Compositions of sequestrated soil carbon in constructed wetlands of Taiwan

Constructed wetlands are an ecological engineering technology that has been widely applied to treat anthropogenic wastewater. Until now, few studies have focused on soil carbon (C) in the constructed treatment wetlands in tropical regions. Therefore, this study provides insight into the changes in soil C composition of tropically constructed wetlands at different ages. Five constructed wetlands were investigated in northern Kaohsiung, Taiwan. Soil C was analyzed at three different depths using an acid-hydrolysable method. The results showed that soil TOC content was highest on the soil surface (0-2 cm) and decreased at greater soil depths (2-5 and 5-10 cm) in all the studied constructed wetlands. There was more soil acid-hydrolysable C in the older constructed wetlands than in the younger ones at all depths. On the contrary, the soil recalcitrant carbon (RP-C) did not vary much across the wetland soils. In addition, the RP-C to TOC ratios were higher in the younger than older constructed wetlands, implying that the soil bioavailable C sources for microbial growth increased with the wetland's age. As a result, the compositions of organotrophic microbes, such as methanogens (mcrA copies), appeared to increase with wetlands' ages (i.e., negatively correlated with RP-C/TOC), while the total microbial abundance (16S rDNA) and abundance of lithotrophic microbes, such as methanotrophs (pmoA copies), were not correlated with RP-C/TOC or AHPI-C/TOC ratios, based on the results of our canonical correspondence analysis. Furthermore, the constructed wetlands accumulated soil RP-C from 2.33 to 0.08 g C m^-2 day^-1 in the constructed wetlands 1 to 30 years old, respectively.