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

With the aggravation of climate change, extreme rainfall events occur more frequently. Further research is needed on the effects of extreme rainstorms on soil organic carbon (SOC) and its different components. In this study, we took the "7.20" extreme rainfall event in Henan Province as an opportunity. Three kinds of farmland with different flooding durations included none 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 in 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. The SOC in the flooded farmland was higher than that in the non−flooded farmland as a whole, and the increasing proportion of soil layer below 40 cm was higher than that in 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 Y1and 4.68−15.15 g∙kg⁻¹ under Y2. As the waterlogging duration increased, the SOC in 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 Y1and 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 Y2 was significant higher than that of CK in 40-60 cm and 80-100 cm soil layers. 2) There were differences in the distribution patterns of SOC and DOC in the vertical profiles of farmland under different waterlogging durations after the extreme rainfall event. With the increase of soil depth, the 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 trend of decreasing with the increase of soil depth, while the changing trend of DOC in Y2 was not obvious within the 0−100 cm soil layer. 3) Soil moisture can significantly affect SOC and DOC/SOC, but there was no significant regression relationship with DOC. The contents of soil nutrients such as total carbon, total phosphorus, and total nitrogen have significant effects on SOC, DOC, and DOC/SOC. There was no significant correlation between the changes in pH and electrical conductivity and the distribution of SOC and DOC.