Does long term fertilization impact soil carbon’s functional group composition and its distribution in aggregates?

The carbon (C) sink capacity of soil hinges its ability to retain C over time. Hence, understanding the factors influencing C stability is crucial for assessing changes in its dynamics due to long-term agricultural practices. The present study was conducted from 51-year long-term fertilizer experiment (LTFE) in maize-wheat system started since 1971 by selecting five treatments consisting of control, 100% N, 100%NPK, 150% NPK, 100% NPK+ farmyard manure (FYM) (10 Mg ha-1) along with undisturbed fallow area. The study determined the soil organic carbon (SOC) and its fractions, including particulate organic matter (POM), light, and heavy C (LFC and HFC), in 0-100 cm soil, and aggregate size and associated C in 0-15 cm and 15-30 cm soil. Fourier Transform Spectroscopy- Attenuated Total Reflectance (FTIR-ATR) was used to analyze functional groups within the SOC. Long-term use of 100% NPK+FYM significantly increased SOC by 150%, even in deeper layers. The concentration of cPOM, fPOM, LFC and HFC, in 0-15 cm layer were 55-147, 65-103, 30-70 and 11-80% higher over control. Micro-aggregates decreased while macro-aggregates increased under 100%NPK+FYM, resulting in improved soil aggregation compared to chemical fertilization-based treatments and control. FTIR revealed that integrated application of chemical fertilizers and organic manures positively impacts the presence of amides, amines, hydroxyl, aliphatic, and carboxylic groups, as well as complex organic molecules like polysaccharides, lignin, and phenols. This study demonstrated the beneficial effects of long-term, balanced, and integrated fertilizer management in maize-wheat system on aggregates associated C, stable C fractions, and functional groups for improved soil and environmental sustainability.