Characteristics of soil organic carbon and labile components at early stage of reforestation of aquaculture pond

Reforest mangrove in aquaculture pond is considered to be an effective way to restore mangrove wetlands and their carbon sink function. In order to reveal the characteristics of soil organic carbon pools and their influencing factors at the early stage of reforest mangrove in aquaculture pond, and to elucidate the dynamic changes of soil carbon pools in the process of mangrove wetland restoration, we collected soil samples from a mangrove reforest aquaculture pond located in the northwest coast of Kaozhou Bay, Huidong County, Guangdong Province, southern China. The mangroves Rhizophora stylosa and Avicennia marina were planted in 2021. Cylindrical soil samples with 0–100 cm in length were collected in three areas: the mangrove plantation island (PI), bare island (BI), and the soil under the water channel (WC) in the pond. The soil sub-samples were compared for the soil organic carbon (SOC), as well as active organic carbon [Easily oxidizable organic carbon (EOC), microbial organic carbon (MBC) and soluble organic carbon (DOC)] mass fractions among different sampling areas and depths, as well as the relationship with soil physicochemical parameters. The results show that the average SOC mass fraction for the 0–100 cm soil depth followed a descending order: BI [(7.92±0.43) g·kg⁻¹]>PI [(7.72±0.35) g·kg⁻¹]>WC [(7.48±0.69) g·kg⁻¹]. Similarly, the average SOC density followed a descending order: BI [(84.56±3.65) Mg·hm⁻²]>PI [(72.01±3.20) Mg·hm⁻²]>WC [(70.12±1.44) Mg·hm⁻²]. The highest SOC mass fraction and density were observed at the 40–60 cm depth in PI and BI areas, and at the 0–20 cm depth in WC area. The EOC, MBC and DOC mass fractions were highest in PI area, followed by WC and BI areas. The highest EOC and MBC in PI area were found at depths of 40–60 and 80–100 cm, respectively, which were deeper than those in WC and BI areas. Soil bulk density, total nitrogen and total phosphorus were identified as key factors influencing the distribution patterns of labile organic carbon. The EOC and DOC contents showed a significantly negative correlation with soil bulk density and total phosphorus, while the MBC content showed a significantly negative correlation with total nitrogen (p<0.05). The relatively high proportion of labile organic carbon components within SOC indicates that the soil carbon pool is still in an active state. The ratio of labile carbon of SOC is relatively high at the middle and deeper soil depth, indicating that the mangrove has promoted presence of labile organic carbon at middle and deeper soil depths, and mangrove facilitate the migration of organic carbon in the deeper soil layers. As the mangrove plants continue to grow, their effects on the carbon sequestration become more significant.