Mollisols (black soils) are the most fertile and productive soils worldwide, yet intensive cultivation has caused severe degradation, including decline in soil organic matter (SOM) content. However, the fundamental processes in the transformation and turnover dynamics of carbon fractions under cultivation remain unclear. Here we used natural stable carbon isotope (δ 13 C) and C/N ratios of bulk SOM and mineral-associated organic matter (MAOM), and accelerator mass spectrometry radiocarbon (AMS 14 C) dating analysis of five profiles and 30 pairs of cropland/natural topsoil samples from Phaeozems along a latitude transect to elucidate cultivation-induced SOM dynamics. Cropland topsoils showed higher δ 13 C values (mean −22.9‰, ranging from −18.8‰ to −25.5‰) than natural topsoils (mean −25.2‰, from −21.9‰ to −27.7‰), suggesting influence of cultivated corn (C4 plant). Below the plough horizon (0–20 cm), grassland-dominated profiles exhibited upward increases in δ 13 C SOC due to late-Holocene C4 grass expansion, while forest-region profiles showed downward increases in δ 13 C SOC , mostly due to increasing contributions of microbial-derived organic matter as evidenced by the downward decreasing C/N ratios of both SOM and MAOM. An average enrichment of 1.5‰ ± 1.3‰ in 13 C of SOC from 20 cm to 0 cm within the plough horizon indicated that corn residues contributed 28% ± 13% to SOC. δ 13 C MAOC was lower than δ 13 C SOC by 0.63‰ at 0–5 cm and by 0.23‰ at 15–20 cm, suggesting progressive transfer of corn-derived C from particulate organic matter (POM) to MAOM from surface downward. Variations in δ 13 C profiles within the plough horizon suggest the various impacts of corn cultivation on C transfer and transformation from POM to MAOM, both temporally along the soil profiles and geographically across Mollisols in Northeast China.
Geng et al. (Tue,) studied this question.