Seasonal and Interannual Dynamics of Soil Organic Carbon and Associated Soil Properties Under Conventional Tillage and No-Till in Continuous Spring Wheat Cultivation in Northern Kazakhstan

Increasing aridity, strong interannual variability in water availability, and limited crop diversification threaten soil organic matter in the chernozems of Northern Kazakhstan, where continuous spring wheat cultivation remains widespread. We assessed seasonal and interannual soil organic carbon (SOC) dynamics in haplic (ordinary) chernozem under conventional tillage (CT) and no-till (NT) during the completed 2023–2025 field seasons at the Karabalyk Agricultural Experimental Station. CT and NT were evaluated without fertilizer and with N30P20 fertilization; bare fallow served as an uncropped reference. In the 0–20 cm layer, SOC concentration, total nitrogen, humus, microbial biomass carbon (Cmic), soil temperature, moisture, and CO2 flux were evaluated; humic acid carbon (HA-C) and fulvic acid carbon (FA-C) were directly measured only in 2024–2025. Mean seasonal SOC concentration was significantly affected by Year (F = 4.622, p = 0.020, partial η2 = 0.278) and Fertilization (F = 4.301, p = 0.049, partial η2 = 0.152), whereas the overall Tillage effect was not statistically significant (F = 0.417, p = 0.525, partial η2 = 0.017), and the Year × Tillage interaction was also nonsignificant (p = 0.909). HA-C and FA-C varied markedly between seasons and years without a consistent NT advantage, while the SOC–Cmic relationship showed a strong positive trend that did not reach the predefined significance threshold (r = 0.809, R2 = 0.654, p = 0.051). These results emphasize dominant interannual variability and the need to distinguish descriptive tillage differences from statistically supported management effects. These findings are relevant to sustainable soil management and the development of climate-resilient dryland wheat production under increasing climatic variability in Northern Kazakhstan. The present study evaluates SOC concentration rather than SOC stock or sequestration.