[Characteristics of Spatial Distribution of Soil Organic Carbon and Its Seasonal Change of Different Vegetation Buffer Zones in Duliujian River].

Abstract

Different vegetation types may affect the accumulation and transformation of soil organic carbon （SOC）, but it is unclear whether the organic carbon fixation is realized by litter input and/or root control of environmental factors and dissolved organic matter （DOM） of soils. In this study, the spatial distribution characteristics of easily oxidizable organic carbon （EOC）, dissolved organic carbon （DOC）, particulate organic carbon （POC）, mineral-bound organic carbon （MAOC）, and their seasonal variations in the surface soil （0-10 cm） were studied in different vegetation zones of the arbor forest （at the upper position）, the mixed forest of arbor and shrub （at the middle position）, and the waterfront vegetation （at the bottom position） in the ecological embankment of Duliujian River, Tianjin, China. The spatial distribution characteristics of soil DOM components and their seasonal changes were also analyzed by combining UV-visible spectroscopy and 3D fluorescence spectroscopy. The results showed that： ① The accumulation of SOC was significantly higher in the waterfront vegetation than in the arbor forest and the mixed forest of arbor and shrub in summer, whereas the opposite was true during the spring season. It was indicated that the root input of the soil was the key driving factor for determining the accumulation of SOC in summer, whereas the input quality of above-ground litters was more important for the sequestration of SOC in spring. ② Differences in DOM fractions explained the fixation and transformation pathways of SOC in different seasons, with humus-derived DOM in spring promoting the transformation of DOC to POC and MAOC and microbial-derived DOM in summer advancing the transfer of EOC to MAOC under the action of microorganisms. ③ Soil physicochemical properties had less direct influences on SOC, which preferentially affected SOC accumulation by regulating the composition as well as the chemical structure of soil DOM. ④ The structural equation modeling indicated that water content （MC） and total phosphorus （TP） were directly involved in SOC transport and transformation, whereas ammonium nitrogen （NH₄⁺-N）, nitrate nitrogen （NO₃^--N）, electrical conductivity （EC）, available phosphorus （AP）, pH, K⁺, and Na⁺ indirectly affected SOC accumulation mediated by DOM from humus and microbial sources. In summary, the present study elucidated that the trade-off mechanisms affecting SOC sequestration in the critical functional zone along the land-river ecotone, and the results can provide theoretical support for further exploring the constructive methods of ecological corridors and the pathways of carbon sequestration and sink enhancement in the "watershed-estuary-offshore" system of the coastal rivers.