Symbiotic N Fixation, C Assimilation, and Water-Use Efficiency in Four Groundnut Varieties (Arachis hypogaea L.) Assessed Using ¹⁵N and ¹³C Natural Abundance Techniques

Groundnut is an essential legume crop vital for global food security, especially in resource-limited areas. However, poor soil fertility and climate change often hinder crop growth in Africa, resulting in low yields, particularly on smallholder farms. This study systematically evaluates, for the first time in the North West Province of South Africa, the inter-annual variations in symbiotic nitrogen fixation and water use efficiency among four major groundnut varieties during the 2023 and 2024 cropping seasons, using ¹⁵N and ¹³C natural abundance techniques. Significant differences were observed in plant growth, nitrogen content, δ¹⁵N, %Ndfa, N-fixed, soil N uptake, and grain yield across both years. Groundnut varieties derived 35.51% to 85.77% of their nitrogen from symbiotic fixation, contributing 22.13 to 168.48 kg of nitrogen fixed per hectare. The grain yield was much higher (4305.45 to 7465.91 kg ha⁻¹) in 2024 cropping season compared to (514.73 to 1207.95 kg ha⁻¹) in the 2023 cropping season. The shoot ¹³C values for the groundnut varieties ranged from −28.57‰ to −28.19‰ in 2023 and −30.17‰ to −28.34‰ in 2024. Notably, varieties Kwarts (85.77%) and Shubert (82.95% demonstrated higher symbiotic dependency in 2023 cropping season, whiles varieties ARC K6 (106.86 kg ha⁻¹) and PC 474 (164.48 kg ha⁻¹) fixed the most N in the 2024 cropping season, along with better water-use efficiency. These varieties namely Kwarts, Shubert, and PC 474 show promise for inclusion in future breeding programs.