The abandoned paddy wetlands are recognized not only for their various functions and high ecological values of fallow lands where cultivation has stopped but also for the carbon function of the soil. The reason for the recognition is that much attention pays to the conservation of biodiversity, the protection, proliferation, and restoration of endangered species and habitats, and carbon absorption concerning climate change. However, farmlands in Korea are rapidly increasing due to the changes in land use, the decrease in farmland area and economic value, and the aging rural population, raising interest in abandoned paddy wetlands. Hence, the study to preserve, restore, and manage abandoned paddy wetlands is needed.
This study aims to provide a scientific basis for the conservation and restoration of abandoned paddy wetlands, which emphasizes their ecological importance and functional characteristics like carbon absorption, to improve their multi-functionality. This study also aims to propose guidelines for analyzing the characteristics of each ecologically environmental factor of the carbon accumulation in abandoned paddy wetlands.
To this end, three methods were used. First, to classify the types of wetlands, after going through steps such as indoor and outdoor surveys and expert opinion convergence (brainstorming) in consideration of wetland discrimination indicators, type classification standards and purposes, a type classification system suitable for the purpose is finally derived.
A type classification system considering biodiversity and hydrology and a classification system considering hydrology and vegetation by land use were derived and applied to 40 preliminary sites.
Second, the hydrological and topographical characteristics of each land use in the ecological influence zone were analyzed.
1) For land cover and land use analysis, land cover maps provided by the Environmental Spatial Information Service (major category, resolution 30 m; middle category, resolution 5 m; sub category, resolution 1 m), and orthographic images provided by the integrated map service of the National Geospatial Information Platform of the National Geographic Information Institute and 1:5,000 digital topographic map and aerial photographs were used. Based on the moving range of small animals and water circulation using GIS, the ecological influence area around the wetland was set to 300m, and the land cover type, area, and ratio were analyzed.
Analysis of time-series changes in land cover analyzed changes over 40 years at 20-year intervals, such as 1980, 2000, and 2021 for the major category, and 20 Yearly changes were analyzed. Because sub-categories were provided only in 2021, the land cover status in 2021 was compared rather than looking at time-series changes. Wetland boundaries are based on serial cadastral maps, but on-site verification of wetland identification factors such as hydrology, vegetation, soil, etc., and area calculation using GIS. It was confirmed and verified through a field survey.
In the three categories (2021) of the land cover ratio of the ecological impact zone by target area, the proportion of forest and farmland was generally high. In the annual change (1980-2021), forest and farmland decreased very rapidly, and artificial grasslands, bare fields, and others. It showed a tendency to change to other types of covering. It is expected that the number of fallow lands will increase and the number of paddy wetland areas will continue to increase. In the case of a short fallow period, there are many cases where artificial grasslands or bare fields remain, but it is judged that the paddy wetlands will be abandoned as time passes.
By land cover types, ratio of urbanized and dried areas, such as single and multi-family dwellings, was severely low. However, the proportion of roads in some areas was relatively high at around 10% compared to other areas. Recently, from urbanization, the ratio of urbanized and dried areas has been increasing. Forest areas tended to be replaced by broad-leaved and mixed forests from coniferous forests. Forest areas tended to be replaced by broad-leaved and mixed forests from coniferous forests.
Based on the 1/5,000 scale digital topographic map from the National Territory Information Platform, the overall hydrological and topographical characteristics of the surrounding 500m area affected by the wetland development process were analyzed. Using programs such as GIS, Auto CAD, and dreamplus that can extract longitudinal and cross-sectional views, the altitude and slope of the surrounding forest or agricultural area were confirmed. Indoor analysis and field investigation was conducted by checking the contour lines (intermediate contour line 7111, half interval contour 7112, supplementary contour 7113, index contour 7114) and water system (explosive river 2111, rill 2112, dry stream 2113, lake·reservoir 2114) layers on the digital topographic map.
Topographic locations were classified into U-shaped valley-type valley head and valley bottom, or linear terrain valley-level plain and reclamation plain. The average altitude was 40 m and the slope within the wetland was 2.3% on average. This topography is an area that was cultivated in the form of paddy fields in the past. It seems that it is at an altitude where farming activities are effortless and that the land was cleared in a terraced horizontal structure. As for the hydrological characteristics, it is closely related to the topographic location. Water continuously or periodically stays in the wetland area due to surface water such as rainfall and underground water in the form of water gathering and eluting.
3) For carbon absorption by land use, based on the LULUCF greenhouse gas emission/absorption calculation method, based on the movement range of small animals and water circulation using GIS, the ecological influence zone around the wetland was set at 300m, and The potential carbon uptake was calculated.
Third, the characteristics of the wetland ecological environment were analyzed.
1) Plant sociological survey was conducted according to the Braun-Blanquet method of the Z-M school for the community constituting the correlation by selecting a point with representative tree species. Vegetation descriptions were marked using seven scales such as 1 to 5, +, and r.
As for the vegetation status, wetland plants such as Persicaria thunbergii, Typha Orientalis, Phragmites communis, and Salix koreensis were distributed in areas with smooth water supply, and some Artemisia princeps, Pueraria montana var. lobata, and Humulus japonicus indicated a tendency to incarnate. This tendency appeared in zones where water sources were not maintained or in edge areas adjacent to forests. It seemed to be a phenomenon that was intensified due to soil loss.
2) As for amphibian reptiles and benthic invertebrates, survey points such as habitats and hiding places in the wetland were selected, and species were identified visually or by collection, trace investigation, and cries. The species list was prepared following the National Biodiversity Inventory (National Institute of Biological Resources, 2018), and the cluster analysis method was based on dominance (DI), diversity (H'), abundance (R1), and evenness (J') by survey site. Four indices were calculated.
Amphibians were not high in the number of species at each survey site, and reptiles were identified in some abandoned paddy wetlands, but the sighting frequency was low.
As a result of investigating benthic macroinvertebrates in water puddles, ponds, and agricultural waterways, the appearance rate of dragonflies, beetles, and stink bugs, which are mainly found in the habitats of water purification areas, was high. It seems to be a result reflecting the characteristics of the aquatic environment. Although the species that appeared by survey site showed some differences, the number of species that appeared in the watershed area was generally high, and the overall species composition was similar.
Most abandoned paddy wetlands are located in the low-lying granary and arable lands that form an agricultural ecosystem, and wetland grasslands have been moved in and formed due to a long period of fallow, and their naturality is being restored from the reduction of the influence of artificial disturbance. The area around the abandoned paddy wetlands forms a biotope in connection with lowland forests, agricultural waterways, and farmland, and serves as a resting place for wild animals and a feeding area. In particular, wetlands such as water puddles and ponds formed in wetlands of abandoned paddy fields were found to serve as significant breeding grounds for insects such as amphibians, dragonflies, butterflies, and flies that spawn in the aquatic environment.
3) For soil analysis, a total of eight items were measured: pH, EC, organic matter content, total nitrogen, ammonia nitrogen, nitrate nitrogen, total phosphorus, and soil by the soil and plant analysis method standards presented by the National Institute of Agricultural Sciences (2000). The potential soil organic carbon storage capacity was calculated using the analyzed soil organic matter content.
The chemical properties of the soil revealed a tendency for weak acidity, and the organic matter was analyzed as 32.71g/kg. The relationship between the organic matter content and the total nitrogen content was also closely related to the inundation of wetlands, and in most cases, the organic matter content showed a difference of about 20 to 25 times the total nitrogen content.
Overall, the soil was loam (L), and soil development proceeded in a stable state in the submerged land without external disturbance. However, in the case of abandoned paddy wetlands, it can be seen as a phenomenon that occurs when the water source is temporarily satisfied rather than continuously maintained. As a result of correlation analysis between soil items, organic matter content and total nitrogen content showed a very high positive (+) correlation, and organic matter content and clay content also showed a positive (+) correlation. In the form of inorganic nitrogen, ammonia nitrogen was found to be higher than nitrate nitrogen, which can be assumed that the wetland's unique reduced soil was formed. The correlation between total nitrogen content, ammonia nitrogen content, and organic matter content and total phosphoric acid content did not appear.
4) For water quality analysis, a total of seven items were measured: pH, EC, TOC, total nitrogen, ammonia nitrogen, nitrate nitrogen, and total phosphorus according to the public test standard for water pollution presented by the Ministry of Environment (2011).
As a result of correlation analysis between water quality items, there was no correlation between TOC and total nitrogen content, TOC and entire phosphoric acid content, EC and ammonia nitrogen, and EC and nitrate nitrogen. Contrary to the results of soil analysis, nitrate nitrogen was more dominant than ammonia nitrogen in the form of inorganic nitrogen present in water. It seemed to be sufficient dissolved oxygen in abandoned paddy wetlands.
5) Eight wetland functions (plant diversity and wildlife habitat, fish and amphibian reptile habitat, flood control, runoff reduction, and water quality) were evaluated using the Rapid Assessment Method(RAM) technique, which is a general function evaluation for each site, through the analysis of the ecological environment characteristics of the wet paddy field. conservation and improvement, revetment and dike protection, aesthetics and recreation, and groundwater maintenance) were evaluated. As a result of the functional evaluation, the average value of function was 2.01 to 2.20, which was to be 'normal.'
This study is meaningful in that it develops a guideline for analyzing the characteristics of the ecological environment of the carbon-accumulated abandoned paddy wetlands and presents essential data for the future creation, restoration, and conservation of abandoned paddy wetlands.
Considering the characteristics of abandoned paddy wetlands with sturdy variability, in-depth research using established traditional research methods and LiDAR and eDNA will be more accurate and sensitive research.