Reforestation based on the ditch-and-embankment technique increased soil carbon stock and alleviated soil salinity in a coastal area subject to land subsidence- a preliminary study

The coastal areas are renowned for their exposure to strong winds, salt sprays, and blowing aeolian sands, collectively posing threats to farming and the health of residents. Well-established coastal forests offer protection to alleviate these adverse effects and provide various ecosystem functions, such as carbon sequestration and soil conservation. Soils account for approximately 74% of the carbon stock in terrestrial ecosystems. Consequently, even a subtle increase in soil carbon can lead to significant carbon sequestration. Reforestation is considered a suitable practice to enhance both above- and belowground carbon sequestration. However, the benefits of reforestation in coastal areas are hindered by land subsidence and seawater intrusion due to over-exploitation of groundwater for fish farming or manufacturing. To overcome these obstacles, a reforestation practice known as the "ditch-and-embankment technique (D-E technique)" is adopted. This technique involves reforesting coastal lands suffering from land subsidence by constructing inter-parallel ditches and hills. By applying the D-E technique, soil properties can be improved through salt leaching, and soil organic carbon (SOC) stock can be enriched by organic matter inputs from reforested trees. However, the effectiveness of this technique in terms of soil carbon and soil amelioration lacks sufficient evidence. In this study, we investigated the soil carbon stock and soil salinity of a 15-year-old coastal plantation, consisting of four dominant species (Casuarina equisetifolia, Millettia pinnata, Melaleuca leucadendra, Cerbera manghas) established by the D-E technique on the western coast of Taiwan. Soil samples from hills (O horizon and mineral soil) and ditches were collected using soil cores and a piston sampler. A proximate submerged forest was used as a reference baseline. Soil carbon was determined as organic, inorganic, and elemental carbon with a TOC analyzer. Soil salinity was measured in terms of soil pH and electrical conductance (EC_1:5). Our results showed that the D-E technique could increase the total SOC stock (O horizon + 0-50 cm mineral SOC) to an average of 48.38 Mg C ha^-1, compared to the submerged forest (12.22 Mg C ha^-1). The total SOC stocks of hills ranged from 38.02-60.33 Mg C ha^-1, significantly higher than the submerged forest, irrespective of species, although there were no significant differences in total SOC stocks between species. Consistent with total SOC stocks, mineral SOC stocks of hills (13.91 -24.49 Mg C ha^-1) were generally higher than the submerged forest, with only those from Cerbera manghas and Millettia pinnata being significantly or marginally higher. The similar amount of total inorganic carbon stock between hills and the submerged forest further supported the contribution of reforestation. Soil pH at the 0-5 cm layer of hills was lower than in the deeper soil layer and soils from the ditch. Additionally, EC generally were lower at soil at 0-5 cm or 5-10 cm layers, suggesting the occurrence of salt leaching. In conclusion, our preliminary study suggests that the D-E technique could be an appropriate reforestation approach to establish coastal plantations in areas subject to land subsidence, meeting multiple objectives, including protecting residents' well-being, soil carbon sequestration, and soil salinity amelioration.