hibberdene and Mzinto showed a high reliance on nitrogen derived from the atmosphere. Furthermore, C. cajan and V. unguiculata accumulated high shoot biomass despite low soil nutrition in the sampled soils. The growth of C. cajan and V. unguiculata in the sampled soils indicates that microbial activity and soil enzyme functions facilitated nutrient mineralisation and solubilisation, making essential nutrients more available for plant uptake and leading to increased total plant biomass. Overall, this study highlights the potential of leguminous cover crops in mitigating soil degradation caused by sugarcane monoculture. The incorporation of C. cajan and V. unguiculata into sugarcane production systems can enhance soil microbial diversity, improve nutrient cycling, increase soil pH, and ultimately contribute to the long-term sustainability of 198 smallholder sugarcane farming in KwaZulu-Natal.