To explore the differences in the responses of soil carbon (C) pools and key C cycle processes following the conversion of cropland to different plantation types, this study adopted the space-for-time substitution method, with three land-use types as research objects including: Masson pine plantation (MAS), mulberry orchard (MUL), and cropland for maize monoculture (MAI). After more than 25 years of cropland conversion, the contents of soil total organic carbon (TOC), particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) were determined. Additionally, the contents of soil lignin phenols and amino sugars were analyzed. Meanwhile, the activities of cellulase, peroxidase, and β-1,4-glucosidase were measured. The results showed that the TOC content in MAS was 21.1 ± 0.5 g kg−1, significantly higher than that in MUL (11.9 ± 0.4 g kg−1) and MAI (10.2 ± 0.1 g kg−1); the POC and MAOC content in MAS were much higher than that in MUL and MAI. The total lignin phenol content in MAS (803 ± 35 mg kg−1) was nearly 7 times higher than that in MUL (116 ± 12 mg kg−1) and MAI (118 ± 6 mg kg−1). The total amino sugar content in MAS (646 ± 25 mg kg−1) was twice as high as that in MUL (361 ± 44 mg kg−1) and MAI (397 ± 40 mg kg−1); the total microbial-derived C in MAS (5.7 g kg−1) was significantly higher than that in MUL (3.2 g kg−1) and MAI (3.8 g kg−1). Structural equation modeling results confirmed that carbon-degrading extracellular enzymes promote the formation of POC and MAOC by regulating the lignin phenol content, with POC acting as the key fraction sustaining soil total organic carbon stocks. This study reveals that converting cropland to MAS can better promote the sequestration and accumulation of soil organic C compared with converting to MUL, and increasing plant-derived carbon inputs is critical for boosting regional soil carbon storage.