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Purpose: To explore the effects of land use patterns on the accumulation of soil organic carbon (SOC) fractions in tropical areas, soil samples from paddy fields, abandoned croplands and vegetable-growing lands were collected at depths of 0–10, 10–20 and 20–30 cm in Hainan province, China. Methods: The particulate organic carbon (POC) and mineral-associated organic carbon (MOC) concentrations in soil samples were determined, and their distribution characteristics were analysed. Results: The results showed that MOC is the main component of SOC, with an average MOC/SOC ratio of 0.65. The POC and MOC concentrations showed a trend of paddy field > abandoned cropland > vegetable-growing land and gradually decreased with an increase in soil depth. The POC concentrations in the 0–10-cm soil of the paddy field and abandoned cropland were significantly higher than those of the 20–30-cm layers, and the MOC concentration in the 0–10-cm soil layer of the paddy field was significantly higher than that of the 10–20 and 20–30 cm layers. Correlation analysis showed that POC and MOC had a significant positive correlation with SOC, a significant negative correlation with soil particulate diameter and a weak correlation with pH and Eh. Conclusions: Therefore, soil tillage and fertilisation under different land use patterns have a great influence on organic carbon fractions, and paddy fields have high carbon sequestration potential. [ABSTRACT FROM AUTHOR]

The mangrove wetland in Dongzhai Harbor, Hainan Province was taken as a research area for the collection of samples of sediments and other media. The Pd content was determined by microwave digestion and inductively coupled plasma mass spectrometry (ICP-MS), and the factors influencing the accumulation of the platinum group element (PGE) Pd in mangrove wetland sediments were discussed. The results showed that the average concentrations of Pd in the surface sediments, plants and water of mangrove wetlands were 4.81 ng/g (1.51~8.15 ng/g), 0.94 ng/g (ND ~ 4.34 ng/g), and 0.11 ng/mL (ND ~ 0.27 ng/mL), respectively. Pd was mainly concentrated in the sediments, which absorb most of the Pd in the mangrove wetland environment. From the wetland estuary to offshore, the sediment Pd showed a decreasing trend. The land origin of Pd in mangrove wetland was explained. Extending from the mudflat to the mangrove, the "V" shape becomes the main distribution pattern of sediment Pd, which is higher in the mudflat and forest, and lowest in the forest edge. In the vertical direction, the average concentrations of Pd were the highest at the surface (10–20 cm) and decreased with increasing depth. Pd concentrations in the sediment in the dry season was higher than that in the wet season. Pd was significantly correlated with pH and Eh. This study can help clarify the environmental geochemical process of PGEs in mangrove wetlands and has important scientific significance and practical value for the environmental protection and utilization of mangrove wetlands. [ABSTRACT FROM AUTHOR]

[Display omitted] • Soil organic carbon (SOC) in tropical rainforests of different origins were compared. • SOC concentrations in plantation forests were higher than those in natural forests. • In the 0–50 cm soil depth, the SOC concentrations decreased with increasing soil depth. • Soil pH and particle size indirectly affected SOC concentration. Tropical rainforests are the most complex forest ecosystem in the world, and play an important role in the global carbon cycle. To investigate the accumulation characteristics of soil organic carbon (SOC) in tropical rainforests of different origins, soil samples from plantation and natural forests of tropical mountain rainforests in Wuzhi Mountain (WM), Qixian Mountain (QM) and Diaoluo Mountain (DM) in eastern Hainan Island, China, were collected at various depths (0–10, 10–30 and 30–50 cm). SOC concentrations of different-forest-origin soils were analysed and compared, and the effects of soil physicochemical parameters on SOC concentration in natural and plantation forests were investigated. SOC concentrations in plantation forests in WM, QM and DM were higher than those in natural forests, although the difference was not statistically significant. In the 0–50 cm soil depth, the SOC concentrations of different-forest-origin soils decreased with increasing soil depth, with SOC concentrations in the surface layer (0–10 cm) of natural forests being significantly higher than those in the middle layer (10–30 cm) and bottom layer (30–50 cm) (P < 0.05). Natural forests have lower pH than plantation forests. Correlation analysis revealed significant negative correlation (P < 0.05) between SOC concentration in both forest origins and soil pH. The proportion of clay and silt is greater in natural forests than in plantation forests, but the proportion of sand is greater in plantation forests. [ABSTRACT FROM AUTHOR]