Organic amendments affect soil organic carbon via changes to microbial necromass and plant litter

The addition of organic amendments to soil plays a crucial role in shaping the formation, turnover, and storage of soil organic carbon (SOC) within agricultural ecosystems. However, it is still unclear how different organic amendments influence the contribution of microbial- and plant-derived C to SOC. To fill this gap, we determined soil microbial necromass and plant litter content (0–20 cm) in a Calcarie-Fluvie Cambisol under five fertilizer treatments: (1) unfertilized control (CK), (2) chemical fertilizer alone (F), (3) cattle manure and chemical fertilizer (FM), (4) pig manure and chemical fertilizer (FC), and (5) maize straw and chemical fertilizer (FS). In treatments FM and FC, the amount of chemical fertilizer applied was half of that applied in F. We used amino sugars and lignin phenols as indicators of microbial necromass and plant litter, respectively. The findings indicated that, in comparison with the unfertilized control, the use of chemical fertilizer alone resulted in significant lower concentrations of SOC (by 7.9 %) (P < 0.05), amino sugars (by 8.3 %) (P < 0.05), and plant lignin phenols (by 12.9 %) (P < 0.05). Compared with chemical fertilizer alone, the organic amendment treatments resulted in significant higher concentrations of SOC (by 42.4 %–55.6 %) (P < 0.05), total amino sugars (by 26.2 %–29.7 %) (P < 0.05), and plant lignin phenols (by 64.3 %–84.1 %) (P < 0.05). The lignin phenol content exhibited significant differences among the three organic amendment treatments (P < 0.05), following the order: FS > FM > FC. Moreover, the application of organic amendments resulted in a significant lower relative content of amino sugar-derived carbon in SOC (by 9.6 %–19.1 %) (P < 0.05) and a significant higher relative content of lignin phenol-derived C in SOC (by 5.5 %–28.5 %) (P < 0.05). Additionally, the ratio of amino sugars to lignin phenols in organic amendments treatments (1.67–1.81) was significant lower than that observed in the chemical fertilizer treatment (2.36) (P < 0.05). At the same time, the ratio of amino sugars to lignin phenols exhibited significant differences among the three organic amendment treatments (P < 0.05), following the order: FC > FM > FS. These results indicate that, when organic amendments are applied, lignin is more likely to be retained in the soil and plant-derived materials make a greater contribution to SOC than microbial necromass. This could facilitate long-term SOC accumulation. • Chemical fertilizer results in lower amino sugars and lignin phenols than CK • Organic materials result in higher amino sugars and lignin phenols than fertilizer • Straw input leads to significant higher lignin phenols than cattle and pig manure • Plant-derived C makes a greater contribution to SOC than microbial-derived C