Distribution characteristics of soil organic carbon fractions in paddy profiles with 40 years of fertilization under two groundwater levels

Soil organic carbon (SOC) storage in paddy profiles plays an important role in regulating terrestrial carbon cycling and global climatic change. The changes in groundwater level may influence the accumulation characteristics of SOC fractions in paddy soil profiles. However, the characteristics of organic carbon storage in paddy profiles under different groundwater levels are not well recognized. In this work, effects of groundwater levels (high and low groundwater levels at 20 and 80 cm depths, respectively) on the distribution of SOC, particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) in red paddy profiles (0–20, 20–40, and 40–60 cm depths) with 40 years of chemical fertilization (CF) and organic fertilization (OF) were investigated. The results showed that compared to a low groundwater level, a high groundwater level resulted in an increase in SOC, POC, and MAOC contents in the profiles. SOC and POC increased by 9.00 ~ 9.11% and 22.89 ~ 26.95% at 0–20 cm depth and increased by 24.02 ~ 52.42% and 36.69 ~ 76.94% at 20–40 cm depth, respectively. MAOC increased by 18.18 ~ 38.28% at 20–40 cm depth. No obvious changes were observed in SOC, POC, and MAOC at 40–60 cm depth between the two groundwater levels. At high groundwater level, the contents of aliphatic and aromatic carbon in soil profiles were higher, but the δ13C values were less than those at low groundwater level. MAOC constitutes main fraction of SOC in all soil profiles, and the values of MAOC/SOC increased obviously with depth. Moreover, the MAOC/SOC values in all soil layers were higher under low groundwater level than under high groundwater level. Correlation analysis showed that SOC, POC, and MAOC were positively correlated with Feo and significantly negatively correlated with Fed-Feo (P < 0.01) under two groundwater levels. Overall, distributions of SOC, POC, and MAOC were strongly influenced by groundwater levels, with higher levels leading to increased storage of these fractions in paddy soil profiles, particularly in 20–40 cm soil layer. MAOC is the main component of SOC in the paddy profiles, and higher groundwater levels are available for POC accumulation, especially for OF treatments. The δ13C value of SOC decreased with increasing groundwater level. These results suggested that groundwater level is a potential strategy to improve carbon sequestration in paddy soils, which offers new insights for understanding the influence of groundwater levels in distribution of SOC fractions in paddy profiles.