Effects of flooding on soil organic carbon and dissolved organic carbon under extreme rainstorm

Extreme rainfall events are occurring more frequently with the aggravation of climate change. Further research is required on the effects of extreme rainstorms on soil organic carbon (SOC) and its components. In this study, we took the “7.20” extreme rainfall event in Henan Province as an opportunity. Three types of farmland with different flooding durations, including non-flooded (CK), flooded for less than 1 month (Y1), and flooded for more than 2 months (Y2) after the rainstorm event, were selected to explore the impacts of extreme rainfall events on SOC, dissolved organic carbon (DOC), and their main influencing factors at 0–100 cm soil depth. The results showed that: 1) After the extreme rainfall event, the changes in SOC and DOC with the duration of flooding presented different rules. Overall, the SOC in the flooded farmland was higher than that in the non-flooded farmland, and the increasing proportion of the soil layer below 40 cm was higher than that in the soil layer above 40 cm. The content of SOC in 0−100 cm soil layer was 3.41−14.25 g∙kg⁻¹ under CK, 5.45−18.11 g∙kg⁻¹ under Y1, and 4.68−15.15 g∙kg⁻¹ under Y2. As the waterlogging duration increased, SOC in the farmland first increased and then decreased. The content of DOC in 0−100 cm soil layer was 414.19−580.39 mg∙kg⁻¹ under CK, 327.99−874.19 mg∙kg⁻¹ under Y1, and 242.34−301.93 mg∙kg⁻¹ under Y2. As the waterlogging duration increased, the DOC showed a trend of first increasing and then decreasing in the 0–40 cm layer, while showing a gradually decreasing trend below the 40 cm layer. The DOC content of CK was significantly higher than that of Y2 in 40–60 cm and 80–100 cm soil layers. 2) Differences were noted in the distribution patterns of SOC and DOC in the vertical profiles of the farmland under different waterlogging durations following extreme rainfall events. With an increase in soil depth, SOC showed a decreasing trend; the DOC in CK showed a trend of first increasing and then decreasing, with the highest content of DOC in the 10–20 cm layer. The DOC in Y1 showed an overall decreasing trend with increasing soil depth, while the changing trend of DOC in Y2 was not obvious within the 0−100 cm soil layer. 3) Soil moisture significantly affected SOC and DOC/SOC; however, no significant regression relationship with DOC was observed. The content of soil nutrients, such as total carbon, total phosphorus, and total nitrogen have significant effects on SOC, DOC, and DOC/SOC. No significant correlation was noted between the changes in pH, electrical conductivity, and the distribution of SOC and DOC.