Soil organic carbon fractions and ¹⁴C ages through 70 years of cropland cultivation

Understanding the biogeochemistry of soil organic carbon (SOC) and SOC components is essential to assess soil quality and management practices in reclaimed croplands. Here we study the radiocarbon (¹⁴C) ages in profiles to a depth of 500 cm for five cultivation timespans: 5, 15, 35, 60, and at least 70 years from the Gutun watershed on the Chinese Loess Plateau. We isolated an acid-extractable SOC component (F1), three temperature stepped-combustion SOC components: F2 (400 °C), F3 (600 °C), and F4 (900 °C), and measured their ¹⁴C ages. Our results show that SOC stocks in reclaimed soils declined from cultivation times of 5 years (R5) to 15 years (R15), then increased with longer cultivation times, until 70 years (R70). The SOC content of the F2 and F3 fractions was higher than F1 and F4 over the past 60 years, while the F4 fraction in R70 had the highest value. Since the newly reclaimed croplands were heavily fertilized, R5 had high SOC and carbon management index (CMI) values, and ¹⁴C ages close to the other SOC fractions in the surface soil. The ¹⁴C ages of SOC fractions in deep soils were older than others, indicating a weak capacity to fix fresh C that infiltrated the deeper soils in R5. R70 contained the highest SOC stocks, but with a large proportion of old, non-labile SOC that contributed to relatively low SOC quality and CMI. Therefore, we predict that the R70 site would be unable to sustain optimal maize productivity. The SOC stocks under R35 were significantly lower than those from R5 and R70, but contained a high proportion of labile SOC, with relatively young ¹⁴C ages (compared to R60), even to a depth of 500 cm. This observation suggests that planting forest nurseries can improve SOC quality and promote SOC sequestration even in relatively deep soils. This study demonstrates the efficacy of labile SOC content and ¹⁴C ages as measures of SOC quality, and our findings offer useful guidance for the management of reclaimed croplands.