Bulk soil stable isotope compositions (δ¹³C, ‰ vs. VPDB; δ¹⁵N, ‰ vs. air N₂) and elemental nitrogen and carbon concentrations (N%, C%, C:N ratio) measured at during different agronomic events from soil cores across the full randomised complete block design in Uasin Gishu County, Kenya. Soil cores were collected at three depths (0–20, 20–40, 40–60 cm) across all treatment plots receiving nitrogen rates of 0, 25, 50, 75, 100 and 125 kg N ha⁻¹. Dried and ground samples were analysed by elemental analyser coupled to isotope ratio mass spectrometry (EA-IRMS), calibrated using alanine (δ¹³C = −27.44‰, δ¹⁵N = +1.18‰), tyrosine (δ¹³C = −23.99‰, δ¹⁵N = +5.30‰), and caffeine (δ¹³C = −34.50‰, δ¹⁵N = −3.18‰) as international reference standards. Data were collected at the University of Eldoret, Uasin Gishu County, Kenya (0°34′45″N, 35°18′22″E; 2,213 m a.s.l.) on Ferralsol soils. Bulk soil δ¹³C and δ¹⁵N integrate the isotopic imprint of long-term soil organic matter dynamics and fertilisation history. This dataset is part of a multi-variable collection archived at PANGAEA.