Depth-dependent responses of carbon fractions and sequestration in a maize-wheat cropping system after 51 years of long-term fertilization

Long-term fertilizer application plays a pivotal role in regulating soil carbon (C) dynamics by altering various soil C fractions. However, the response of these C fractions to long-term fertilization across the soil profile remains insufficiently explored under real-time farmland scenarios. This study evaluated the long-term impacts of organic and inorganic fertilizer application on C fractions and C sequestration to a 1 m depth over five decades (1971–2022) and compared equivalent mass and fixed-depth approaches for quantifying total soil organic carbon (TSOC) stocks. Treatments included control, 100 % N, 100 % NPK, 150 % NPK, and 100 % NPK + FYM. Fertilizer applications significantly altered TSOC and its fractions relative to the control. 100 % NPK + FYM resulted in higher TSOC by 92 % over the control and 30 % over 100 % NPK, with the highest TSOC stocks (70 Mg ha −1 ) observed in the 0–100 cm soil profile. Labile C fractions like potassium permanganate-C (KMnO4-C), water-soluble C (WSC), hot water-soluble C (HWSC), and microbial biomass C (MBC) were higher by 14–60 %, 50–176 %, 60–304 % and 31–136 % respectively in 100 % NPK + FYM. Both acid hydrolyzable C (AHC) and acid non-hydrolyzable C (ANHC) were also elevated, with the greatest increases under 100 % NPK + FYM. Labile C fractions accumulated predominantly in the topsoil (0–15 cm), whereas more recalcitrant fractions were enriched in the deeper layers. Equivalent mass-based C sequestration was 21–39 % higher in the topsoil and 5–15 % higher in the entire profile compared to fixed-depth calculations. Overall, this study provides new insights into fertilization induced deep soil C sequestration and its persistence, offering valuable implications for soil C monitoring frameworks, C farming initiatives, and sustainable soil management strategies.