Soil carbon fractions and carbon management index after 26 years of continuous organic and mineral fertilization

This study investigated the impacts of long-term application of mineral and organic fertilizers on soil organic carbon (SOC) fractions and carbon management index (CMI) under a faba bean cropping system. The experiment was established in 1996 at the Çukurova University Research Centre in Adana/Türkiye and is still ongoing. Since establishment, five fertilizer treatments have been applied each cropping season, including control (no fertilizer), mineral fertilizer (100N–26P–83K kg ha −1 ), animal manure (25 t ha −1 ), compost 25 (25 t ha −1 ), and compost 10 (10 t ha −1 ) with mycorrhizal fungi. In the present study, faba beans were grown and harvested in 2022 cropping season. At harvest, soil samples were collected to a depth of 0–20 cm and analyzed for SOC and its fractions, such as permanganate oxidizable “labile” (POXC) and particulate organic carbon (POC). The soil CMI and carbon sensitivity index were also estimated. Animal manure and compost 25 increased bulk SOC and its fractions relative to other treatments. Manure increased bulk SOC, POC, and POXC concentrations by 74%, 226%, and 85%, respectively, relative to the control, while compost 25 increased these fractions by 57%, 54%, and 59%. The CMI was also increased by 87% and 57% over the control under manure and compost 25 , respectively. Cumulative soil CO 2 flux did not differ among treatments. In conclusion, long-term manure and compost 25 use increased CMI and the labile and particulate SOC pools. The POC fraction showed the highest sensitivity compared to POXC, particularly under manure, indicating its potential as an early indicator of SOC change.