Impacts of converting coastal natural wetlands to mariculture ponds on soil dissolved organic matter.

Coastal wetlands are important "blue carbon" ecosystems for mitigating climate change. The rapid expansion of mariculture has driven extensive conversion of coastal wetlands into aquaculture ponds. We investigated the impacts of such conversion on soil dissolved organic matter (DOM) by collecting soil samples from a <i>Spartina alterniflora</i> saltmarsh and four saltmarsh-converted mariculture ponds with the cultivation ages of 7, 10, 12, and 15 years in Yancheng, Jiangsu Province. The sources and components of DOM were characterized using ultraviolet-visible spectroscopy and three-dimensional excitation-emission matrix fluorescence spectroscopy combined with parallel factor analysis. Results showed that DOM concentrations in the ponds were lower than that in <i>S. alterniflora</i> saltmarsh and declined progressively with increasing pond age. Four fluorescence components were identified, including three humic-like substances (C1, C2, C3) and one protein-like substance (C4). Terrestrial humic-like components (C1 and C3) dominated the DOM pool (58.7%-60.0%) but declined most sharply following the cultivation age (26.2%-51.7% for C1 and 26.6%-61.9% for C3). After seven years of conversion, DOM stability significantly decreased, and then exhibited an upward trend. Redundancy analysis revealed that soil organic carbon content, electrical conductivity, and water content were the main factors driving DOM changes during the conversion. In conclusion, the conversion of coastal wetlands to mariculture ponds reduced soil DOM content and stability, thereby threatening their carbon sink function. Future mariculture development should minimize the conversion of natural coastal wetlands.